Mark Donohue and 930 rear suspension

[David]

I just got finished reading "The Unfair Advantage" by Mark Donohue and I highly recommend it.

My question is: is the rear suspension pickup modification that he had Porsche do on the 911 the same one that was done on the 930 rear suspension?

Donohue said they raised the inner pivot points which didn't improve the bump steer (why would the rear suspension affect this?) but the car was a lot flatter in the turns.

He had very high praise of the 911 Carrera. When choosing the first IROC cars he wrote: "... the Carrera had to be the most reliable car available."

It's really amazing how little they knew about suspension back then, especially in the late 60's.

[Jack Olsen]

What's amazing is how much Donohue figured out in such a short time. As I understand it, Turbo trailing arms are shorter, and have a slightly different geometry than the standard ones. Smart Racing sells a kit to raise the rear suspension mounting points, either for Turbo trailing arms or stock ones.

In BB2, TRE installed the Smart Racing pieces for my (non-Turbo) trailing arms.

Here's a thread on it.

Donohue also extended the front control arms for a wider track and better geometry, like we did on BB2.

[jluetjen]

Donohue said they raised the inner pivot points which didn't improve the bump steer (why would the rear suspension affect this?)

Rear suspensions can have bump steer just as much as front suspensions. The 911 tends to toe in under bump and toe out under droop which is the primary contributor to the car's "Trailing Throttle Oversteer". In most rear suspension designs (including live rear axles in one wheel bump), it's really a factor which is a function of the suspension design and can't be tuned very well. In the case of double-A arm suspensions I believe that it can sometimes be adjusted, but in general it is designed to be zero by having the rear toe link on the same plane as one of the A-arms.

[David]

John, thanks for the insight on the suspension.

Donohue, talks about the car "falling" coming out of a corner due to the bump steer and low roll center in the rear. Any idea what he means by "falling"?

[RazorRacer]

I think he means the falling of the front inner wheel.