Re: Tire wear, all-seasons, and stuff

[Luis Marques <marques@ti.com>]

Michael Williams <urquattro@surfree.com>
> John Paul Karasaki decided to speak these words:
> >I'm not sure what you mean by technically, but if you run higher tire 
> >pressures (or spring rates for that matter) in the rear than the
> > front, you will help REDUCE understeer by increasing the ability to
> > oversteer.
> 
> Interesting, in my experience, it has been the exact opposite.  

I think you guys need to make a distinction between higher-profile
street tires on street suspensions and lower profile track tires in
track suspensions.  It's not a simple straight-line tire pressure - grip
relationship.  It's more of an inverted U shape.  There is an optimum
air pressure point that maximizes grip.  Go below, or above, and you
loose grip.  So both Michael and John are right, depending on where in
the graph you are.

On street tires, street suspensions, etc, the car rolls more than the
suspension can compensate with increasing negative camber and the result
is that rarely the tires are perpendicular to the road under cornering. 
More often than not, you will be cornering on the shoulders of the
tires.  In this situation, the tire rolling into the sidewall is the
limiting factor and more pressure tends to improve the situation. 
Usually it gets better and better as you increase pressure and minimize
the tire deformation.  Frequently, you reach the max. pressure limits
before you reach the max. lateral grip point.  But there could be some
cases where the optimum pressure is below the max. inflation pressure. 
In this case, things will get better up to a point (say for example, 44
psi) and anything above that causes you to loose grip.  That's because
the limiting factor is no longer the tire deforming and rolling into the
shoulder, but the contact patch itself.  More on that to follow.

On very wide, low profile tires (or tires with built-in camber
compensation like the discontinued R1's), and cars with the suspension
upgraded to the point that the roll is so little and/or the camber is
negative enough that the tires are actually perpendicular to the road
while cornering, there is no rolling-into-the-sidewall issues to be
concerned with (unless you go to some ridiculously low pressures like 15
psi) and grip is mainly dictated by contact patch size.  Bigger contact
patch = more grip.  Lower pressure = bigger contact patch.  So in this
case, lower is better.  But again, if you go too low, you can go past
the optimum pressure and begin to loose grip due to the tire deforming
and rolling onto the shoulder, etc.  

So there you have it.  It can go both ways depending on your setup.  The
easiest way to know for sure is to test, test, test.  No equations or
computer simulations can be of any real help here since there are so
many variables.  HTH and hope I haven't confused you all even more.

Luis Marques
'87 4kcsq