V1.2 Nov 22 2006
This is a work in progress, and it has been a very long time coming. I intend on working on it for the next little while, dumping as much of my brain and hard-earned knowledge into the public record. It is, however, NOT public domain; I reserve the copyright for myself. You may NOT duplicate this elsewhere without my express permission - and this is the age of Google, folks, I WILL find you if you post it elsewhere. Otherwise, read and enjoy, and put this all to good use. Go fast!. DG
The single most important part of the whole suspension system is THE TIRE. Tires are absolutely, positively, hey-Joe-no-foolin THE most dominant part of the suspension. Being on the right tire for the job at hand will buy you more time than any single other suspension improvement, and in terms of autocrossing (and road racing, to a point) tires buy you more time than anything outside of more seat time - PERIOD.
So half, maybe even 3/4 of the battle is getting on the right tires. If you have the right tires and everybody else is on the wrong tires, you win.
If there are multiple tires eligible for your class, then it sucks to be you, because you're going to have to test and try them all - because being caught on the wrong tires means you lose. And when the tires change throughout the season - as they most certainly do; no race tire stays the same between build codes because the manufacturers are in a constant scramble to stay ahead of the other guy - then it really sucks to be you if you are behind the curve. Tire. Wars. Suck.
Anyway, the first step is to determine the tire you will be using; all else falls out of that.
Picking the winning tire is part luck, but mostly testing. Buy as many different sets of tires as you can afford, and then test them, back-to-back, in the same conditions. Try and get a handle on what makes the tire work, particularly in terms of temperatures and pressures. The trend has been towards tires that start working at lower temperatures (and go off at lower temps as well; the tire will overheat and get "greasy") but there is no promise that this trend will hold and you cannot make any assumptions about anything. Particularly, this year's wonder tire can easily become next year's dogshit (see BFG G-Force and Hoosier A3S04 (dogshit), vice BFG R1 and Hoosier A3S03 (wonder tire)
If you cannot afford to test, the next best thing is to keep an eye on the guys with money and watch what they do. They can afford to test, and they are doing so. If they suddenly get faster, that's probably tire at work and you should try and beg, borrow, or steal a run on the same tires to see what they feel like.
Normally, the difference between a good tire and a better tire is immediately obvious to the driver; you won't need many tries to figure out what tire you need to be on. (My first time on Hoosiers - old, slightly worn-out Hoosiers at that - compared to the Kumho V700s I had been running was night and day. It took exactly two corners to see just how much better the tire was)
Note too that there's more to a tire than just raw grip. There is transitional response (how fast the tire will cut from full left to full right), recovery time (how fast the tire regrips once you cross the line and start it sliding) temperature sensitivity, and grip itself can be broken up into lateral and longnitudnal grip. Different cars will respond differently to different aspects of the tire's performance envelope, and what might work on one car might not work on another.
For example, a tire that trades a little lateral grip and transitional response for forward bite will tend to work better on a high powered rear wheel drive car (that typically has a hard time putting all its power to the ground) than on an underpowered front wheel drive car that cannot power the tires loose when parked on an ice floe. The only way to know is to test, test, test, and pay attention to what is going on around you.
Once you have the tire, there is a particular envelope that it needs to be kept in to maximize its performance. There is a particular camber angle, pressure, slip angle, temperature, age, loading yadda yadda that puts the tire in its Happy Place where it provides the most grip, stability, response, etc to get the most out of it.
This is important - if everybody is on the same tires, he who keeps his tires in their Happy Place the largest percentage of the run time, wins. When you have an advantage in WHICH tire you are on, you can get away with being sloppy here and still win (assuming a large enough delta between "right tire" and "wrong tire") But once you are all on a level playing field, tire-wise, the difference between the champ and the also-ran is in the execution of keeping that tire happy.
Any given tire, once in its happy place, produces a certain maximum amount of grip. That max amount of grip has an enormous amount of influence on suspension tuning. There are things you can get away with on a low-grip street tire that will just ruin your day on a high-grip race tire - and the higher the grip, the more true this becomes. High grip == more force, and the suspension needs to be able to deal with it.
Certain tires are more finicky about their happy place than others. Tire A may work well from 50 deg F to 250 deg F. Tire B may be a rock from 50-120 deg F and a greaseball from 180 deg F on up. Tire A may lose 20% of its grip at camber angles +/- 1 degree from optimal, where tire B may only change 3% over a 5 degree camber range. Oh, and it may be surface dependent too. A big part of the battle is to figure out exactly where your tires are sensitive/insensitive and control the parts that make the biggest difference.
...and those parameters need no be STATIC either. A given tire may not care what the camber angle is within 5 degrees, but it may not tolerate CHANGES in camber angle faster than 1 deg/sec. Etc etc etc.
So then, we want to find the best tire we can, and we want to keep it happy, and we have a ton of testing to do in order to discover exactly where the Happy Place is, and how sensitive the tire is to changes.
One last rule of thumb: wider is almost always better. It is usually worth doing whatever you have to do to get a wider tire under the car, even if it means a larger tire diameter. Every single time we put a wider tire under the car (even if it was taller) we went faster.
For my opinions on "street tires", see this article. In a nutshell, I think they are a viable option for people looking to play at a local/regional level. At a National level though, they don't get you out of the tire testing game, the number of tire brands/models that need testing is actually LARGER than the number of R compounds normally available, and the tires themselves are usually just as expensive as the R compounds. The extra wear you get out of the (notionally) harder compound is a false economy, if you are really set on winning.
Until the day when the SCCA comes up with a spec tire class, a large portion of the efforts of a winning team will be spent on tire testing, and that is independant of if the tire is a "race tire" or a "street tire" - because the rewards of being on the "right tire" (and the penalty associated with being on the "wrong tire") are so large. If you aren't testing, you're losing - or you're getting lucky, because everybody else in your class is lazy too.