DeTomaso Mailing List: June 2001, Message #48

I'm a little embarrassed that I hadn't thought or recognized this before.  I
figured I'd share a recent learning.  I just fit new and larger wheels to my
lowered car.  As a consequence, it seemed appropriate and wise to raise the
car's ride height.  This would afford me additional suspension travel in the
rear before the dreaded tire to inner frame rail interference occurred.

Digression:  When checking rear wheel fitment, I removed the shock and put
my old 255/60/15 tire on a stock wheel through it's full range of motion.
If the rear suspension is compressed far enough, the wheel will contact the
inner frame rail.  Ah ha! No surprise to me that increasing wheel
backspacing and tire width makes this episode more common.  Frankly, the
geometry of the rear suspension is such that if the rear suspension is
compressed far enough, any size wheel/tire combination will hit the frame
rail.

I had aggressively lowered and added rake to my car to optimize track
performance.  As a consequence, I frequently dragged my lowered floor pans,
destroyed my front spoiler and compressed my right rear suspension far
enough on turn 1 at Texas World Speedway to cause my narrow 255/60/15s to
contact the inner frame rail.

Since I'm now fitting taller and wider tires, a new approach is warranted.
After raising the car approx. .75" at all four corners, (It is still lower
than a car with stock spring/shock combo) I rechecked my camber adjustments.
The front wheels went from .5 degrees negative to .5 degrees positive.  The
rear wheels went from 0 degrees to .5 positive.  I am surprised to learn
that the ride height adjustment influences camber so significantly.  I knew
that some change was inevitable; but, I thought it would remain more
constant through it range of motion.

Learning:  I know recognize why I could not eliminate my negative camber
with stock length upper A-arms on my lowered car   The common explanation on
the list had been it was caused by a collapsed wheel house.  A condition
that I knew did not apply to my car.  Rather, I submit on cars with
adjustable suspension, it is a consequence of lowering your car's ride
height.  Lowering generates more negative camber that at some point can no
longer be exorcised with the stock length upper A-arm and removing lower
A-arm shims.

Practically speaking,  I suppose there are benefits to the suspension
increasing negative camber under compression.  High speed cornering that
would cause the suspension to compress and induce more negative camber would
occur in an environment when the tire's sidewall/carcass is more apt to roll
and therefore, benefit the most.

JT