Premier Motorsp

That is exactly what I am saying. The amount of deflection needs to be tailored to the amount of grip and the cornering stiffness of the tires. 18 inch tires likely have much more cornering stiffness than the 17s the early 993s came with.

They may have also decided to use a rubber compound with some damping to reduce oscillations like you describe.

Without changing the hub carriers or subframes, Porsche could not have adjusted the suspension geometry.

Chris Cervelli
Spline Technologies

TheOtherEric

Cool, thanks Chris. Interesting theory, and it sounds plausible. However, you say:
Why not? I.e. let's say they just lengthened the lower a-arm and shortened the 2 upper arms. Isn't that possible? Someone said that they noticed a difference in geometry of the old and new parts.

(p.s. I suspect you know that PCA did change the subframe somehow, but I'm guessing that's not relevant to your point)

Premier Motorsp

I don't think the length of the arms changed. We know that if there was a length change it would have been quite small, say under 5mm. (Otherwise there would have been problems with alignments, tire clearance, etc) The slots for the alignment adjustments are about 12mm long, at least. So making small length changes on the arms would not have helped anything.

The lower a-arm may have been changed. I believe that all A-Arms fit all subframes though, so the pickup points on the subframe and the arm lengths couldn't have changed.

If somebody can confirm that they have an A-Arm that won't fit a subframe then I am wrong about this.

Chris Cervelli
Spline Technologies

TheOtherEric

Chris- do you or anyone else know WHICH of the bushings in the lower a-arm are the ones that allow for radial deflection, thus causing the kinematic toe? Frere's book says it's the rear one, but if so I'm surprised that people are putting monoballs in that joint. That would reduce the KT effect drastically. Perhaps he was mistaken and it's actually the front bushing.

Premier Motorsp

It is the rear bushing of the lower A-arm. The forward bushing is already a monoball in stock form.

The toe correction this bushing gives you is not really appropriate for a race car so I encourage people to install monoballs etc. On the other hand, the 993 Cups used 993 RS lower A-Arms and they handle ok. They do not exhibit the oscillations that have been discussed.

The kinematic toe adjustment is separate from all this and should be covered in an entirely new discussion.

Chris Cervelli
Spline Technologies

TheOtherEric

Thanks Chris. (Joel stated the opposite above and confused me a bit) Looking at a diagram it's obvious how a soft rear lower a-arm bushing results in additional toe due to either cornering forces or braking forces.

I can't help wonder if simply updating the lower a-arm would address this occassional instability issue. I'd love to see if they changed the bushing stiffness.

jmreiser

I stand corrected on the end that pivots. Chris just explained to me that the rubber on that end is just an enclosure for a monoball on all versions.

TheOtherEric

Couldn't it be different for RSRs versus street cars? Perhaps everybody is right.