FAQ: About Chassis and Suspension (Reading Required)

[treekiller]

I am beginning to realize the lack of good information available in respect to this forum. In an attempt to bring the education level of this forum to the level it should be. I am going to over the next couple weeks the following.

Basic Theory:

Why do I even Need suspension
Diffrent types of suspension (What do I have?)
Under/Oversteer (I can make my car do that?)
Sprung Vs. Unsprung Weight (lighter is better)
Slip vs. Grip angle (when turning the wheel is bad)
Tires (tires are a spring? and the "Gription theory")
Basic Math (I'm no engineer, how do I figure this out)
Tyre-road friction and tyre slip (tyre loading as requested)
Centrifugal Force (the imaginary cornering force)
Traction Circle (spending money in the RIGHT places)
Center of Gravity, (why lower is not ALWAYS faster)
Weight transfer (the springrate and swaybar guide)
Dampers (Shocks, struts, what do these things do, and how)
Suspension Travel (the rally diaries)
Putting power down (FWD, AWD, RWD Vs. Jet power)


Allignment:

Kingpin Angles (steering axis inclination)
Toe In or out (Of the swimming pool?)
Roll Centers (camber gain under compression)
Castor (Scrub Radius)
Squat and Dive (and how they change the dynamics)
Spring Pre-load (I may not need it, do I want it?)
Stock vs. mild street vs. racing (my car stinks on ice)

Using this Knowledge on track:

Racing Line (how can I corner faster)
Horsepower (how a Miata "can" beat a corvette)
Race Braking theory (Threshold braking vs. ABS)
Tuning Dampers and Spring Rate (Spotting BS from fact)


I will update this post with the information as I get to it, the following thread will be for questions and answers so I can update this post and then make a proper FAQ out of it. if we can keep the questions to the current topic I'd appreciate it.. when we are done we should have a decent FAQ

THIS IS NOT A THREAD FOR VENDORS TO SELL THEIR SERVICES. DON'T EVEN THINK ABOUT IT!

[treekiller]

Well... you don't NEED suspension, Lets look at a go-kart. one of the best handling vehicles in the world... Their suspension is fixed, yes there is still alignment, however as we will discuss later in this article, A go kart tyre and wheel is specifically designed to act as the only vertical motion between the chassis and the ground. and the tyre acts as a Spring AND a shock, but like I said we will touch on that later. so the spring rate on a go kart is infinity, there is a small bit of chassis flex, but for our purposes we will assume ZERO spring rate or a rate of infinity. (maybe there is more to suspension thing then I assumed)

Now if we all drove around on perfectly flat smooth roads, we could get away without suspensions, Bicycles made due for centuries with out suspensions but recently it's hard to buy one without. so I don't NEED a suspension? RIGHT But I want one? YES, if you want to travel at any speed.

so since we don't live in a world with perfectly flat smooth roads we need to compromise, our tires need to travel up and down, they need to follow the pavement, allow us to traverse curbs, handle the occasional pothole at 75mph without spearing off into the tree line, but what else does my suspension do?

1: It carries the weight of the car, and allows changes in weight, and cargo.
2: It keeps the wheels perpendicular to the road, regardless of what the car is doing
3: It allows us to accelerate and brake with great force.
4: it handles large torque loads without twisting the wheels off the car
5: It handles large lateral loads, cornering forces, (much more when we are done)

[treekiller]

(content and pictures used with thanks to carbibles.com)

There are several diffrent types of suspension design, the earliest vehicle suspensions are thousands of years old found on OX carts (wagons) and chariots. now they were basically a solid axle between 2 wheels and some rudimentary form of spring. there was no damper other then the friction within the spring, and cars used friction dampers well into the 1930's. so our modern suspension design is a very new science. (in the grand scheme of things)

What are the major suspension types? since we know no cars we drive are using dead solid axles there must be something diffrent. and there are several variants More then I can list so I am going to focus on the "TOP 9"

1. Live Axle + 4 Link Live axle
2. De Deion
3. McPherson and Chapman Strut
4. Double Wishbone
5. Multi Link
6. trailing Arm + Semi Trailing Arm.
7. Transverse Leaf Spring
8. Beam axle (more often twist beam)
9. Ford Control Blade

Solid-axle, leaf-spring

http://www.carbibles.com/images/solidaxleleafspring.jpg
This system was favoured by the Americans for years because it was dead simple and cheap to build. The ride quality is decidedly questionable though. The drive axle is clamped to the leaf springs and the shock absorbers normally bolt directly to the axle. The ends of the leaf springs are attached directly to the chassis, as are the tops of the shock absorbers. Simple, not particularly elegant, but cheap. The main drawback with this arrangement is the lack of lateral location for the axle, meaning it has a lot of side-to-side slop in it.

Solid-axle, coil-spring

http://www.carbibles.com/images/solidaxlecoilspring.jpg

This is a variation and update on the system described above. The basic idea is the same, but the leaf springs have been removed in favour of either 'coil-over-oil' spring and shock combos, or as shown here, separate coil springs and shock absorbers. Because the leaf springs have been removed, the axle now needs to have lateral support from a pair control arms. The front ends of these are attached to the chassis, the rear ends to the axle. The variation shown here is more compact than the coil-over-oil type, and it means you can have smaller or shorter springs. This in turn allows the system to fit in a smaller area under the car.
4-Bar

http://www.carbibles.com/images/4barparallel.jpg http://www.carbibles.com/images/4bartriangle.jpg

4-bar suspension can be used on the front and rear of vehicles - I've chosen to show it in the "rear" section of this page because that's where it's normally found. 4-bar suspension comes in two varieties. Triangulated, shown on the right here, and parallel, shown on the left.
The parallel design operates on the principal of a "constant motion parallelogram". The design of the 4-bar is such that the rear end housing is always perpendicular to the ground, and the pinion angle never changes. This, combined with the lateral stability of the Panhard Bar, does an excellent job of locating the rear end and keeping it in proper alignment. If you were to compare this suspension system on a truck with a 4-link or ladder-bar setup, you'd notice that the rear frame "kick up" of the 4-bar setup is far less severe. This, combined with the relatively compact installation design means that it's ideal for cars and trucks where space is at a premium. You'll find this setup on a lot of street rods and American style classic hot rods.
The triangulated design operates on the same principle, but the top two bars are skewed inwards and joined to the rear end housing much closer to the centre. This eliminates the need for the separate panhard bar, which in turn means the whole setup is even more compact.

Derivatives of the 4-Bar system

There are many variations on the 4-bar systems I've illustrated above. For example, if the four angled bars go from the axle outboard to the chassis near the centreline, this is called a "Satchell link". (Satchell is a US designer, who used the above linkage on some of Paul Newmans Datsun road racers some years back.) It has certain advantages over the above examples. Both of the these angled linkages can be reversed to have the angled links below the axle and the parallel links above. The roll centre will be lowered with the angled bars under the axle, a function which is difficult to accomplish without this design. The other variation on the "four bars" not shown are the Watts and Jacobs bar linkages to replace the Panhard rod for lateral positioning. Another linkage is the two parallel bars above the axle and a triangulated link underneath - a design you will find on the Lotus 7 - where the lower link has its base on the chassis and the apex under the differential. Then there is the Mallock Woblink, which could be described as half way between a Jacobs ladder and a Watts link, and makes it possible to place the rear roll centre quite low without sacrificing ground clearance.
Watts links are pretty popular with the hydraulic lowrider/truck bed dancer types. The Jacobs ladder is used almost exclusively on US midget and sprintcar dirt track rear ends. The Mallock Woblink is used mostly on the Mallock U2 Clubman cars in Great Britain.
de Dion suspension, or the de Dion tube
de Dion suspension, or the de Dion tube

