Contact Patch Discussion

[Fobio]

Several things I learned at track school, as taught by Ian Law's Track School:

1. The contact patch of each tire is approximately the size of an average hand. You have 4 of these on a car.

2. F/R weight transfer, done through braking and accelerating/throttling is essentially done to maximize the contact patch and grip at the end of the car that needs it most.

3. All else being equal, increasing tire width DOES NOT increase the size of your contact patch. It turns you OEM length wise rectangle to a wider but shorter rectangle. Total area is the same, but the goal is to trade some forward grip for more lateral grip, aiding the car have more grip in the corners.

They didn't actually teach us anything about adjusting tire pressure, which was diappointing, but I guess there are other courses that'll focus more on that.

The RT-615's hold up very well now that I got some mileage on them...and the tires have never rolled even when pushed. The tire has the very thin bead at just the right spot so there's no need for chalk-marking. Anxious to see the difference with sways.

[Fuman]

They didn't actually teach us anything about adjusting tire pressure, which was diappointing, but I guess there are other courses that'll focus more on that.
it's hard to teach you about adjusting tire pressure.
In short, you are trying to find a balance between contact patch and lateral grip.
Which is different depending on tire, driving style, drivetrain and the weight distribution of your car. what psi were you running? did you feel you were at the right PSI?

[Fobio]

well...I wasn't expecting them to teach me how to adjust tire pressure for my car, but as far as they're concern, if it's rolling it's too soft...that's that.

What I was hoping was just pointers...like little tidbits on reading tire and maybe even if they talked about chalk-marking...

When I got to the track and realized I had to get gas, I actually made sure I was at 38 psi up front...I think I'm going to go a little higher next time, prolly 40 psi...but like I'm talking about it here, it sounds very arbitrary...can't wait for more testing this Sat.

[Fuman]

What I was hoping was just pointers...like little tidbits on reading tire and maybe even if they talked about chalk-marking...
true, i see what you are getting at now. they didn't tell me that either when I went =(

[Marsh]

Gentlemen (and ladies I suppose).

Adjusting tire pressures for autoslalom is something of a black art. The traditional chalking of the sidewalls and monitoring how far you scrub is how every should start. But you'll likely notice that very experienced competitors don't do this, but are still changing their pressures.

The problem is that, especially on street tires, you can jack the pressures WAY too high and still roll onto the sidewall if you're pushing too hard in the corner. If you're doing this it's faster to take some pressure out and drive more gently. How much is too much will very from car to car and tire to tire. I used to compete in a 97 Prelude (similar weight and distribution to a 3) and found that anything more than 45 psi cost me speed. But I could still roll the factory tires at well over 50 psi. I would suggest 45 is probably a good pressure for street tires. I very high performance tire (ie R-compound) should be able to get away with less. Again some race tires need more, some don't. Eventually you will develope a feel for how the tire is responding.

Also you will here a lot of debate on whether to go lower in the rear or higher in the rear. I personally say higher. I prefer a car setup this way and recommend it to rookies because it's easier to drive than higher. The reason you don't use the same pressure in the rear is to help reduce understeer.

[Fobio]

Also you will here a lot of debate on whether to go lower in the rear or higher in the rear. I personally say higher. I prefer a car setup this way and recommend it to rookies because it's easier to drive than higher. The reason you don't use the same pressure in the rear is to help reduce understeer.

Please clarify...do you mean higher or lower?

[Elder_MMHS]

I always thought higher rear pressure = floatier, less grip, but easier to rotate (possibly too much).

[Marsh]

Please clarify...do you mean higher or lower?
Lower pressure.


I always thought higher rear pressure = floatier, less grip, but easier to rotate (possibly too much).

Well yes, but you can achieve the same result by going lower as well. There are advantages and dissadvantages to both. I prefer, and strongly recommend for rookies, going lower.

[Fuman]

I prefer, and strongly recommend for rookies, going lower.
Agreed, especially if you tend to go too hot into a corner.

you think 1 psi higher in the back will do anything, in terms of feel? (i've tried lower in the back, want to see what it is like if it is higher)

[Marsh]

Agreed, especially if you tend to go too hot into a corner.

you think 1 psi higher in the back will do anything, in terms of feel? (i've tried lower in the back, want to see what it is like if it is higher)

That depends. On my '90 miata 1 psi can make a big difference. But on my Prelude I could range around by 5 psi before I notice any real difference. Higher is easier to rotate than lower, but easier to loose control of as well.

[Fuman]

That depends. On my '90 miata 1 psi can make a big difference. But on my Prelude I could range around by 5 psi before I notice any real difference. Higher is easier to rotate than lower, but easier to loose control of as well.
the two times I went, I was in my friend's car (very similar setup to mine), the back was 2 psi lower. What would you recommend me to try?
even psi?

[Fobio]

last wknd, I ran 41.5 front and 39 rear...next time out, if I can get my own compressor, I'll try out a different setup...maybe even all around...and higher in the rear.

for the record, I think I'm a lot more front-heavy...

[mleblond]

Yeah different tires equal different pressure. You got to feel them (hence test and tune days) But I usually have a difference of 2-3 psi lower in the back wheels. Im running 38-39 front and 34-35 back COLD. Pressure will elevate after a couple runs of warming up.