# RE: Contact patches

```no scott you're wrong again, (your semantics to one side).  what does 32psi
really mean?  you need to figure that out and then you'll find that you've
got the answer to the question about bigger tyres having the same sized
contact patch as smaller tyres.  change the *pressure* and the contact patch
size changes.  certainly if you do stupid things with rims, the *kinematics*
of the contact patch might not be optimal, but physics says that the tyre
*pressure* is what supports the car, *not* the rubber.  it's simple physics.
action and reaction.  equilibrium.

what you need to understand is that the contact patch shape *will* change
with the modification of the tyres (generally wider and shorter for plus
sizes).  that is where your improvement in cornering (lateral grip) can come
from, but at the expense of (longitudional) traction/braking because the
contact patch has become shorter as well.  its the old friction circle
thing.

perhaps the problem scott, is that you're taking your baseline understand
from tyrerack?  it's interesting that they don't mention simple contact
patch size physics.  same as them not listing the weights of the wheels and
tyre they sell.  i wonder why?

hth,
dave
'95 rs2
'90 ur-q
'61 mb fintail

-----Original Message-----

Date: Sun, 23 May 1999 22:15:51 EDT
From: QSHIPQ@aol.com
Subject: Contact patches

Dave E. writes:
>no jeff, a narrower wheel does *not* change the *size* of the contact
patch,
>unless tyre pressures are different.  again, this is elementary to the
>wheel/tyre debate.

Sure it does, Dave.  FE, put a 255/50 15 @ 32psi on a 15X6 stock audi turbo
quattro coupe wheel.  What happens to the tread near the sidewall?  What
about overall diameter?  Put the 255/50 15 @ 32psi on a 15x9 Urq racing rim.
Any difference in contact patch?  Elementary to the proper sizing of rim
width to tyre width.  We could speak of the contact patch of a 245/40 17 on
a
17X7 in RS2 wheel too I suppose. ;)

Try www.thetirerack.com.  They do a fine job of explaining the elementary
wheel and tyre questions.  Plus sizing is in there for you too.

Scott Justusson

```