Spring rates

[BeQuietAndDrive]

When looking at various coilover systems for my car, and springs as well, I've found that nearly all JDM coilover manufacturers (Tien, JIC, Tanabe, etc.) have higher spring rates in front than in the rear.

This seems to differ from what I've read about reducing understeer on FF cars.

Anyone have any input?

[Leonard]

Spring rate by itself isn't a good number to compare front/rear rates.

What you are concerned with is the wheel rate (or effective spring rate at the wheel). Changing the spring rate affects the wheel rate, but how much it affects it is dependent upon the motion ratio (or where the spring interfaces with the control arm).

Essentially, the control arm is a lever and gives you a mechanical advantage when compressing the spring. The closer the spring is to the end of the control arm (the lever) the less advantage you have. The further inboard it mounts, the more mechanical advantage you have.


wheel rate = spring rate * MR^2

If the coilover mounts to the control arm in the exact center (50% of the way from the pivot point to the wheel), the MR is .5. If it were to attach at the pivot point the MR=0 and if it attaches at the end of the control arm, the MR=1.

So, here's an example.

Say the coilover mounts in the middle of the control arm, (MR=.5) and you have a 500lb/in spring.

500*.5^2 = 500*.25 = 125lb/in wheel rate.

Take another example with the same spring 500lb/on, but the coilover mounting to the control arm very close to the wheel (say MR=.9)

500*.9^2 = 500*.81 = 405lb/in

[Leonard]

Also, the average car has more weight up front than in the rear, so you're going to need more wheel rate to control the weight of the car.

[BeQuietAndDrive]

I see. Also, spring rates are dependant on vehicle weight, correct? As in, springs with a 5 kg/ 4kg rate will be much stiffer on a 2300 lb vehicle than a 3500 lb vehicle,correct?

[PseudoRealityX]

every one of the true coilover systems is made for STREET cars, and therefore is setup to understeer. And it's TRUE in every single case that I've driven.

[BeQuietAndDrive]

Well,that sucks...

[Loren]

It's not all gloom and doom. The aftermarket coilover suspension probably won't understeer AS MUCH as the stock suspension. And you'll still have the option (if you're careful and make sure the kit you get either uses standard springs or has different spring options that can be purchased) of changing out the front or rear springs later if you want to to improve balance.

For a front driver, a good coilover kit and a big rear swaybar coupled with a really good alignment can do wonders.

If you want to get closer to an optimal autocross setup on the first try, look at other FWD cars of similar weight, power and suspension design and see what they're using for spring rates. I doubt that you'll want higher spring rates in the rear, but you might end up with the difference between front and rear a bit less than anyone's standard "street" setup.

[BeQuietAndDrive]

Yeah,those were exactly my plans.

I believe I'll end up going with Tein Basic coilovers. I'm not sure if I should spend the extra few hundred for shock dampening adjustment,though.

[Leonard]

Dampening adjustment=yes. You can use it to fine tune the setup (within limits) for various surfaces.

Whatever you do, as Loren said, try to go with something that uses standard spring sizes. My FFR uses 2.5" diamater 10" long springs. I can get springs in 25lb/in increments for $110 a pair.

[jaball77]

If you already know what spring rates you want, why not put something together yourself? Some Ground Control coilovers/Eibach ERS springs, and some Konis would probably come in cheaper than the ///JDM brands you listed. :-)

And a side note, there are at least a couple FWD cars that buck the "higher rates in front" trend. The Integra R has 250lb LINEAR springs in the front, and 250lb progressives in the rear. And the newest Celicas have like 130f/160r springs. :-)

[MrSideways]

As Jesse already said. Tein, Tanabe etc. Those companies are for clueless people that will probably never understand what they actually have under the car for suspension. They are made for the street and these companies make a suspension that will cause the car to understeer ie. Saftey to keep them from being sued. You want to make your car handle. Start doing research order yourself a set of Koni Yellows. Send em to pro parts, True Choice, or even koni them selves. Pick your spring rates you want. and tell the people at whatever company you send em to that you want the shocks valved for X spring rate and converted to Coil overs, and please put X springs on the shock. I just did this with the M3. I got the shocks used for free but figure.... $700 for the shocks.
$100 in shipping. (both ways)
$1100 in valving and converting to coilovers
$60-80 a spring.
True Choice is a little on the $$$ side but they had the quickest turn around time. I think Koni told me they could do it for $800 on the valving, but they wanted 3 months to get it back.

[BeQuietAndDrive]

I definatly don't know enough about suspension tuning to get custom everything. JIC makes a really good, 15 way dampening adjustable system for my car, that I have been looking into.

This car is also driven by me every day, so I don't need something TOO crazy.

Ground Control,how do those work? You pick the shock and they provide the spring and everything else?

[Leonard]

You might want to take a look at the Carroll Smith books

Tune to Win
Drive to Win
Engineer to Win

They'll really get you pointed in the right direction as far as understanding how to accomplish what you're trying to achieve. (More importantly, you'll understand why certain changes need to be made and how it really works)

[KJ]

Find a set of used Koni Yellow's on ebay and have them revalved for the highest springrates you see yourself possibly using. Order a set of Ground control's and several sets of springs at different rates. Test them out and see what works best with how you drive.

[MrSideways]

JIC 15 way adjustable. HEHE it cracks me up when I read that stuff. First of all you only get rebound adjustment. Which is no big deal because 90% of people have no idea how to adjust their shocks, let alone compression adjustment. But the 15 points are within a range and just clicks. Koni's are infinite adjustment within that range!