http://www.carbibles.com/images/dediontube.jpg

The de Dion tube - not part of the London underground, but rather a semi-independent rear suspension system designed to combat the twin evils of unsprung weight and poor ride quality in live axle systems. de Dion suspension is a weird bastardisation of live-axle solid-beam suspension and fully independent trailing-arm suspension. It's neither one, but at the same time it's both. Weird! With this system, the wheels are interconnected by a de Dion Tube, which is essentially a laterally-telescoping part of the suspension designed to allow the wheel track to vary during suspension movement. This is necessary because the wheels are always kept parallel to each other, and thus perpendicular to the road surface regardless of what the car body is doing. This setup means that when the wheels rebound, there is also no camber change which is great for traction, and that's the first advantage of a de Dion Tube. The second advantage is that it contributes to reduced unsprung weight in the vehicle because the transfer case / differential is attached to the chassis of the car rather than the suspension itself.
Naturally, the advantages are equalled by disadvantages, and in the case of de Dion systems, the disadvantages would seem to win out. First off, it needs two CV joints per axle instead of only one. That adds complexity and weight. Well one of the advantages of not having the differential as part of the suspension is a reduction in weight, so adding more weight back into the system to compensate for the design is a definite distadvantage. Second, the brakes are mounted inboard with the calipers attached to the transfer case, which means to change a brake disc, you need to dismantle the entire suspension system to get the driveshaft out. (Working on the brake calipers is no walk in the park either.) Finally, de Dion units can be used with a leaf-spring or coil-spring arrangement. With coil spring (as shown here) it needs extra lateral location links, such as a panhard rod, wishbones or trailing links. Again - more weight and complexity.
de Dion suspension was used mostly used from the mid 60's to the late 70's and could be found on some Rovers, the Alfa Romeo GTV6, one or two Lancias a smattering of exotic racing cars and budget sports cars or coupes.
More recently deDion suspension has had somewhat of a renaissance in the specialist sports car and kit car market such as those from Caterham, Westfield and Dax. These all uniformly now use outboard brake setups for ease-of-use, and a non-telescoping tube, usually with trailing links and an A-bar for lateral location (rather than a Watts linkage or Panhard rod.) Whilst a properly setup independent suspension system will always win hands-down on poorly maintained roads, when you get on to the track, the advantage is not so clear cut and a well set up deDion system can often match it turn-for-turn now, espeically for flyweight cars.

MacPherson Strut or Chapman strut

http://www.carbibles.com/images/mcpherson.gif

This is currently, without doubt, the most widely used front suspension system in cars of European origin. It is simplicity itself. The system basically comprises of a strut-type spring and shock absorber combo, which pivots on a ball joint on the single, lower arm. At the top end there is a needle roller bearing, which allows the assembly to pivot as a single solid unit. In the picture here, you can't see the shock absorber because it is encased in the black gaiter inside the spring.

The steering gear is either connected directly to the lower shock absorber housing, or to an arm from the front or back of the spindle (in this case). When you steer, it physically twists the strut and shock absorber housing (and consequently the spring) to turn the wheel. Simple. The spring is seated in a special plate at the top of the assembly which allows this twisting to take place. If the spring or this plate are worn, you'll get a loud 'clonk' on full lock as the spring frees up and jumps into place. This is sometimes confused for CV joint knock.

Double wishbone

http://www.carbibles.com/images/coilspring1.gif http://www.carbibles.com/images/coilspring2.gif
Type 1 Type 2
Coil Spring type 1

This is a type of double-A or double wishbone suspension. The wheel spindles are supported by an upper and lower 'A' shaped arm. In this type, the lower arm carries most of the load. If you look head-on at this type of system, what you'll find is that it's a very parallelogram system that allows the spindles to travel vertically up and down. When they do this, they also have a slight side-to-side motion caused by the arc that the wishbones describe around their pivot points. This side-to-side motion is known as scrub. Unless the links are infinitely long the scrub motion is always present. There are two other types of motion of the wheel relative to the body when the suspension articulates. The first and most important is a toe angle (steer angle). The second and least important, but the one which produces most pub talk is the camber angle, or lean angle. Steer and camber are the ones which wear tyres.

Coil Spring type 2 This is also a type of double-A arm suspension although the lower arm in these systems can sometimes be replaced with a single solid arm (as in my picture). The only real difference between this and the previous system mentioned above is that the spring/shock combo is moved from between the arms to above the upper arm. This transfers the load-bearing capability of the suspension almost entirely to the upper arm and the spring mounts. The lower arm in this instance becomes a control arm. This particular type of system isn't so popular in cars as it takes up a lot room.

Multi-link suspension
http://www.carbibles.com/images/multilink2.jpg
This is the latest incarnation of the double wishbone system described above. It's currently being used in the Audi A8 and A4 amongst other cars. The basic principle of it is the same, but instead of solid upper and lower wishbones, each 'arm' of the wishbone is a separate item. These are joined at the top and bottom of the spindle thus forming the wishbone shape. The super-weird thing about this is that as the spindle turns for steering, it alters the geometry of the suspension by torquing all four suspension arms. They have complex pivot systems designed to allow this to happen.
Car manufacturers claim that this system gives even better road-holding properties, because all the various joints make the suspension almost infinitely adjustable. There are a lot of variations on this theme appearing at the moment, with huge differences in the numbers and complexities of joints, numbers of arms, positioning of the parts etc. but they are all fundamentally the same. Note that in this system the spring (red) is separate from the shock absorber (yellow).

Trailing-arm suspension

http://www.carbibles.com/images/trailingarm.jpg
The trailing arm system is literally that - a shaped suspension arm is joined at the front to the chassis, allowing the rear to swing up and down. Pairs of these become twin-trailing-arm systems and work on exactly the same principle as the double wishbones in the systems described above. The difference is that instead of the arms sticking out from the side of the chassis, they travel back parallel to it. This is an older system not used so much any more because of the space it takes up, but it doesn't suffer from the side-to-side scrubbing problem of double wishbone systems. If you want to know what I mean, find a VW beetle and stick your head in the front wheel arch - that's a double-trailing-arm suspension setup. Simple.

Transverse leaf-spring

http://www.carbibles.com/images/transverseleafspring.jpg
This system is a bit odd in that it combines independent double wishbone suspension with a leaf spring like you'd normally find on the rear suspension. Famously used on the Corvette, it involves one leaf spring mounted across the vehicle, connected at each end to the lower wishbone. The centre of the spring is connected to the front subframe in the middle of the car. There are still two shock absorbers, mounted one to each side on the lower wishbones. Chevy insist that this is the best thing since sliced bread for a suspension system but there are plenty of other experts, manufacturers and race drivers who think it's junk. It's never been clear if this was a performance and design decision or a cost issue, but this type of system is very rare.
http://www.carbibles.com/images/herald_tilt.jpg Historically, Triumph used transverse leaf spring suspension on their small chassis cars (Herald, Vitesse, Spitfire & GT6). In the good old British school of thought, they did this because it was cheap. The spring was bolted to the differential, rather than the chassis, and under (very) hard cornering you got jacking and tuck-under. If you got this whilst driving and panicked enough to let off the gas, or worse, step on the brake, you got massive over-steer, and pirouetted off into the nearest tree. There were plenty of complaints about this suspension system in the late 60's, so Triumph changed to a 'swing spring' system on some cars (no longer bolted to the diff), and what they called 'rotoflex' on the GT6. Again from the good old British school of thought, the replacement system was unnecessarily complicated and allegedly very fragile.

Beam Axle

http://www.carbibles.com/images/beamaxle.jpg


This system is used in front wheel drive cars, where the rear axle isn't driven. (hence it's full description as a "dead beam"). Again, it is a relatively simple system. The beam runs across under the car with the wheels attached to either end of it. Spring / shock units or struts are bolted to either end and seat up into suspension wells in the car body or chassis. The beam has two integral trailing arms built in instead of the separate control arms required by the solid-axle coil-spring system. Variations on this system can have either separate springs and shocks, or the combined 'coil-over-oil' variety as shown here. One notable feature of this system is the track bar (or panhard rod). This is a diagonal bar which runs from one end the beam to a point either just in front of the opposite control arm (as here) or sometimes diagonally up to the top of the opposite spring mount (which takes up more room). This is to prevent side-to-side movement in the beam which would cause all manner of nasty handling problems. A variation on this them is the twist axle which is identical with the exception of the panhard rod. In a twist axle, the axle is designed to twist slightly. This gives, in effect, a semi-independent system whereby a bump on one wheel is partially soaked up by the twisting action of the beam. Yet another variation on this system does away with the springs and replaces them with torsion bars running across the chassis, and attached to the leading edge of the control arms. These beam types are currently very popular because of their simplicity and low cost.

