hotmayo

I was going to do this in the Saab school thread, but it's degraded into a bit of a mess. There were a few debates about what you want to do to tire pressures. Whether you want to increase them for track/autocross use or decrease them. There was a chart that basically implied that for whichever side you want to increase traction, you just increase pressures.

Those educated guesses might be right in many applications, however, using them is like using the answers right out of the back of an algebra textbook. It might be right, but how did you get there? And what happens when one of the variables changes. Will you know where to compensate?

PART ONE - Tire pressure

Your goal will always be even tire wear. When the tire wears flat, you know you've been using all the available tire optimally. When you're adjusting tire pressure, you want to make the bottom of the tread as flat as possible. When there are areas that are receiving too much pressure, they will produce more heat.

It is SUPER helpful to use a pyrometer when setting up your car. It really is essential if you want it done properly.

I liked that tire pressure chart from the other thread, so I added some temperature numbers to it.

If you're running with too little tire pressure, the tires will be hotter on the inside and outsides, while cooler on the insides.

Figure 1 - too little tire pressure

If you're using the correct pressure the tire temperatures will be equal from the inside to the outside.

Figure 2 - correct tire pressure

If you're running with too much tire pressure, the middle will be getting hotter than the inside and outside.

Figure 3 - too much tire pressure

So a blanket statement like "increasing rear tire pressure will decrease oversteer" will only apply if the rear tires are already underinflated. The tires have an optimal pressure, and using too much or too little pressure will prevent them from having optimal traction.

The tires are inflated to their optimal pressure for driving in a straight line, but what happens when you're at the track or an autocross, where you're always turning? The outsides edges of the tires will get hotter, while the insides don't get as much. All the weight is transferring to the outside edge of the tire, so the heat distribution ends up looking more like this:

Figure 4 - temperatures after cornering
The temperatures are not even from outside to inside, but if you notice the temperature in the middle is exactly the average of the outside and the inside. This is good, because it means our tire pressure is correct, even though our camber could use some work (more on that later).

If you're underinflated it will look more like this:

Figure 5 - underinflated tire after cornering
If you average out the temperatures between the outside and the inside you would get 212.5, which is much higher than our reading of 150. This would tell us that we need to increase the tire pressure.

If you're overinflated it will look more like this:

Figure 6 - overinflated tire after cornering
Here are some really crazy numbers. You don't need to do the math to figure out something's wrong here. Since the middle number is higher than the average of the other two, you know you should be decreasing the pressure.

Part 2 - adjusting camber

You've probably figured out that you probably want to optimize the contact patch from what you have in Figure 4. The tire pressure is good, but the temperatures are not even. The fix is simple, just move the top of the tire in.

Figure 7 - proper camber settings
The temperature is equal across the tread. everything is perfect... more or less, but more on that later.

If you have too much negative camber the temps will look more like this:

Figure 8 - too much negative camber

Usually when you pit and check your temperatures you're going to have to adjust more than one variable at a time. It's possible you can get a tire that looks like this:

Figure 9 - too much negative camber with an overinflated tire

Other considerations

It can be a bit tough to get your tire temperatures at the right time. Every corner will have different camber requirements. On some tracks you might want to properly set up the car for a specific corner, but by the time you've gone around the rest of the track to the pit, the temperatures have changed. It's always a bit of hit and miss.

Also, the more negative camber you use, the more traction you're taking away from accellerating and braking. Figure 2 with zero camber was the optimal setting for straight line traction. It's especially important for braking, so no matter what you're making a tradeoff.

Toe can also have an effect on tire temperatures. If you've seen a tire corded on just a think 1" strip on the inside or the outside, you can picture what it does to tire temperatures:

Figure 10 - toed in
It can really confuse things when you're trying to diagnose tire temps. It's easy enough to spot it here where everthing else is equal, but what would happen if you didn't have enough camber and your outside temps were already hot? Would you compensate by increasing tire pressure and camber?

That's about all I can think of right now. I won't be offended if anyone points out where I'm wrong.

comets

Thanks man-syrup. good info