Re: Contact patch again???

[Robert Wilkinson <wilk@domain.elided>]

The discussion below entices me to make one additional point.  Grip is
perhaps most affected by weight.  The "law" stating that frictional force =
friction coefficient x normal force (weight) is only approximately correct.
 While more weight gives more grip [grip being frictional force that
resists loss of traction, either forward (acceleration or braking) or
lateral (cornering)], the increase in grip is not quite proportional to the
increase in weight.  Thus the lighter a car is, the better its tires grip,
relatively speaking.  This is also the touchstone of chassis tuning--the
best handling is with all corner weights the same.  Thus, if an ideal car
with 500 lbs on each rear wheel corners at 0.5 g, a similar car with 400
lbs on one wheel and 600 on the other (due to lateral weight transfer)will
corner at less than 0.5 g--the 600 lb side will have more grip and the 400
lb side less, but their sum will be less than the case with both tires at
500.  So total weight (including of course unsprung weight) enters
prominently into handling, perhaps more so than some of the tire issues
being considered.  Sorry if everyone already knows this, but it seemed to
come up naturally in this discussion.

While I agree with Mr. Smith's comments below, I would add that short stiff
sidewalls, and consequent low slip angles, are always an advantage on
smooth pavement but with bumps, the suspension must behave properly in
order to keep the tire flat on the ground.  Older tires are more forgiving
of camber change or roll with bumps/cornering.  I don't know how my 72 does
in this department compared to newer cars that come stock with low aspect
ratio tires.

Bob Wilkinson
72 Spider


Date: Sun, 24 Nov 2002 14:39:14 -0700
From: C M Smith <cmsmith@domain.elided>
Subject: Contact patch again???

Tire pressure affects contact patch size in proportion to vehicle weight. 
if it did not, then the vehicle would sit on flat tires. The tire pressure 
does indeed exert itself in all directions, including upwards to match the 
force of gravity on the vehicle.

Funniest of all is the knowledge that contact patch area does not affect 
tire grip. The coefficient of friction operates proportionally to pressure 
on the contact patch and the area thereof, with one variable going up while 
the other goes down. So, wider tires do not grip better because of anything 
that happens to the size of the contact patch. Only the shape changes. Grip 
changes primarily because of the slip angle effects due to tire carcass 
design, nothing to do with contact patch shape except incidentally. Lower 
profile tires of the same rolling radius will have shorter wider contact 
patches of approximately the same are as those of taller tires on the same 
vehicle. The additional grip afforded by wider tires is due to shorter 
stiffer sidewalls. If you don't  change the suspension geometry or the 
tread compound, you won't get much performance increase solely from lower 
profile tires.
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