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Re: Tire Pressure for Better Handling Balance
Posted by Lane Dexter (more from Lane Dexter) on Tue, 19 Feb 2002 23:57:55
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

Not to play games in semantics, but let's take a look at 'ultimate grip' and 
make sure we're saying what we want others to understand.  If we're looking 
for 'grip' on pavement, increasing pressure tends to keep tire distortion 
from pulling part of the tread patch off the road and decreasing 'grip.' But 
only up to the point where overinflation begins to reduce contact patch and 
'ultimate grip.'

Most of us know that at very low speeds, low pressures can be used to 
improve both floatation and 'grip,' allowing tires to conform to irregular 
surfaces.  But that's a different game than on-road traction and handling.

Suspension design introduces a big variable: Does camber change greatly 
under hard cornering?  If so, then plenty of pressure to hold a 'squarish' 
profiled tire in shape may simply result in camber changing pulling half the 
contact patch off the road.  Keep this in mind when selecting tires.  Going 
back in time to when I was more actively studying such things, one test 
found the Pirelli P7 held better than the Phoenix Stahlflex 3011 on a car 
that did not change camber much when cornering - but the results were 
reversed on a car that showed substantial camber change.

To finally and mercifully cut short my verbiage, and offer some data that 
may be of use to drivers of nose-heavy FWD cars -- back when I ran the 
Phoenix 3011 'Racer' tires on my 99 Turbo (and I RAN it), I usually kept 36 
psi in front, and 30-32 in the rear.  This did not magically turn understeer 
into oversteer, but it reduced understeer and I could get the tail out a bit 
in a hard left foot-braking situation if I wanted to.  (OK, OK, only old 
timers remember the 'Thunderbolt and Leadfoot' article in Road and Track, 
all about Stig Blomquist and the Saab factory rally 99 Turbo -- but it was a 
hell of a car and he was a hell of a driver.)

My advice is to get a good tire gauge, find a place where you can do repeat 
comparison testing, and experiment a bit.  Have fun.

Lane
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