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      04-09-2008, 03:30 AM   #20
MrSteak
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Drives: 08' 335i Coupe (6-speed)
Join Date: Mar 2007
Location: New York

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Quote:
Originally Posted by Chowbow View Post
I disagree on some points. I agree that manufacturers often state specific pressures for comfort. Hence, I don't follow them. The more a tire is inflated, the "stiffer" the sidewall is per se. It doesn't make it stiffer, but there is simply less roll over when the tire is loaded. But it does act like running a tire with a stiffer sidewall.

I have experienced many "over inflations" where the tire is inflated to the point that the contact patch is reduced because the tire starts to get ballooned. Try this... Inflate your tires to 10psi over the max stated rating on one side, and 5psi under the max rating on the other. Put a stripe of paint on the tread of all 4 tires, from inner edge to outer edge. Take a quick drive on the freeway (less than a mile), and take a look at the paint stripes on the tires. The ones which are inflated higher will have a lot of paint left on the insides and outsides of the tire due to the ballooning. The contact patch of the tire is reduced, and braking performance is worse.

It is better to run slightly higher pressures tires in the rain, as it reduces the contact patch on the road, which reduces chances of hydroplaning. It's like driving on 315mm tires in the rain vs 225mm tires. Even though there's less contact patch, there's a huge reduction in hydroplaning because there's simply less surface area for hydroplaning.

I also find better cornering performance with more pressure in the tires, up to a certain point. Once there's a lot of air and the tire is ballooned up too much, there's too little contact patch on the road for grip, even though there is hardly any rollover because there's a lot of air in there. I try hard to find the balance between the two conditions at the track. Since I'm usually autocrossing, braking performance is not quite as important as on the track so I don't mind sacrificing some braking and accelerating grip for cornering.

Harsher ride and wear... true but not the most important to me. Performance and safety above all.

A lot of stunt cars run very high PSI because they incur a lot more load than a typical car does. Driving on 2 wheels and jumps are some things I can think of. Also, if doing a lot of J-turns and drifts, it's best to inflate really high to minimize the tire from rolling over and scrubbing all the tread off the sides of the tires... They would wear VERY quickly if inflated normally.

In the end, do what suits your driving style best. It's just in my experience, I have not had the best performance by inflating the tire so much.

Tires don't balloon up like you think. I used to think that way until this past September when I took a stunt driving class. The instructor (Bobby Ore) gave us at least two hours of classroom time on tires, which pretty much covered everything here, then throughout the course he showed us more... One thing I was surprised to see were the tires on his truck with a maximum psi rating of 44 inflated to 100psi. They didn't balloon, up, or get distorted in any way. (BTW, all the cars we used were stock.. even the truck.)

As far as wet weather traction goes... he explained how the faster you go, the middle part of the thread of a tire, bends upward into the tire. (Hard to explain half this stuff without him, or the tires/examples he used in class.) One example he gave was how he was working with a tire manufacturer and they'd have him drive over a piece of plexiglass with dye on it, and a high speed camera under it. At 70mph the tire inflated to a recommended psi would retain 70% of it's contact patch, where one inflated 5psi or so above max psi would retain 90%. (I don't know the exact #'s, but at each speed, the tire with recommended psi would retain significantly less contact than the one inflated higher, about a 20% difference.)

Don't get me wrong, this may or may not be the best set up for autocross, but for daily driving and everything in between I think it is. The stunt driving school was the most thorough driving school I've been to yet, and I plan to do more, and attend some more racing schools in the future.
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