Ford Control Blade™ Suspension

http://www.carbibles.com/images/controlblade1.jpghttp://www.carbibles.com/images/controlblade2.jpg
A lot of attention and marketing has been coming out of Ford recently about their new Control Blade™ rear suspension. Details and engineering facts are predictably sketchy but the glossy marketing brochures will tell you this revolution in rear suspension will make your Ford Focus handle better, grip the road better, and brake better than everything else on the road. It warrants some investigation when they make claims like that, but it turns out what they mean is "we've got a new suspension system", and not much else. It actually started out its life sometime around 1998 in Ford of Australia and I believe Holden had something to do with it too. Since then its become far more mainstream.
So "Control Blade™" is the snappy marketing name that Ford use to describe their new system. It sounds good, looks good on paper, and has an aura of 21st century-ness about it. "Blade". Ooh. Cool.
The reality isn't quite so cool though - control blade is basically an evolution of trailing-arm suspension. However its still an interesting development and it does serve the purpose for which Ford designed it. The primary purpose of Control Blade suspension is to increase the interior space available in the vehicle. Most suspension systems used in daily drivers have strut towers front and rear. In the front it's not really a problem, but in the rear it impedes on boot (or trunk) space. Ford wanted to give more space in the back and needed to find a good way to remove or reduce the size of the strut towers. The result is their Control Blade™ system which in essence separates the shock absorber from the springs. To do this, Ford needed to use a trailing-arm type suspension so that they didn't have swingarms up under the wheel arches. The springs were shortened and moved inboard and underneath. In one variation, the shock absorbers still sit vertically but the space they take up now is hugely reduced because they no longer have the coil springs around the outside. In the second variation the shock absorber is a subminiature unit mounted inboard of the springs underneath the vehicle. I'm not sure of the merits of the super-short shock absorber but Ford seem to think it works. The control blades themselves are basically the trailing arms which give lateral support and provide the vertical pivot point for the entire unit.
The Ford spiel says this about Control Blade™: "It has the key function of promoting ride and reducing road noise transmission, while providing the freedom to let the lateral links define toe and camber by absorbing any rearward forces and allowing the rest of the suspension to do it's job uninterrupted. Effectively isolating the handling components of the new IRS from the road noise and impact harshness components of the suspension.". In English? It means better handling and less road noise. Looking at the basic design it's not difficult to see that this system has a much lower centre of gravity than a Macpherson strut (for example). Lower C-of-G in a vehicle is always a good thing. The geometry of the Control Blade™ system also provides significant 'anti-dive' under braking force, which means a the car body will dive less when you jump on the brakes which in turn translates into more well-behaved braking response. Lower C-of-G, less roll and less pitch during braking all add up to better handling, althouth whether the average driver would notice or not is a different matter.
Another function of this system is that they've separated the two basic functions of suspension. With the springs and shock absorbers being mounted in different places, Ford have managed to optimise the function of these components. It's similar in concept to what BMW did with the telelever front suspension on motorbikes - separating braking from suspension forces, only in the control blade system, it separates the springing support of the suspension from the shock reducing functions of the shock absorbers.
The images below are currently from other sources as I've not had the time to render up my own just yet, but they show the basic layout of each variation of control blade suspension and I've annotated them accordingly.

Aftermarket on Control Blade™ vehicles.

There's one thing worth noting about this suspension system. Because the spring and shock are in different locations, and because of the reduced or removed strut towers, it makes it very difficult to bolt-on aftermarket suspension kits to these vehicles. For the daily driver, that's probably not an issue but if you're looking at spiffing up the suspension on a Ford Focus for track days or racing, it's not going to be quite so straightforward as it is on other cars. Just so you know.

[treekiller]

This is one of the more fun basic topics in relation to suspension.

Keep in mind while some of the methods expressed here are for "TRACK USE ONLY" while when perfected you will find them helpful in street driving do not practice agressive tactics in the street as there are no sand traps or runoff room on the street and messing up a complex turn will put you off the track, or on the street, into a tree. (don't ask me how I know, however I did not get the nickname treekiller because I am unfamiliar with mother nature)

There are a million catch phrases that explain these diffrent types of handling but one truth remains self evident, if your not experiencing under or oversteer then your not driving hard enough.

Some of my favorites,

NASCAR: under(tight) is when you hit the wall with the front of your car
NASCAR: over(loose) is when you hit the wall with the rear of the car

"Understeer is when the driver is scared, oversteer is when the passenger is scared" -unknown

Understeer is a car that has a tendency to "push" or want to go straight at the limit, where if you add more steering input the car just will not turn any more,
http://www.f1-country.com/f1-engineer/understeer.gif

Oversteer is a car that has a tendency to be loose or want to "come around" -Spin.
http://www.f1-country.com/f1-engineer/oversteer.gif

The easiest way to explain dramatic oversteer is by watching a drift or rally driver, the rear begins to rotate the corner faster then the front giving the car the ability to turn faster, or "go sideways".... but it's not always faster in lap times. turning rubber into smoke only wastes horsepower that could be used to accelerate the car out of the corner.

http://i.a.cnn.net/nascar/.element/img/2.0/sect/kyn/101/glossary/oversteer-loose.gif

This is called Power induced oversteer, which is only possible when at least a portion of the power is going to the rear wheels. and those rear wheels are able to break traction with their surface (much easier on dirt) a front wheel drive car will never experience power on oversteer.

Now there are other forms of oversteer more common to the FWD community, almost everyone in their teens experiments with the first or I call it "kindergarten" method of oversteer, the E-Brake turn. By locking the rear wheels while the suspension is loaded in a corner the rear looses grip and you get instant and violent oversteer, unless the surface is VERY loose and the turn is Very sharp this is hardly the "FAST" way around a corner, but the E-brake with an immediate and violent follow up of acceleration in a Rwd car is often used to initiate a "Drift" or as us civilized people like to call it a "power slide".

Also and more commonly used in racing is what is called "trailing throttle oversteer" where as you lift off the throttle, the weight shifts forward to the front wheels allowing you to control the mid corner dynamics of the car with a slight and smooth input of the throttle the car returns to "neutral". all fast track drivers with cars set up to handle near "neutral" use this method to attack many corners.

There is a third method of inducing oversteer in a car that has a tendency to want to push (most any FWD car) and it's called "Trail Braking" just like trailing throttle oversteer letting the car slowly move the weight over the front wheels, "Trail Breaking" is mostly used on cars that have a tendency to push. it's done by carrying your braking later then you normally would often with your left foot, (which we will explain later) and by still applying the brakes AFTER you've initiated the corner, this forces much more weight onto the front wheels (and in effect off the rear wheels) and allows the driver to make the rear of the car "light" and oversteer. now when the driver wants the car to settle he just smoothly applies the throttle and the weight go back over the rear wheels increasing their grip and lessening the front's grip.

Here is myself trail breaking the entrance to a corner, notice all he weight of the car shifted to the front wheel, and the inside rear wheel has gotten so light it has left the ground completely

http://www.floridaracing.org/auto-x/bmw031608/images/IMG_4324.jpg

Understeer,

Most commonly experienced understeer is "throttle on understeer", when in a corner A car tuned for mild street driving, or a FWD car adding more power will only cause the car to turn less, I can recall countless students in my drivers clinics complaining about mid corner understeer.. or commonly I get the statement "My car pushes like a pig" and this is from drivers of well tuned BMW M3's or cars of that nature. much of that is driver training. (which we will get to at the end of this lesson.) more often then not they are not carrying enough speed into a corner, and then trying to accelerate too hard or too early since the entry speed was too low. You will learn in driving dynamics the effects of under vs. oversteer favor the later much greater with increased speed. where a car that pushes at 30mph might be a tail happy monster at 90. (this is why you'll see areodynamic wings int the back of cars.) and if there is a wing on the back of a civic you can now laugh at the driver since you know it's most likley hurting the handling of the car.

I won't spend too much time on understeer since 99.99 percent of all the cars on the road understeer and battling it with the techniques I just provided will be more important. but understand WHAT the difference is.

If you go to your local club autocross you will see people in stock cavaliers, yaris, sentras turning in lap times that best the Porsche. Corvettes, and BMW's. and you'll ask HOW? it's because understanding the dynamics of the car, and knowing what to do , and when, is 100 times more important then a trick swaybar, spring kit, or set of dampers. if there is anything you take from this lesson it should be to learn to drive and what the car is doing, then modify to suit the shortcomings. Too many times a driver will modify a car first then learn to drive the car modified.. which in turn they never learned proper vehicle dynamics. and have to re-learn when they buy a new car.

A truly GREAT driver can get in any car and drive it at 90% of it's capability. (I my friends am not a GREAT driver, just good)

Now our focus in this session is the basics and explanations, we will get into detail about how to tune a car to do what you want, and what changes to make in later sessions.


A little added bonus I found:

Here is a nascar Fact sheet about adjusting spring preload (they call it wedge) to increase under or over steer (thght vs. loose)
http://i.a.cnn.net/si/2006/racing/02/13/tight.vs.loose/nascar101-tightvsloose.jpg

[gritt]

The strut itself is the load-bearing member in this assembly, with the spring and shock absorber merely performing their duty as oppose to actually holding the car up.

can you explain this a little better for me. maybe i`m not thinking straight, but by this description, you could take the spring and shock out of the assembly and the strut (now just a strut tube as the shock absorbing member is removed) would hold the car up? all the McStrut systems I have worked on, if you took those 2 components out and tried to drive the car you wouldn`t make it off the lift.

