THSCC Forums

Setting Tire Pressures

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OK,I've been meaning to post this since Danville,but with overloads at work and putting the Neon back together ,it took some time.
Here goes. First off I recommend starting your tire pressures at 45lbs.(Unless you have run these tires before or know someone w/a car similiar to yours running these tires and then you can start with their recommended pressures)
Next mark your sidewalls w/ shoepolish or chalk about 1inch onto the tread and up the sidewall.Take your First run and check the chalk,if it is still visible on the tread , let air out.If it has worn the chalk off the sidewall ,put air in.You want the wear to just start up the shoulder of the tire a 1/16 of an inch,more than that and you could damage the sidewalls.
Make your changes in 4-5lb increments untill you get close to the desired results.If you have access to a pyrometer or digital thermometer ,you can more accurately adjust these pressures.You want to take the tire temps immediately after a run( have a friend assist you).Take the temps 1inch in from each edge and in the center.You will have maximum grip when the temps are even all the way across the tire.If the edges are hotter,you need to add air .If the center is hotter, you need to drop air pressures.If the outside is hot and the inside cool you need to add camber(that comes later). Always adjust your front tires for maximum grip,then adjust the rears to your handling preference.
AS a general rule you add air to increase grip ,and reduce pressure to lessen grip but this is only true to a point.After you have reached maximum grip if you continue to add air the tire starts to balloon and loose grip again,conversely if you are already too high on pressures,reducing increases grip.
Many people try reducing pressures in the rears to make the car "free" or help it"rotate".While this does work,its been my experience that this causes "snap" oversteer, and is not very predictable.I preffer to add air past the point of maximum grip,so that the tire just starts to balloon.This reduces traction but in my opinion seems to be more predictable.
Each tire and car is going to be different,and each driver is going to like a different "feel" and have his own opinion.This should get you close and keep you from trashing a new set of "EXPENSIVE" tires. The rest is just Trial and errror.
One more hint.When you come to a course with lots of grit or little grip, reduce the pressures in ALL tires by a couple of lbs.Dont upset the balance youve worked so hard to find.On grippier surfaces,increase the pressures of ALL tires by a couple of lbs.You would be surprised at how much more a tire will "rollover" on a good concrete surface.Add the air and save your sidewalls !!!!!
If I've missed anything I'm sure I'll hear about it.I hope I've helped a few of you out.This is just my experiences and its not written in stone.
OH YEAH !!! One more thing.Loose is fast!!! Its very uncomfortable the first time you feel the back end sliding through a turn,but this IS the fast set-up.You have to find that edge without going too far over it,so dont make the car completely neutral just because its comfortable,set the pressures so the car is just slightly "free".You WILL get used to it!!

There is one optimal tire pressure for a given set of conditions, anything above or below will result in less grip.

I've gone both ways to adjust grip (higher and lower).

Chalk on the sidewalls is still valuable info, what does it tell you? Years ago I want as low as 17 psi in rear of the celica to keep it from spinning all the time. I know Jason M. has dropped the pressures in the 240 a bunch lately, each time getting more grip.

Scott

I recomend picking up "How to make your car handle" by Fred Puhn. Its a bit dated in parts but tires are the FIRST thing he touches. Its also not tooooo technical for the most part, so its comprehendable by average Joe.

Like others have said, think of grip like a bell curve, anything above or below the peak of the curve will reduce grip.

At sanford I dropped the pressure down to 22lbs cold front and rear. It seemed to work fairly well in terms of adding grip. Didn't help me stay off the cones though.