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Old 10-08-2012, 10:31 PM   #43
fiveightandten
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Join Date: Jan 2007
Location: Connecticut
Posts: 1,297
My Ride: '03 330ci / '98 M3
Quote:
Originally Posted by dmax View Post
fiveeight...

You said to increase the rear tire pressure to reduce understeer...and I'd said just the opposite. I wonder if AutoX is that different from 'regular' driving...because in DD, increasing the front tire pressure relative to the rear increases oversteer. For the same degree of turn in the tires, with more air, the tire is bigger, and thus more turning is done.

Are the physics in AutoX that different? I'm confused...or you just made a typo.
The tire has a range of pressure in which it will obtain optimal grip. Under-inflate the tire and it will roll over on the sidewall too much, decreasing grip. Over-inflate the tire and you will balloon out the center slightly, decreasing grip.

Because of this, there is no blanket statement that will *always* be true, as the balance between the front and rear are relative to one another. But in a situation like Mango described, where he's running 40PSI, running slightly higher in the rear will move the car's tendency more towards oversteer. If his pressures were lower (say 30 PSI), the same might not be true.

Every tire is different. But from my experience, when you're at the high end of your optimal range of pressures like that, running higher in the rear will loosen the back end up. I do this in my E36 on tighter courses where I want to steer more with my right foot. I run star specs, which have a very stiff sidewall and squared shoulder. Other tires would take more pressure to slide off in grip on the top end. But regardless, 40 cold is on the high side.
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