[treekiller]

I agree that section was written in poor English, so I cut it down. and eliminated that statement. while accurate, it's meaningless without a diagram. but a quick and easy example of a NON coil over Strut is the 79-93 mustang. you could use a eye to pin shock absorber and the inboard spring. and the strut would just be a locating and alignment component. Just Ignore that one for now. it's not used often enough to leave in.

[gritt]

gotcha, thanks for clearing that up.

[Outkasted24]

Your are the Man!!!

[treekiller]

In a ground vehicle with a suspension, the unsprung weight (or, more properly, the unsprung mass) is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them, rather than supported by the suspension. (The mass of the body and other components supported by the suspension is the sprung mass.) Unsprung weight includes the mass of components such as the wheel spindles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.

http://upload.wikimedia.org/wikipedia/commons/e/ed/Car_diagram.jpg

Effects of Unsprung Weight
The unsprung weight of a wheel controls a trade-off between a wheel's bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression--which induces a force on the unsprung weight. In time, the unsprung weight then responds to this force with movement of its own. The amount of movement is inversely proportional to the weight - a lighter wheel which readily moves in response to road bumps will have more grip when tracking over an imperfect road. For this reason, lighter wheels are often sought for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality is deteriorated.
Pneumatic or elastic tires help by providing some springing for most of the (otherwise) unsprung mass, but the damping that can be included in the tires is limited by considerations of fuel economy and overheating. The shock absorbers, if any, damp the spring motion also and must be less stiff than would optimally damp the wheel bounce. So the wheels execute some vibrations after each bump before coming to rest. On dirt roads and perhaps on some softly paved roads, these motions form small bumps, known as washboarding or "corduroy" because they resemble smaller versions of the bumps in roads made of logs. These cause sustained wheel bounce in subsequent vehicles, enlarging the bumps.
High unsprung weight also exacerbates wheel control under hard acceleration or braking. If the vehicle does not have adequate wheel location in the vertical plane (such as a rear-wheel drive car with Hotchkiss drive, a live axle supported by simple leaf springs), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control.
Though this is usually not considered important, at least in the popular literature, there is a positive effect. High frequency road irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the body more completely because the tires and springs act as separate filter stages, with the unsprung weight tending to uncouple them. This can improve overall safety.

Unsprung Weight and Vehicle Design
Unsprung weight is largely a function of the design of a vehicle's suspension and the materials used in the construction of suspension components. Beam axle suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung weight than independent suspension systems, in which the wheels are suspended and allowed to move separately. Heavy components such as the differential can be made part of the sprung weight by connecting them directly to the body (as in a de Dion tube rear suspension). Lightweight materials, such as aluminum, plastic, carbon fiber, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility.
Inboard brakes make a big difference, but put more load on half axles and (constant velocity) universal joints and require space that may not be easily accommodated.

[treekiller]

In car handling, slip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing (i.e., the angle of the vector sum of wheel translational velocity vX and sideslip velocity vY). This slip angle results in a force perpendicular to the wheel's direction of travel -- the cornering force. This cornering force increases approximately linearly for the first few degrees of slip angle, then increases non-linearly to a maximum before beginning to decrease.

what this means in English for you and me... You turn into a corner, a slight amount of steering wheel input and the slip angle is near zero. so the car turns faster and faster with more steering wheel input. until eventually you exceed the slip angle your tires operate at then you begin to loose sideways (cornering) force.. in addition as your wheels near 45 degrees a good portion of that sideways cornering force becomes braking force pushing backward against the forward progress of the car. (this is often called "scrubbing speed" almost never a good thing when trying to set a lap time) and turns your momentum into heat and tire dust.

Just like when accelerating or braking the tire generates it's optimum force (Gription) when it's sliding but only a small percentage of it's overall movement.

This is where things start getting fun for the tuner since you can adjust the optimum slip angle with alignment, Tire loading (spring rate or sway bars)

A non-zero slip angle arises because of deformation in the tire carcass and tread. As the tire rotates, the friction between the contact patch and the road result in individual tread 'elements' (infinitely small sections of tread) remaining stationary with respect to the road. If a side-slip velocity u is introduced, the contact patch will be deformed. As a tread element enters the contact patch the friction between road and tire means that the tread element remains stationary, yet the tire continues to move laterally. This means that the tread element will be ‘deflected’ sideways. In reality it is the tire/wheel that is being deflected away from the stationary tread element, but convention is for the co-ordinate system to be fixed around the wheel mid-plane.
As the tread element moves through the contact patch it will be deflected further from the wheel mid-plane: (and as you can guess a slick tire does not suffer this deformation)



http://upload.wikimedia.org/wikipedia/en/e/e5/TreadDeflected1.jpg (http://en.wikipedia.org/wiki/Image:TreadDeflected1.jpg)
This deflection gives rise to the slip angle, and to the cornering force.
Because the forces exerted on the wheels by the weight of the vehicle are not distributed equally, the slip angles of each tire will be different. The ratios between the slip angles will determine the vehicle's behavior in a given turn. If the ratio of front to rear slip angles is greater than 1:1, the vehicle will tend to understeer, while a ratio of less than 1:1 will produce oversteer. Actual instantaneous slip angles depend on many factors, including the condition of the road surface, but a vehicle's suspension can be designed to promote specific dynamic characteristics. A principal means of adjusting developed slip angles is to alter the relative roll couple (the rate at which weight transfers from the inside to the outside wheel in a turn) front to rear by varying the relative amount of front and rear lateral load transfer. This can be achieved by modifying the height of the Roll centers, or by adjusting roll stiffness, either through suspension changes or the addition of an anti-roll bar.

Now I can include all the math it takes to calculate these items, but for our purposes it's not required, just understand that whenever trying to make a suspension behave in a desired way the slip angle of the tires must always be taken into account. since exceeding the design of your tire will result in slower lap times and make your tuning much harder.

A good quote I was reminded about is "if your tire is growling you are close, if your tire is howling you've exceeded the slip angle" Back off a bit. we will also discuss later how driving style, by allowing the car to load up the tires properly, and using the dynamic weight shift of the vehicle. you will get increased performance out of the same exact car.

[treekiller]

I'll say it now. tires are the MOST import part of any suspension. and should be the FIRST modification... then setting the suspension to make those tires work should follow. buy the best tires your racing class or budget will allow. and if you tune your car to take advantage of those tires you will ALLWAYS be faster then someone that is chasing the math.

My tires are a spring?

Yes the only TRUE unsprung weight on any car is the tire itself, not only is that weight truly unsprung, but it also has the most rotational inertia in the entire drive train system, This is why you'll see "True" racing tires as light as possible. one lb of tire weight is equal to 8 lbs off a flywheel. so that fancy flywheel does not seem so important now. even some R-compound tires continue to use old fashion cloth radial bands to keep the weight down. while this practice was okay with flobbery bias ply tires, when applying it to radial tires it tends to make them less then streetable. since the radial bands actually support the weight of the car and do not just hold the tire together. but we are not here to talk about tire technology details. that could take a whole other FAQ.

What you need to know is the stiffness of the tire directly effects the dynamic spring rate. A stiffer carcass will deform less under cornering and transfer more of that weight to the contact patch as will stiffer springs. in retrospect a softer or taller sidewall will transfer less road vibration into the suspension and offer a softer ride.

what is best. well for performance as light as possible and as rigid as possible while these things might seem not to go hand in hand a true racing tire can achieve both of course it does not have to worry about sustaining it's rigidity over 30,000 miles or handling potholes, neither of which racing tires are known for.

A new terminology has cropped up since I first started tuning cars. it's called "tire spring rate" and it took the invention of the run-flat tire and it's effect on ride quality to raise the eyebrows of tire engineers.

Keep in mind a tire IS a spring, and that spring rate is variable with pressure. however also keep in mind a tire is almost a living breathing part of the suspension, it has an internal damping, which can be overcome essentially with too much air pressure, in addition the tread surface has a tendency to bow out (reduce contact patch) with too much air pressure. so while stiffer tire spring rate can help handling... like EVERYTHING else we discuss here there are limitations. and that all depends on the construction of the tire, rim width, and what you are asking it to do. This is why racing tires cost up to $600 each and you can buy a pep-boys special for $25. there is as much technology in a set of tires as in the rest of the suspension of your car. and this can be proven by taking a standard road car and fitting racing tires on it. it will lap a track 5-7 seconds faster then the standard car, where a full suspension kit might only provide 3-4 seconds on the same car and track.


