Street-optimal or lower pressures on the track? WTF?

I got into an argument with a guy on another forum about tire pressures. He's saying that the optimal tire pressures for driving on track are similar to the optimal tire pressures for the street or even lower, and that I am full of E36 M3 for saying that you can improve grip by running higher than street-optimal pressures, which he has never heard of. He also said that the angle between a tire and the ground has no effect whatsoever on the contact patch, which if I'm correct, goes against the very foundations of suspension design theory. This dude also says he has 20 years of experience in some kind of SCCA racing (clearly not autocross where high pressures are the norm).

Who's right here? Either this guy's epically trolling me, is the Inspector Clouseau of racing, or I'm going insane.

Epic Clouseau.

I don't think you can make a blanket statement about higher pressures or lower pressures being better for any form of racing.

If it bugs you, get a pyrometer and prove him wrong.

Maybe all of his experience is in drag racing. Otherwise, he's completely wrong.

Given the "angle of tire doesn't change contact patch", I'd guess he's trolling.

+1.....I've met a few people convinced that putting 50psi in their E36 M3ty all seasons made their car "handle like on rails"......I very rudely tell them to buy better tires.

He is technically correct on one thing. The camber of the wheel doesn't change the contact patch (surface area that touches the ground) of the tire. It affects the slip angle of the tire which itself influences the amount of lateral force.

As far as tire pressure, it really depends. My experience with running high-performance tires on track was that they did require a little more pressure (about 4 psi). But I've never run full comp slicks, so...

The pressures are determined by measuring them hot at what the tire manufacturer say they should be if you are using R comps or slicks. When I run Hoosier R6s that is 38-40. When I run Hoosier slicks it's 28.

Street tires all seem to come in around 36-42 or so and most don't come with a hot temp from the mfg so you have to experiment a little to see where they work best. I always work +/- from 39.

The point is - measure them hot as quickly as you can after a run once they are at full temp. The cold pressure is whatever you have to start them at so they warm evenly to the right number. They typically gain 5-7 psi from cold to hot but higher loads will start at different cold temps depending on outside temps, track temps, load and proportion of left to right turns, etc.

So... maybe you do need to add a little to a street tire to get to what works best... but maybe only if its cool outside. Can't really make a blanket statement like that.

fanfoy wrote: He is technically correct on one thing. The camber of the wheel doesn't change the contact patch (surface area that touches the ground) of the tire. It affects the slip angle of the tire which itself influences the amount of lateral force. As the car enters a corner, the inside of the tread tends to lift, decreasing the contact patch . Camber helps to correct this. Slip angle is a composite of several things. As far as tire pressure, it really depends. My experience with running high-performance tires on track was that they did require a little more pressure (about 4 psi). But I've never run full comp slicks, so...

If tires were nothing more than rubber balloons full of air, camber would not matter.
But reality is a bit more complex.
Tire fabric, configuration and belts have a tremendous effect, as well sidewall stiffness.
Ask any racer, particularly Showroom Stock, what the effects of camber control (or lack of) has on handling and cornering capability.

As far as pressures go, many road racing cars do tend to run lower pressures than many street cars, but that's not an apples-to-apples comparison. Bias-ply cantilever slicks on 1200 lb formula cars are VERY different from street radials on 4000 pound street cars. The slicks may want pressures in the teens, the street tires more likely in the 40s.

Yeah in general he is correct, but you aren't wrong. Running UHP street tires on track I generally am right around 34-36 cold but this year I will be runnin NT01's so I know it will be totally different. I use a pyrometer and adjust accordingly for track and conditions. The one time I ran 205/55/16s with soft sidewalls at deals gap I needed close to 50psi to keep off the sidewalls. It's all relative to the tire.

iceracer said: As the car enters a corner, the inside of the tread tends to lift, decreasing the contact patch . Camber helps to correct this. Slip angle is a composite of several things.

While this may seem intuitive, W. Milliken, who wrote THE books on suspension design, tells us that the tire print (its contact patch) is a function of load, tire pressure and tire construction. See "Race Car Vehicle Dynamics", chapter 2. He spends 69 pages on tire behavior. Dry but interesting reading.

During cornering, the print is deformed, but its area stays pretty much constant. This deformation introduces the camber force which is a component of the slip angle.