Suspension & Steering
Weissach Axle "Pinning"
"Pinning" the Weissach axle involves negating the flex in the rear suspension caused by the rubber bushing in the "joint" of the front portion of the lower control arm where it connects to the chassis. Hard or solid bushings are probably used to do the trick. The Weissach axle causes rear tire toe in under deceleration. The patented method introduces a link in the front of the A arm that causes the front of the A arm to move toward the centre of the car when rearward force is applied to the rear axle. The rear of the A arm must also be designed to be flexible to allow this movement. Yes this is a type of four wheel steering.
"Normal" rear suspension on road cars usually experience toe out under deceleration (leading to the infamous "trailing throttle oversteer" in the ill handling early 911's). If you think of an "A" arm connecting the rear wheel via rubber bushings, pulling back on the tire (same as decelerating) will cause the front of the tire to point out away from the car.
Because we like our road cars (928's) smooth and quiet, we use soft bushings which allow toe in or out of the rear tire. Because we don't care if racecars are harsh and noisy, we use extremely hard or solid bushings that do not allow toe out to occur via suspension flexing.
IMOSHO, every track car should have rock hard suspension bushings everywhere. Yes, these would effectively "pin" the Weissach rear axle.
On street cars, I think there is a general consensus on this list that when rear tires exceed a certain width, the toe in provided by the Weissach rear axle is counter productive; therefore, it should be pinned.
78 5 speed
Marc Anderson doesnt pin his rear end, nor do I for track events. It is the most well behaved car on the track and, I think, the easiest to race.
(I'll get back to you when I get a little more power, but handling is definitely awsome)
Check out some of the cds if you dont believe me.
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 a lot 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 allmade 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 can't figure out why the outer edges or inner section of the rear tires are wearing "funky", this is why!
Hope this helps to 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!
Marc M. Thomas
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