So what is the "Gription" thing you keep mentioning. well it's a combination of traction and grip. neither word perfectly explains what is going on when a tire is being used in a racing environment. when a tire reaches it's optimum temperature, there is actually a physical bond between the road surface and the tread surface. many times this exceeds the friction provided between rim and the tire so in some racing they actually use glue or screws to fasten the tire to the rim. this is also why you will see black marks left from a car when not visibly sliding left from the tires. the friction between the surface of the tread and the road has exceeded the friction between the rubber molecule and it's neighbor molecule. so that small part of rubber stays bonded to the road surface instead of staying with the tire. as you can imagine that's A LOT of grip and really only common with true racing tires and only in a small temperature range.

this also explains racing tires terrible reputation of being unforgiving at the limit. as the tire reaches it's optimum tempature it grips more and more (a street tire is almost never designed to reach this temp) and when you exceed that temp it begins to slide, well. then it gets hotter and slides more. this is commonly referred to as a tire getting "greasy" so when it lets go it usually does so in a speticualar display of smoke and in RWD a spinning car , or fwd a car that cannot stop occasionally into a hard object. so we don't want that.

This is why you'll see racing tires available in several compounds, with engineering data so you can tailor the tire to the track and driving style as to approach the limit of grip but not exceed it. This is also why you'll see a Drag car do a smokey burnout, to bring the temp beyond the designed maximum so when they line up it will have cooled close to the optimum. providing a much cleaner getaway and in turn a lower ET. and yes a burnout can help a street car launch, but without the technology and perfect temp known, it's very hard to predict how much heat and when to line up. done right it can shave a lot of time from a drag racing time, done wrong... well a powerful car will spin the entire quarter mile.

[treekiller]

Thanks to: Brian Beckman
©Copyright 1991
In the last two articles, we plunged right into some relatively complex issues, namely weight transfer and tire adhesion. This month, we regroup and review some of the basic units and dimensions needed to do dynamical calculations.



Physics is the science of measurement. Perhaps you have heard of highly abstract branches of physics such as quantum mechanics and relativity, in which exotic mathematics is in the forefront. But when theories are taken to the laboratory (or the race course) for testing, all the mathematics must boil down to quantities that can be measured. In racing, the fundamental quantities are distance, time, and mass. This month, we will review basic equations that will enable you to do quick calculations in your head while cooling off between runs. It is very valuable to develop a skill for estimating quantities quickly, and I will show you how.

Equations that don't involve mass are called kinematic. The first kinematic equation relates speed, time, and distance. If a car is moving at a constant speed or velocity, http://www.miata.net/sport/Physics/_7731_tex2html_wrap111.gif, then the distance http://www.miata.net/sport/Physics/_7731_tex2html_wrap113.gif it travels in time http://www.miata.net/sport/Physics/_7731_tex2html_wrap115.gif is http://www.miata.net/sport/Physics/_7731_tex2html_wrap95.gif or velocity times time. This equation really expresses nothing more than the definition of velocity.
If we are to do mental calculations, the first hurdle we must jump comes from the fact that we usually measure speed in miles per hour (mph), but distance in feet and time in seconds. So, we must modify our equation with a conversion factor, like this http://www.miata.net/sport/Physics/_7731_tex2html_wrap96.gif
If you ``cancel out'' the units parts of this equation, you will see that you get feet on both the left and right hand sides, as is appropriate, since equality is required of any equation. The conversion factor is 5280/3600, which happens to equal 22/15. Let's do a few quick examples. How far does a car go in one second (remember, say, ``one-one-thousand, two-one-thousand,'' etc. to yourself to count off seconds)? At fifteen mph, we can see that we go http://www.miata.net/sport/Physics/_7731_tex2html_wrap97.gif or about 1 and a half car lengths for a 14 and 2/3 foot car like a late-model Corvette. So, at 30 mph, a second is three car lengths and at 60 mph it is six. If you lose an autocross by 1 second (and you'll be pretty good if you can do that with all the good drivers in our region), you're losing by somewhere between 3 and 6 car lengths! This is because the average speed in an autocross is between 30 and 60 mph.

Everytime you plow a little or get a little sideways, just visualize your competition overtaking you by a car length or so. One of the reasons autocross is such a difficult sport, but also such a pure sport, from the driver's standpoint, is that you can't make up this time. If you blow a corner in a road race, you may have a few laps in which to make it up. But to win an autocross against good competition, you must drive nearly perfectly. The driver who makes the fewest mistakes usually wins!
The next kinematic equation involves acceleration. It so happens that the distance covered by a car at constant acceleration from a standing start is given by http://www.miata.net/sport/Physics/_7731_tex2html_wrap98.gif or 1/2 times the acceleration times the time, squared. What conversions will help us do mental calculations with this equation? Usually, we like to measure acceleration in http://www.miata.net/sport/Physics/_7731_tex2html_wrap117.gifs. One http://www.miata.net/sport/Physics/_7731_tex2html_wrap117.gif happens to be 32.1 feet per second squared. Fortunately, we don't have to deal with miles and hours here, so our equation becomes, http://www.miata.net/sport/Physics/_7731_tex2html_wrap99.gifhttp://www.miata.net/sport/Physics/_7731_tex2html_wrap121.gif, which is a typical number for a good, stock sports car, will go 8 feet in 1 second. Not very far! However, this picks up rapidly. In two seconds, the car will go 32 feet, or over two car lengths. roughly. So, a car accelerating from a standing start at
Just to prove to you that this isn't crazy, let's answer the question ``How long will it take a car accelerating at http://www.miata.net/sport/Physics/_7731_tex2html_wrap123.gif to do the quarter mile?'' We invert the equation above (recall your high school algebra), to get http://www.miata.net/sport/Physics/_7731_tex2html_wrap100.gif and we plug in the numbers: the quarter mile equals 1320 feet, http://www.miata.net/sport/Physics/_7731_tex2html_wrap125.gif, and we get http://www.miata.net/sport/Physics/_7731_tex2html_wrap127.gif which is about 13 seconds. Not too unreasonable! A real car will not be able to keep up full http://www.miata.net/sport/Physics/_7731_tex2html_wrap129.gif acceleration for a quarter mile due to air resistance and reduced torque in the higher gears. This explains why real (stock) sports cars do the quarter mile in 14 or 15 seconds.
The more interesting result is the fact that it takes a full second to go the first 8 feet. So, we can see that the launch is critical in an autocross. With excessive wheelspin, which robs you of acceleration, you can lose a whole second right at the start. Just visualize your competition pulling 8 feet ahead instantly, and that margin grows because they are `hooked up' better.

For doing these mental calculations, it is helpful to memorize a few squares. 8 squared is 64, 10 squared is 100, 11 squared is 121, 12 squared is 144, 13 squared is 169, and so on. You can then estimate square roots in your head with acceptable precision.
Finally, let's examine how engine torque becomes force at the drive wheels and finally acceleration. For this examination, we will need to know the mass of the car. Any equation in physics that involves mass is called dynamic, as opposed to kinematic. Let's say we have a Corvette that weighs 3200 pounds and produces 330 foot-pounds of torque at the crankshaft. The Corvette's automatic transmission has a first gear ratio of 3.06 (the auto is the trick set up for vettes-just ask Roger Johnson or Mark Thornton). A transmission is nothing but a set of circular, rotating levers, and the gear ratio is the leverage, multiplying the torque of the engine. So, at the output of the transmission, we have http://www.miata.net/sport/Physics/_7731_tex2html_wrap101.gifhttp://www.miata.net/sport/Physics/_7731_tex2html_wrap102.gif Now, at rest, the car has about 50/50 weight distribution, so there is about 1600 pounds of load on the rear tires. You will remember from last month's article on tire adhesion that the tires cannot respond with a forward force much greater than the weight that is on them, so they simply will spin if you stomp on the throttle, asking them to give you 2870 pounds of force. of torque. The differential is a further lever-multiplier, in the case of the Corvette by a factor of 3.07, yielding 3100 foot pounds at the center of the rear wheels (this is a lot of torque!). The distance from the center of the wheel to the ground is about 13 inches, or 1.08 feet, so the maximum force that the engine can put to the ground in a rearward direction (causing the ground to push back forward-remember part 1 of this series!) in first gear is

We can now see why it is important to squeeeeeeeze the throttle gently when launching. In the very first instant of a launch, your goal as a driver is to get the engine up to where it is pushing on the tire contact patch at about 1600 pounds. The tires will squeal or hiss just a little when you get this right. Not so coincidentally, this will give you a forward force of about 1600 pounds, for an http://www.miata.net/sport/Physics/_7731_tex2html_wrap131.gif (part 1) acceleration of about http://www.miata.net/sport/Physics/_7731_tex2html_wrap133.gif, or half the weight of the car. The main reason a car will accelerate with only http://www.miata.net/sport/Physics/_7731_tex2html_wrap135.gif to start with is that half of the weight is on the front wheels and is unavailable to increase the stiction of the rear, driving tires. Immediately, however, there will be some weight transfer to the rear. Remembering part 1 of this series again, you can estimate that about 320 pounds will be transferred to the rear immediately. You can now ask the tires to give you a little more, and you can gently push on the throttle. Within a second or so, you can be at full throttle, putting all that torque to work for a beautiful hole shot!

