Originally Posted by
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?
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.