From: Marc Thomas [mmthomas@ix.netcom.com]
Sent: Wednesday, June 13, 2001 12:12 PM
To: 928
Subject: [928] Tire and rim info

Two thing not to do - do not mount a too narrow tire onto a wide rim and do
not mount too wide a tire onto a narrow rim.  Both of these conditions can
peel the tire off the rim.

The ratios to keep the tire width to rim width is .~.8 (too low in my
opinion:)) - .85 min and 1.1 - 1.15 max.

At a ratio of 1:1, the sidewalls are parallel to each other and the tire
tread will remain as close to parallel to the rim under cornering, and will
gain or lose camber depending on suspension geometry.

When the ratio goes below 1:1, the tire will lift on the inside and rise on
the outside...draw a picture and load it on the side.  think trapazoid

When the ratio goes above 1:1, the opposite occurs. (this is more racer
friendly...look at f1 or indy cars)

Traction during cornering, tread shape under varying pressures, ride
quality, aspect ratio, diemater, manufacturer recommendations, suspension
geometry, the effect of camber gain /loss on movement under cornering are
all factors in choosing the ratios.

So, a suspension designer will choose a tire/rim width that most suits their
goals for the suspension and overall handling and takes into account all the
above factors and more....

Without getting into the details and analysis, in general, the closer you
are to 1:1 the more precise and better your 928 will handle.  And consult
the manuf tire guide for proper fitment or tested fitment.

some simple factory selected ratios ratios:

225 on a 7      .79 a s4 front
225 on a 8      .90 a gt front
225 on a 7.5    .85 gts front

245 on a 9      .93 gt rear
255 on a 9      .89 gts rear

285 on an 11    .98  turbo!

275 on a 9.5    .87 corvette zr1
315 on a 11     .88 corvette zr1

Beware of folks selling wheel fitments for a 928 that include a big, poor
fitting rim and to make it work, using a toooo narrow a tire!!


Marc
DEVEK