In a rear drive car, weight transfer acts to make the driving wheels capable of withstanding greater forward loads. In a front drive car, weight transfer works against acceleration, so you have to be even more gentle on the throttle if you have a lot of power. An all-wheel drive car puts all the wheels to work delivering force to the ground and is theoretically the best.

Technical people call this style of calculating ``back of the envelope,'' which is a somewhat picturesque reference to the habit we have of writing equations and numbers on any piece of paper that happens to be handy. You do it without calculators or slide rules or abacuses. You do it in the garage or the pits. It is not exactly precise, but gives you a rough idea, say within 10 or 20 percent, of the forces and accelerations at work. And now you know how to do back-of-the-envelope calculations, too.

[Nereth]

You should probably mention tyre load sensitivity at some stage. It's one of the most important things in vehicle dynamics that I ever learned.

It's probably much more important to the average person than slip angles.

[treekiller]

Tyre-road friction and tyre slip (thanks to tut.fi)

Let us consider the truck-tyre in detail.
When braking the tyre in stationary conditions, the normal force acts just in front of the wheel centre. Within the contact area a shear stress arises that increases until the adhesion limit is reached and sliding occurs, after which it decreases broadly proportional (Coulomb friction assumed) with the locally occurring normal stress.
The speed of the tyre relative to the wheel centre first increases. When the adhesion limit is reached, a sliding speed arises locally, resulting in a nonzero average speed Vslip of the rubber within the contact area. The ratio of this speed and the wheel speed in percent is designated the longitudinal slip sx = - k:
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_1_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_1)
If a side force operates on the tyre, a lateral deformation appears in the tyre belt and its tread. Points on the running surface experience the belt deformation before they make contact with the road at which point the tyre first attempts to maintain contact with the road surface.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_2_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_2)
Figure 1 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_2) - Braking the tyre
This corresponds to a gradually increasing shear stress in lateral direction. Once the adhesion limits are reached, the rubber will start to slide relative to the road with a lateral motion, perpendicular to the wheel plane. The asymmetry in the distribution of shear stress causes the resulting force not to grip exactly in the middle of the contact area just under the wheel centre. Rather there is a pneumatic trail which, in combination with the side force, produces an aligning torque which tries to push the tyre in the direction of the wheel speed. The tangent of the slipangle between wheelplane and wheel speed, denoted as side slip -sy :
sy = tan(a)
in conjunction with the wheel load and the camber angle, are decisive for the side force and the aligning torque.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_3_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_3)
Figure 2 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_3) - Cornering of a tyre
The relationships between the position of the tyre, in terms of the slipvalues, and tyreresponse in terms of longitudinal and lateral force, pneumatic trail and aligning torque are of essential importance in studying vehicle behaviour. Without a good description of these tyrecharacteristics, such kind of research ìs impossible.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_4_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_4)
Figure 3 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_4) - A tyre under combined slip conditions
Besides the properties described in pure slip conditions, one is also interested in situations of combined slip which are pertinent to braking when cornering. The maximum shear force between tyre and road surface is given by the existent coefficient of friction multiplied by the wheel load. The implication for braking in a bend is that the possible maximum brake force relative to a situation of straight line braking will be reduced. One has therefore sacrificed braking potential which is indicated by the friction ellipse, in which realistic combinations of brake or drive force and side force are shown separately.
Tyre characteristics
The above discussion finally leads to relationships between:
Side force versus lateral slip
Pneumatic trail versus lateral slip
Aligning torque versus lateral slip
Brake or drive force versus longitudinal slip
under pure slip conditions (only lateral or longitudinal slip). In case of combined slip, the side force also depends on longitudinal slip, etc. Typical examples of these pure slip characteristics are shown in figure 4 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_5). One observes a strong nonlinear behaviour for larger slip. These relationships are of essential importance in studying vehicle behaviour. Without a good description of these tyre characteristics, such kind of research is impossible. The slope of the side force Fy vs. slip angle a near a = 0 (the cornering stiffness) is the determining parameter in the linear basic handling and stability theory of automobiles, as we shall see later.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_5_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_5)
Figure 4 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_5) - Some typical tyre characteristics
Under combined slip conditions, typical plots of Fx (brake or drive force) versus Fy (cornering force) are shown below for fixed values of slip angle a (taken from DELFT-TYRE). For small values of a, the side force almost vanishes. As a increases, the side force Fy becomes apparent at the cost of a lower maximum value of the longitudinal force Fx.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_6_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_6)
Figure 5 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_6) - Combined slip, Fx versus Fy
It is important to note that tyre shear forces depend on tyre load. This dependence is usually nonlinear, where for increasing tyre load, the absolute slope of the tyre force versus tyre load reduces. This is the reason why during cornering, the average lateral tyre force per axle reduces due to force redistribution from inner to outer wheel. Consequently, as we shall see later, the steering performance of the vehicle is changed which might even lead to yaw-instability (oversteer conditions).
An example is shown in figure 6 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_7) with Fy depicted vs. load Fz for three different slip angle. One observes the decreasing absolute slope of these curves, meaning that under load transfer of for example 1500 N (being the increase, decrease of the tyre load at outer and inner tyre, respectively) and with an axle slip angle of 0.08 rad., the total lateral force is reduced. In other words, the cornering stiffness is reduced due to this load transfer.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_7_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_7)
Figure 6 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_7) - Load sensitivity lateral force
The illustration is for passenger cars, for which a restricted load variation is apparent. This is different for truck tyres where large variations in payload and thus large variations in tyre load will occur. This is illustrated in figure 7 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_8) where the normalized cornering stiffnesses (cornering stiffness coefficient: tyre cornering stiffness divided by the tyre load) for typical truck and passenger car tyres are depicted vs, tyre load. One oberves a decreasing trend for both passenger and truck tyres, with a much smaller sensitivity for truck tyres compared to passenger tyres.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_8_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_8)
Figure 7 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_8) - Load sensitivity passenger car and truck tyres
We close this section with some remarks concerning factors that influence tyre characteristic curves. The variations of the longitudinal force coefficient (defined as Fx/Fz) and longitudinal force versus longitudinal slip of two truck radial tyres are depicted in figure 8 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_9) and figure 9 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_10). The peak value is the maximum that can be reached without wheel locking, while the slide value is obtained during the locked wheel. Figure 10 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_11) presents some typical ranges of values of the longitudinal force coefficient obtained on a dry concrete surface for bias and radial tyres with two type of tread patterns design.
The dependencies of the longitudinal force coefficient to the load and speed are presented in figure 11 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_12) and figure 12 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_13).
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_9_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_9)
Figure 8 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_9) - Longitudinal force coefficient vs. slip
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_10_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_10)
Figure 9 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_10) - Braking force vs. slip of a truck tyre [3.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_11_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_11)
Figure 10 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_11) - Longitudinal force coefficient on dry road [2.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_12_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_12)
Figure 11 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_12) - Effect of load (radial tyre) [2.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_13_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_13)
Figure 12 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_13) - Effect of speed (radial tyre) [2.1]
Next, we consider the lateral force in more detail. This force is a function not only of simple friction but also of the size, design, construction and operating condition (i.e. load and inflation pressure).
The influences of the tyre construction and loading condition to lateral force coefficient (defined as Fy/Fz) are presented in figure 13 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_14) and figure 14 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_15).
It must be noticed that radial truck tyres are more responsive than bias truck tyres and the low profile radial truck tyre has a more constant lateral force coefficient through the load change (important in suspension design).
Combining braking and cornering by adding braking force to a tyre which rolls with slip angle results the friction ellipse concept. This ellipse envelops all the plotted curves of lateral forces versus longitudinal forces for different slip angles, as discussed above. Figure 15 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_16) and figure 16 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_17) show some combined forces for two truck tyres (bias and radial) while figure 17 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_18) shows the influence of slip angle on braking forces.
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_14_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_14)
Figure 13 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_14) - Effect of tyre construction [2.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_15_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_15)
Figure 14 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_15) - Effect of load [2.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_16_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_16)
Figure 15 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_16) - Braking and cornering (bias tyre) [2.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_17_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_17)
Figure 16 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_17) - Braking and cornering (radial tyre) [3.1]
http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/kuvat/pic3_1_18_thumb.gif (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_18)
Figure 17 (http://www.tut.fi/plastics/tyreschool/moduulit/moduuli_10/hypertext/3/3_1.html#3_1_18) - Influence of slip angle on braking forces [8]

