mikymu

99% of us that DE have no idea why we check our tire pressure other than the fact your buddy is doing it. When the rear swing around we blame it on *** heavy 911 and buy a 50/50 M3 to solve the problem. There is alot more to the science of go fast on the track and here is a simple chart to review the basic. Next time some one lower their car and wonder why there are so much understeer then you can tell them to lower rear tire pressure a bit to balance things out and look plenty smart

CRex

Great 101, but I'm afraid this cliff notes version is still too much for the regular guy to understand. You'll inevitably end up with questions like: "but what if I soften the front bar and decrease rebound..."

One of the better resources I've come across is SAFEISFAST.COM and particularly this series of videos:
http://safeisfast.com/sections/8-rac...s#.UCN-rE1Dy5I

It does take some amount of mechanical sympathy, but if anyone cares to drive properly they should really LEARN what the machine is designed to do and why it does what it does...

997gt3north

I agree with everything except the first 2

If I want to reduce the understeer I would soften the front tire pressures not raise it

If I want to reduce the oversteer, I would soften the rear tire pressure not raise it - this one actually seems bizarre to me as anyone who has driven a 911 knows when the rear tires get too hot and pressures go to 40+ psi, the rear starts to get very loose and you can't get the pedal down on trackout as the rear is just constantly coming around and you are counter-steering

I'm sure I'm missing something but the tire pressure stuff I think is backwards.

ir_fuel

I was thinking the same.

AudiOn19s

I too think the pressue thing looks backwards.

however, I guess you don't really know where the pressures are relative to ideal either. Too many assumptions on what pressure changes do when you try to generalize it.

Jake951

Here is my read on things. I think there are at least two factors related to tire pressure. Lower pressure increases contact patch size, which should increase grip and reduce slip angles. (Larger slip angles in the front will increase understeer. Larger slip angles in the rear will increase oversteer). However, if the pressure is too low (in the extreme, a flat tire), then the tire becomes so deformed that the net result is loss of grip. Higher pressures reduce contact patch size but I think too high a pressure is not good either because the tire loses some of the compliance it needs to maintain conformity (and grip) with the road surface. So somewhere in between the extremes of too high and too low, there must be an "optimal" pressure, depending on what you are shooting for. The decision to add or reduce tire pressure to fine tune understeer/oversteer depends on which side of optimum you are.

Then to complicate things even more, the tire is also somewhat like a spring in that it flexes under vertical load. More pressure reduces flexing and makes it behave somewhat like a stiffer spring. Stiffer spring rates create the effect of stiffer anti-sway bars.

There are a number of things going on here at the same time and I think in the end, it depends on the characteristics of the particular tires.

GT3DE

tire pressure is more about SPRING RATE than about patch size. Increase tire pressure is the same result as making the spring stiffer.

FFaust

Re-watched the 3 videos on setup yesterday, and they are a great resource. Concepts are explained with such clarity, and really broken down to basics. It also explains how everything is part of a system; it all works together and nothing should be taken in isolation.

mdrums

Is this the same as..." man she's pushing rollin through turn 3 and 4?

mdrums

Right...and the chart says to increase front tire pressure to increase over steer...but further down the chart says the soften front spring to increase oversteer. HUH?

GT3DE

that chart is wrong about pressures. lowereing the pressure in a tire makes it grip better. I have stock shocks/springs, the front sway is full soft, rear sway is factory (middle hole). My car (all three GT3's are setup the same, toe-links, camber, rake, etc.) setup is "loose" aka oversteer. I aim for equal pressure tires when hot. If I feel that car is not loose enough, I LOWER the front tire pressure. FWIW I like a VERY loose car -- see videos. lol

mikymu

this chart is a general guide. There is variable in every car and if you have tire pressure all the way up in the 40 psi hot then nothing works. We typically do very fine adjustment such as 1-2 psi off the tire pressure (at 28 psi hot) and very few mm change in sways and shocks

TRAKCAR

You need dirt track experience.

mikymu

too dirty

TRAKCAR

Than drink more beer. Are you an enjuneer?