NI_racing

If anyone knows what im talking about i would love to get a lil insight into IPR and how it actually works on the track. Thanks

XxXJokerXxX

What do you mean by IPR?

NI_racing

Inches per revolution when setting up the circumfrence of the rear tires.
Simply the advantages of running smaller or larger tires?

XxXJokerXxX

In reference to staggered tire sizes? (I.E. smaller tires on left side)

Or are you talking tire sizes in general referring to gearing and the effects of different tire sizes?

I'm assuming you're talking about staggered tire sizing since you're asking this question in a circle track thread and the latter question is more general mechanical knowledge.

On dirt we run anywhere from 1/2" to 1 1/2" across the front and 2" to 7" across the rear (full bodied cars only as sprints run way more).

In circle track racing staggered tire sizes do a few different things.

A larger right rear tire (running a larger difference across the rear) allows the car to "roll" into the corner under throttle, it will also make the car feel "loose" upon exiting the corner on the throttle.

When running mismatched tire sizes you inherently loose straight away speed as the right rear will always be covering more ground than the left rear and the force of the rear is always trying to force the car to turn left.

The "correct" stagger is the amount of stagger that allows the rear tires to turn at the same revolution while going through a turn.

With modern chassis technology much of the effects of tire stagger can be tuned out of the car rather easily as long as you are not running an extreme amount.

All of this is also true for asphalt except they just run smaller amounts of stagger.

Running a spool vs running a locking rear will produce different stagger effects.

I'm not really sure exactly what you want to know or what you're trying to do so that's just very general information. I can help you some more if you can give me a specific question.

NI_racing

This is exactly what i am after. SO, this is a bit diff than your typical circle track classes, so bear with me.

All competitors are running the same engine make, with the same tech rules (like a crate), internal diferences vary as long as they dont exceed tech. Usually the engines are withing 1 -2 hp of eachother across the board (well the good ones atleast). The minimum weight is of course the same since evryone is using the same engine. Now here is where the real difference comes in.... We are running fixed gears. The front (trans gear) is a class tech item. It has to be the same front gear for every competitor. However, you have a range of rear gears to choose from to get your ratio where you want it. Once you run out of gear to play with, you have to find another way to get more out of the engine, usually regarding RPMs. Now... a smaller tire circumfrence will yeild you less recipicating weight, which creates less parasitic drag, possibly less friction, which all will allow the vehicle to accelerate faster with less stress on the engine, and yeilds a higher RPM. That seems to be a general consensus....

The question at hand is this. A smaller circumfrence tire will travel a certain amount of inches per revoltion (IPR). A larger circumfrence tire will travel more inches per revolution. So in theory, i assumed that the larger tire would travel farther in the same amount of time, because it could cover more ground per revolution, thus making it faster?

However, it appears that this theory is not all inclusive, and depends on the other conditions and setup of the car, engine range, and so on. So my question is this. How would you determine the advantage or disadvantage of running varying tire circumfrences without buying a bunch of sets and blowing em up to different sizes, and testing out the results?

XxXJokerXxX

A larger tire will travel more distance in the same amount of time, yes. It will also take more power to get a larger tire moving as opposed to a smaller set and you therefore will loose some acceleration (depending upon the difference in sizes we may be talking an almost negligible amount).

As far as calculating what tire size is going to yield you the best results, you can calculate gearing and tire circumference changes all day long to get a certain RPM range which you will be running. This really won't translate into real world data on how the circumference change is going to effect your car, your setup, the track, your engine, and your driving style. There are a lot of different factors that will play into the results.

The only suggestion I can make to at least get you headed in the right direction with as little money wasted as possible is to make a baseline run on the tires you are running now. Then run a set at the lowest circumference you can get out of whatever tires your series allows and another set at the highest circumference you can get. I'm not really suggesting you go with a 10" difference but use your best judgement as to what you think you can run as this will depend entirely on the rubber and the properties of the tires your series allows. Some stretch a lot and others very little. Make sure you also try to keep the differences similar (if you run a circumference 2" larger than baseline then run a circumference 2" smaller than baseline). You will then be able to get some feedback from the stopwatch and also your driver on any improvements you have made and which direction in circumference you should go. Likely you will have went too far one way or the other but at least you will then know what direction produces better results and a better feel for your driver.

As far as changing rear gear goes, any change you make in tire circumference can either be multiplied or negated by a change in rear gear ratio. If you go to a smaller circumference and then a taller (3.80 to a 3.40) ratio you are working against the tire change you just made. The opposite also hold true as if you go to a smaller circumference and then a shorter gear (3.80 to 4.10) you are multiplying the effects of the tire change and will be left with more acceleration and lower straight away speed.

It is best to change one thing at a time as making multiple adjustments may yield good results, but you will have no idea what it actually was that got you there.

NI_racing

Got ya..

XxXJokerXxX

I hope that somewhat answered your question and I didn't totally miss your point.

NI_racing

No you answered. Alot of that i already knew. just wasnt looking forward to buying more tires

XxXJokerXxX

I kind of figured as you seemed to know what you were talking about so sorry if I dumbed it down a little too much for you. I hate buying tires more than anything else but you know how it goes, if you want to fly you got to buy.

What series do you run?