[treekiller]

Centrifugal Force - The False Force
An evil word has worked its way into our daily vocabulary, and with it, an incorrect understanding of the way physics works. "Centrifugal Force (http://regentsprep.org/Regents/physics/glossary.htm#Centrifugal%20Force)" ( Latin for "center fleeing") is often used to describe why mud gets spun off a spinning tire, or water gets pushed out of the clothes during the spin dry cycle of your washer. It is also used to describe why we tend to slide to the outer side of a car going around a curve. It is a common explanation...the only problem is all of it is absolutely wrong!!! Centrifugal force does not exist...there is no such thing...it is a ghost we tend to blame odd behavior on.
Take for example this common situation. You are riding in a car going around a curve. Sitting on your dashboard is a cassette tape. As you go around the curve, the tape moves to outside edge of the car. Because you don't want to blame it on ghosts, you say "centrifugal force pushed the tape across the dashboard."--wwrroonngg!! When we view this situation from above the car, we get a better view of what is really happening. The animation below shows both views at the same time. The top window shows you the bird's eye view of the car and the tape, while the bottom window shows you the familiar view from the passenger.
http://regentsprep.org/Regents/physics/phys06/bcentrif/casette.gif
The car tires on the road have a enough static friction to act as centripetal force which forces the car to go around the curve. The tape on the slippery dashboard does not have enough friction to act as a centripetal force, so in the absence of a centripetal force the tape follows straight line motion. The car literally turns out from underneath the tape, but from the passenger's point of view it looks as though something (a ghost force?) pushed the tape across the dashboard. If the car you are riding in has the windows rolled down, then the tape will leave the car (or does the car leave the tape?) as it follows its straight line path. If the windows are rolled up, then the window will deliver a centripetal force to the tape and keep it in a circular path.
Any time the word Centrifugal Force is used, what is really being described is a Lack-of-Centripetal Force.


or as Brian Beckman puts it


©Copyright 1991
One often hears of ``centrifugal force.'' This is the apparent force that throws you to the outside of a turn during cornering. If there is anything loose in the car, it will immediately slide to the right in a left hand turn, and vice versa. Perhaps you have experienced what happened to me once. I had omitted to remove an empty Pepsi can hidden under the passenger seat. During a particularly aggressive run (something for which I am not unknown), this can came loose, fluttered around the cockpit for a while, and eventually flew out the passenger window in the middle of a hard left hand corner.
I shall attempt to convince you, in this month's article, that centrifugal force is a fiction, and a consequence of the fact first noticed just over three hundred years ago by Newton that objects tend to continue moving in a straight line unless acted on by an external force.
When you turn the steering wheel, you are trying to get the front tires to push a little sideways on the ground, which then pushes back, by Newton's third law. When the ground pushes back, it causes a little sideways acceleration. This sideways acceleration is a change in the sideways velocity. The acceleration is proportional to the sideways force, and inversely proportional to the mass of the car, by Newton's second law. The sideways acceleration thus causes the car to veer a little sideways, which is what you wanted when you turned the wheel. If you keep the steering and throttle at constant positions, you will continue to go mostly forwards and a little sideways until you end up where you started. In other words, you will go in a circle. When driving through a sweeper, you are going part way around a circle. If you take skid pad lessons (highly recommended), you will go around in circles all day.
If you turn the steering wheel a little more, you will go in a tighter circle, and the sideways force needed to keep you going is greater. If you go around the same circle but faster, the necessary force is greater. If you try to go around too fast, the adhesive limit of the tires will be exceeded, they will slide, and you will not stick to the circular path-you will not ``make it.''
From the discussion above, we can see that in order to turn right, for example, a force, pointing to the right, must act on the car that veers it away from the straight line it naturally tries to follow. If the force stays constant, the car will go in a circle. From the point of view of the car, the force always points to the right. From a point of view outside the car, at rest with respect to the ground, however, the force points toward the center of the circle. From this point of view, although the force is constant in magnitude, it changes direction, going around and around as the car turns, always pointing at the geometrical center of the circle. This force is called centripetal, from the Greek for ``center seeking.'' The point of view on the ground is privileged, since objects at rest from this point of view feel no net forces. Physicists call this special point of view an inertial frame of reference. The forces measured in an inertial frame are, in a sense, more correct than those measured by a physicist riding in the car. Forces measured inside the car are biased by the centripetal force.
Inside the car, all objects, such as the driver, feel the natural inertial tendency to continue moving in a straight line. The driver receives a centripetal force from the car through the seat and the belts. If you don't have good restraints, you may find yourself pushing with your knee against the door and tugging on the controls in order to get the centripetal force you need to go in a circle with the car. It took me a long time to overcome the habit of tugging on the car in order to stay put in it. I used to come home with bruises on my left knee from pushing hard against the door during an autocross. I found that a tight five- point harness helped me to overcome this unnecessary habit. With it, I no longer think about body position while driving-I can concentrate on trying to be smooth and fast. As a result, I use the wheel and the gearshift lever for steering and shifting rather than for helping me stay put in the car!
The `forces' that the driver and other objects inside the car feel are actually centripetal. The term centrifugal, or ``center fleeing,'' refers to the inertial tendency to resist the centripetal force and to continue going straight. If the centripetal force is constant in magnitude, the centrifugal tendency will be constant. There is no such thing as centrifugal force (although it is a convenient fiction for the purpose of some calculations).
Let's figure out exactly how much sideways acceleration is needed to keep a car going at speed http://www.miata.net/sport/Physics/_7932_tex2html_wrap98.gif in a circle of radius http://www.miata.net/sport/Physics/_7932_tex2html_wrap100.gif. We can then convert this into force using Newton's second law, and then figure out how fast we can go in a circle before exceeding the adhesive limit-in other words, we can derive maximum cornering speed. For the following discussion, it will be helpful for you to draw little back-of-the-envelope pictures (I'm leaving them out, giving our editor a rest from transcribing my graphics into the newsletter).
Consider a very short interval of time, far less than a second. Call it http://www.miata.net/sport/Physics/_7932_tex2html_wrap118.gif (http://www.miata.net/sport/Physics/_7932_tex2html_wrap104.gif stands for ``delta,'' a Greek letter mathematicians use as shorthand for ``tiny increment''). In time http://www.miata.net/sport/Physics/_7932_tex2html_wrap118.gif, let us say we go forward a distance http://www.miata.net/sport/Physics/_7932_tex2html_wrap128.gif and sideways a distance http://www.miata.net/sport/Physics/_7932_tex2html_wrap136.gif. The forward component of the velocity of the car is approximately http://www.miata.net/sport/Physics/_7932_tex2html_wrap140.gif. At the beginning of the time interval http://www.miata.net/sport/Physics/_7932_tex2html_wrap118.gif, the car has no sideways velocity. At the end, it has sideways velocity http://www.miata.net/sport/Physics/_7932_tex2html_wrap120.gif. In the time http://www.miata.net/sport/Physics/_7932_tex2html_wrap118.gif, the car has thus had a change in sideways velocity of http://www.miata.net/sport/Physics/_7932_tex2html_wrap120.gif. Acceleration is, precisely, the change in velocity over a certain time, divided by the time; just as velocity is the change in position over a certain time, divided by the time. Thus, the sideways acceleration is http://www.miata.net/sport/Physics/_7932_tex2html_wrap90.gif How is http://www.miata.net/sport/Physics/_7932_tex2html_wrap136.gif related to http://www.miata.net/sport/Physics/_7932_tex2html_wrap100.gif, the radius of the circle? If we go forward by a fraction http://www.miata.net/sport/Physics/_7932_tex2html_wrap126.gif of the radius of the circle, we must go sideways by exactly the same fraction of http://www.miata.net/sport/Physics/_7932_tex2html_wrap128.gif to stay on the circle. This means that http://www.miata.net/sport/Physics/_7932_tex2html_wrap130.gif. The fraction http://www.miata.net/sport/Physics/_7932_tex2html_wrap126.gif is, however, nothing but http://www.miata.net/sport/Physics/_7932_tex2html_wrap134.gif. By this reasoning, we get the relation http://www.miata.net/sport/Physics/_7932_tex2html_wrap91.gif We can substitute this expression for http://www.miata.net/sport/Physics/_7932_tex2html_wrap136.gif into the expression for http://www.miata.net/sport/Physics/_7932_tex2html_wrap148.gif, and remembering that http://www.miata.net/sport/Physics/_7932_tex2html_wrap140.gif, we get the final result http://www.miata.net/sport/Physics/_7932_tex2html_wrap92.gif This equation simply says quantitatively what we wrote before: that the acceleration (and the force) needed to keep to a circular line increases with the velocity and increases as the radius gets smaller.
What was not appreciated before we went through this derivation is that the necessary acceleration increases as the square of the velocity. This means that the centripetal force your tires must give you for you to make it through a sweeper is very sensitive to your speed. If you go just a little bit too fast, you might as well go much too fast-your're not going to make it. The following table shows the maximum speed that can be achieved in turns of various radii for various sideways accelerations. This table shows the value of the expression http://www.miata.net/sport/Physics/_7932_tex2html_wrap93.gif which is the solution of http://www.miata.net/sport/Physics/_7932_tex2html_wrap142.gif for http://www.miata.net/sport/Physics/_7932_tex2html_wrap98.gif, the velocity. The conversion factor 15/22 converts http://www.miata.net/sport/Physics/_7932_tex2html_wrap98.gif from feet per second to miles per hour, and 32.1 converts http://www.miata.net/sport/Physics/_7932_tex2html_wrap148.gif from gees to feet per second squared. We covered these conversion factors in part 3 of this series.
http://www.miata.net/sport/Physics/_7932_tabular65.gif
For autocrossing, the columns for 50 and 100 feet and the row for 1.00 http://www.miata.net/sport/Physics/_7932_tex2html_wrap150.gif are most germane. The table tells us that to achieve 1.00 http://www.miata.net/sport/Physics/_7932_tex2html_wrap150.gif sideways acceleration in a corner of 50 foot radius (this kind of corner is all too common in autocross), a driver must not go faster than 27.32 miles per hour. To go 30 mph, 1.25 http://www.miata.net/sport/Physics/_7932_tex2html_wrap150.gif is required, which is probably not within the capability of an autocross tire at this speed. There is not much subjective difference between 27 and 30 mph, but the objective difference is usually between making a controlled run and spinning badly.
The absolute fastest way to go through a corner is to be just over the limit near the exit, in a controlled slide. To do this, however, you must be pointed in just such a way that when the car breaks loose and slides to the exit of the corner it will be pointed straight down the optimal racing line at the exit when it ``hooks up'' again. You can smoothly add throttle during this maneuver and be really moving out of the corner. But you must do it smoothly. It takes a long time to learn this, and probably a lifetime to perfect it, but it feels absolutely triumphal when done right. I have not figured out how to drive through a sweeper, except for the exit, at anything greater than the limiting velocity because sweepers are just too long to slide around. If anyone (Ayrton Senna, perhaps?) knows how, please tell me!
The chain of reasoning we have just gone through was first discovered by Newton and Leibniz, working independently. It is, in fact, a derivation in differential calculus, the mathematics of very small quantities. Newton keeps popping up. He was perhaps the greatest of all physicists, having discovered the laws of motion, the law of gravity, and calculus, among other things such as the fact that white light is made up of multiple colors mixed together.
It is an excellent diagnostic exercise to drive a car around a circle marked with cones or chalk and gently to increase the speed until the car slides. If the front breaks away first, your car has natural understeer, and if the rear slides first, it has natural oversteer. You can use this information for chassis tuning. Of course, this is only to be done in safe circumstances, on a rented skid pad or your own private parking lot. The police will gleefully give you a ticket if they catch you doing this in the wrong places.

