From: Marc Thomas [mmthomas@ix.netcom.com]
Sent: Thursday, February 17, 2000 2:17 PM
To: 928
Subject: [928] Pinning the Weissach joint..

Folks,

Hopefully, this will answer some questions about the the Weissach rear
joint.

This joint controls the amount of toe (in or out) the rear tires experience
based upon the load the rear tires/joint experiences.

For a given load condition, the joint deflects and causes the tires to
change toe to maintain traction under cornering and changing conditions.
The rear tires sees an amount of change of toe, which is then translated to
the tire patch.  The tire contact patch will twist slightly with changes in
toe and the goal is to keep the patch in contact with the road and maintain
traction.  And generally, on a narrow tire, a small change in toe is ok and
provides the wonderful Weissach effect.

To further understand this concept, you must draw a line down the center of
the tire patch, and visualize the toe change and its effect on the tire
contact patch.  On a narrow tire, the toe change creates an amount of scrub
on the outside edges that is small...just like a poor alignment!  Not alot
of wear in the center, but more towards the edges.  And with a narrow tire,
and a good alignment, this scrub is within the normal range of tire
compliance and as such provides altered suspension/tire geometry that
maintains tire contact.  The benefit of the toe change outweighs the scrub
of the tire.  In general, narrow tires lose little to no grip due to this
scrub.

But what happens with a larger/wider tire?  The scrub at the outer edges of
the tires is increased for a given degree of toe change, and is generally
beyond the compliance of the tires material!  You end up with too much toe!
And that outer section of the tire patch is no longer gripping the ground,
but turning into heat!  You now have lost some percentage of tire contact
patch.  And the gripper the tire, the more the load, the more the toe
change, the less the grip!  The effect is that the amount of grip you have
from increased tire size is mot maximized!

There is the problem!  Now the solution.

We worked on this problem when we were building horsepower and traction,
since the problem was more pronounced with more power, cause with more
power, you need more traction.  We also found that the problem was more
pronounced with wider and sticker tires.  Both of these problems were due to
more load on the joint = greater deflection = greater toe change.  More toe
change, more scrub..less traction!

We did some tests...wide tires on wide wheels on a stock engines S4, power
on/off during cornering, developed a test that was repeatable!  And then
pinned the joint in a few places.  Repeated tests and it stuck....!!  When
combined our data with the tests of Bob DeVore on his 700 hp 928, it all
made sense....it was a combo deal...too much stick and too much power (we
are in favor of this position), makes for too much toe....

So, we developed the DEVEK Weissach Pin Kit, which does not eliminate the
toe change, but rather reduces the amount of toe at any given load...with
this kit you will be able to run with wide or sticky tires and achieve
maximum grip.  It has the effect of keeping the toe to tire geometry the
same or similar to stock when tire size or grip or power is increased.  And
most importantly, you will keep the positive effect of the original concept
of the Weissach toe control...and this is a good thing!

Our testing showed that a 928 with 275 street rubber or with very sticky 255
rubber is the point that you begin to lose traction due to tire scrub at the
outside of the tire.  If is not a hard fast rule, but a guide.

If you track the car and cant figure out why the outer edges or inner
section of the rear tires are wearing "funky", this is why!

Hope this helps understand the basics of the Weissach joint and its effect
on handling/traction.

Remember, regardless of what you use your 928 for, be it street or track
driving, the goal is to maximize traction.....it keeps you and others safe!

BTW, if you have not put your name in for our Vol 3 catalog, give a call as
they are shipping now.....a little overdue, but worth the wait!


--
Marc M. Thomas
DEVEK

650-592-5287 phone
650-610-0557 fax
http://www.devek.net