Rufus Sanders

Ok, I've wondered this for years, and finally remembered that I could bounce it off all you Shark-bitten bros.

So, ya do some suspension work on your Shark, maybe new shocks/springs etc. Afterward you go to set the toe and camber etc. but you need to drive the car to settle the suspension over and over before it's right

Why is this the case? What makes the front suspension not settle, and what can be done to shorten the vicious spiral of adjust and drive, adjust and drive? Any insight on this?

hacker-pschorr

Few different theories have been floated around. I remember years ago talking to a Koni engineer who stated the 928 is the only car he's ever come across that takes so long to settle.

One little tid-bit in the WSM got me thinking a while ago. When installing the front lower control arms you are supposed to leave the bolts loose until the car is back on the ground. Then with the car at proper ride height should the bolts be fully torqued.
Thread I started on this: https://rennlist.com/forums/928-foru...lt-torque.html

This has always led me to believe the culprit is the lower control arm bushings. I've always wondering how many 928's have the lower control arms "clocked" incorrectly which will make this problem even worse.

The best person to conform or deny this theory is Mike Simard with his lower control arms with spherical bearings installed.

I've worked on other cars with specifications on what angle the control arms should be in before torqueing the control arms down to avoid having the rubber bushing "clocked" incorrectly which would put them in a permanent state of bind, which is very bad.

AO

I remember someone way smarter than Hacker or me did a writeup (here on RL) on why the 928 front suspension is so unique. If i can find it, I'll post the link here.

M. Requin

If you ever take the lower control arm off you will immediately realize why, and why the WSM calls for leaving those bolts loose as Hacker points out! It is interesting though that even if you do, settling in still occurs...

Cheburator

Hacker,

My race car has spherical bearings everywhere. At present the front suspension moves up and down with no resistance whatsoever.

On my 1993 5spd GTS, which is on the stock rubber bushes, there is a lot of resistance, almost "memory" in them and you can see that Porsche incorporated the twisting of the rubber in its suspension design. If you think about it - the spring rates for stock rubber bushed suspension are quite low. Partly due to Double wishbone designs, but also a lot to do with incorporating rubber twist into the damping. There is a similar, although less pronounced effect at the back too.

All Porsche documents to do with racing 928s on full spherical bearings call for spring rates which are up to 7 (Seven) times higher...

KenRudd

Those would be an interesting read. Do you have copies you could post?

Rufus Sanders

Cheburator and Hacker, those are the most reasonable explanations I've heard to date. 7 times more spring rate! That sounds insane.

I've heard that loosening "something" made a difference, so the lower arms must be what I heard about. So what can you guys tell us about spherical bearings? I've never heard of that before. What are the pros and cons? Thanks!

Lizard928

Replace the upper a-arms with a bearing designed unit and the settling practically goes away......

I believe that the rubber bushing actually manages to rotate slightly in the arm which then takes longer for things to settle back into place.

OTR18WHEELER

The 928 front suspension is designed for Indiana Interstate bridges. When the the car becomes airborne the oil pan won't scrub the pavement upon landing.

Ad0911

So applying some kind of lubricating agent like white grease prior to installing the lower control arms will take away the memory spring effect of the rubber, which it is upposed to do?

hacker-pschorr

The suspension is designed to have this rubber in a constant state of bind, no amount of grease or lube will change that. The clamping force is just too great. Even if it did help loosen up things, the effect that might have on the overall spring rate as mentioned above might not be a good thing.

This is why simply replacing rubber with polyurethane can be such a horrible idea, you are actually making it worse. Most feel "stiffer = better handling" which is false.

IMO the only change from stock rubber should be to 100% free flowing heim style joints like Mike Simard made for his race car.

Rufus Sanders

Lizard928, OK, I'm interested in this. but most of the comments here seem to be about the lower arm. You're saying that a different type of upper arms. You're saying that a different type of upper arm can solve this? I'm all ears.... - Ruf

Lizard928

Rufus,

I bought one of the first pairs of 928 motorsports steel upper a-arms, and tested with those.
They failed in less than 3k miles, on the inner joint.
But with them installed, there was virtually no suspension settling.

On the factory upper arms, you can loosen the nuts in the inner joint which will reduce settling time as well.

I got given another set of these arms (keep in mind you must shim these to get a good camber for street use (min -2.5 deg)), I put them in my car as a test with other suspension components, and again with stock lowers had no ride height settling, it was lowered and stayed at that set ride height. I was also using 1000# springs in the front, both times, but I doubt that would have much if any of an effect.

None of these suspension parts though should be used on a primary street car for the average user, they add a great deal more rigidness and need to be checked frequently for wear. Same with heim jointed suspension.

Speedtoys

Rufus:

What do you believe needs solved?

Nothing's broken.

Rufus Sanders

Speedtoys , I would just like to know if there's away to avoid this cycle. Every time my car comes off the ground, it takes days for the front suspension to go back where it belongs. I want to know how to avoid or improve this situation. If there's really nothing to be done, so be it, but if sounds like some of the guys here have some stuff they've done (be it race/exotic etc.) that helps this.

I guess it's not really a huge problem, and I now understand that the bushings in the lower arms are loaded and this contributes to this (more than I knew earlier this week...and that's an improvement in itself) I do like hearing about stuff that most of us never heard of. It just adds to the general base of useful info. - Ruf