[vivin]

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[playbodys]

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[kevinthenerd]

Centrifugal force DOES exist, but you first have to assume a non-inertial (accelerating) frame of reference. The math gets ugly for anything more than a simple case, yes, but it's doable. Centripetal acceleration is definitely a better concept to explain to early physics students, but it is incorrect to say centrifugal force doesn't exist at all.

For anyone willing to look at the ugly math, it's all explained in the book Vector Mechanics for Engineers: Dynamics by Beer, Johnston, Clausen, and Staab.

[treekiller]

The traction circle is way of thinking about the grip that a particular tire has on the road and how you can use it.
Visualize, if you will, a circle, with an x-axis and y-axis running through the center. The edges of the circle denote 100% utilization, the x-axis represents lateral grip (or cornering grip), the y-axis represents longitudinal grip (or braking/accelerating grip). This area in the circle represents the domain of your tire's grip. Points inside the circle are possible combinations of acceleration, braking, and turning; points outside cause the tire to lose grip and slide. If you're a real geek, you'll realize that this is a vector whose magnitude is always less than 1.
This traction circle is a way of teaching people the basics of tire grip, the essential limiting factor in performance driving. In the traction circle, you can be either turning left, turning right, accelerating, braking, or a combination of turning one way and accelerating and braking. Duh. The important thing the traction circle illustrates is that you can combine turning and speeding up or slowing down, but the less of one you do, the more of the other. This explains why you can't go through a hairpin at 100mph, but more importantly, it tells you why you can't go through a 51mph turn at 52. It also helps explain why turning strategies tend to tell you to brake, turn, accelerate rather than to brake through the turn, then accelerate through it.
It is also an important way of expressing driving cues. When a racer brakes at 80%, that 80% is not putting 80% of the pedal to the floor, nor is it a constant. The percentage depends on the overall size of the traction circle, which is the amount of grip available in the tire. If conditions change (such as cresting a hill or running over a bit of gravel), that 80% may become 110%, and the tire will slide. Of course, a racer's job is to always maximize available grip through a turn, so she would brake at 95% unless she was worried about weight transfers (which also affect available grip) or needed to turn.
Of course, the best strategy would be to follow very closely to the edge of the traction circle at all times so that one is not taken off guard by sudden changes, but you risk being passed by someone using 98% of their grip. Traction becomes an issue when following behind someone, as you will most likely be forced into the line they choose, so outbraking and pushing the car becomes the strategy to pass. However, because the difference between 100% and 95% is never more than a few seconds at the end of a race, it is more important to focus on cutting good lines and following the track than to focus on pushing the car.

From Wikipedia, the free encyclopedia

The Circle of forces or Traction circle is a useful way to think about the dynamic interaction between a vehicle's tire and the road surface. In the diagram below we are looking at the tire from above, so that the road surface lies in the x-y plane. The vehicle that the tire is attached to is
moving in the positive y direction.

http://upload.wikimedia.org/wikipedia/en/thumb/9/93/Circle-of-Forces.gif/180px-Circle-of-Forces.gif http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png

Circle of Forces


In this example, the vehicle would be cornering to the right (i.e. the positive x direction points to the center of the corner). Note that the plane of rotation of the tire is at an angle to the actual direction that the tire is moving (the positive y direction). That angle is the slip angle.
A tire can generate horizontal force where it meets the road surface by the mechanism of slip. That force is represented in the diagram by the vector F. Note that in this example F is perpendicular to the plane of the tire. That is because the tire is rolling freely, with no torque applied to it by the vehicle's brakes or drive train. However, that is not always the case.
The magnitude of F is limited by the dashed circle, but it can be any combination of the components Fx and Fy that does not exceed the dashed circle. (For a real-world tire, the circle is likely to be closer to an ellipse, with the y axis slightly longer than the x axis.)
In the example, the tire is generating a component of force in the x direction (Fx) which, when transferred to the vehicle's chassis via the suspension system in combination with similar forces from the other tires, will cause the vehicle to turn to the right. Note that there is also a small component of force in the negative y direction (Fy). This represents drag that will, if not countered by some other force, cause the vehicle to decelerate. Drag of this kind is an unavoidable consequence of the mechanism of slip, by which the tire generates lateral force.
The diameter of the circle of forces, and therefore the maximum horizontal force that the tire can generate, is affected by many things, including the design of the tire and its condition (age and temperature, for example), the qualities of the road surface, and the vertical load on the tire.

http://www.snowtire.info/References/Push-Pull/Push_Pull_or_Both_html_64b4e5f9.gif