Wandering, Negative Steering Roll Radius?

["Scott Mo." <scottmo@teleport.com>]

Audi published a Sales Training book called the "1991 Engineering Fact
Book, Audi 100/200 Series" . This book has a few new tidbits of information
and discusses the Negative Steering Roll Radius Geometry design used in the
steering/suspension system.

I am no suspension expert, so  I will leave the following excerpt as food
for thought and ask does a change in wheel offset/width alter this Negative
Steering Roll radius enough to make the car more susceptible to wandering?

"Front Suspension with Negative Steering Roll Radius Geometry"

"The front suspension used on C platform Audis utilizes the same proven
configuration found in previous models: a full independent arrangement
consisting of lower control arms attached to shock-spring struts which
define the upper portion of the suspension geometry. The precise steering
characteristics of this arrangement are attributable to the struts ability
to bring steering and shock absorbing forces into the same plane. Built
into the front suspension of the 100, 100 Quattro, 200 models is a
geometric relationship of components we call Negative Steering Roll Radius.
The term is admittedly an unwieldily one, but the engineering it represents
helps make a car in trouble act less wieldy.

NSRR places each front wheels pivot point outside the center of the tires
axis. This has the effect of making the front wheels want to return to
center-to turn in the direction opposite that of the pivoting force-even
under conditions when one front tire has far better traction than the
other. When this happens on a conventional automobile, as when the driver
must brake hard in a corner, has a blow-out, or has one front wheel drop
off the road, the front tire that retains traction has greater stopping
power."

Some other questions:
The V8 and 1991 200TQ 20V have different front hubs (441-407-615C) than
the 1989-90 200 (447-407-615B) which I believe moves the wheel out further?
What is the over all effect with the 35mm offset 15X7.5 wheels used on the
V8 and 91 200TQ 20V?

I have felt the same steering wandering effect on my 89 200TQ when I run
the 16X7.5 wheels with 35mm offset, versus the stock 15X6 wheels with 45mm
offset. I have considered having 5mm shaved off the wheel mounting face to
get the offset at 40mm. Softer side wall tires lessen the wandering effect,
as does adjusting the steering rack preload. The addition of the steering
damper does help stabilize the steering wheel in your hands when you hit a
rut in the road.

Steering out of trouble......

Scott Mockry

http://www.teleport.com/~scottmo