FlyingDog

Why didn't you go with Monoballs for the upper and lower mounts? At least the lowers would make sense since you were customizing them.

BC

I think a search on here would reveal some pictures or at least some drawings. I can't remember the gentleman's name right now.

Okv

I assume you mean the coilover upper and lower mounts:

I do not think there is any performance gain on the 928 by using better mounts for the coilovers; the geometric motion of the suspension is guided by the upper and lower a-arms, which are mounted on rubber bushings on to the chassis.

The lower mount is almost similar to a ball becuse there is no rubber bushing at all, the upper mount is stock and consist of a very big rubber mount that support the forces from the spring and shock.
May be the performance of the damping would be compromised on very hard spring set-ups for pure racing, with rates at least 4 times what I have now.

Any movement in these attachment points will not lead to dislocation of the suspension geometry.

FlyingDog

Most racers actually don't use much stiffer springs than you (600/400). Some people have had the upper rubber mounts fail. I think Carl posted pics of his front upper rubber mount failure.

Okv

Would really appreciate feedback from any of the following users on this issue:
Bill Ball,
Dr Nick,
Cheburator,
Carl Fausett,
Mark Kibort.
(Of course I welcome comments from others as well, these are users have noticed have had similar issues or have experience with suspension setup for the 928).

Reading posts about suspension and hypercoils and bushings again, and find several posts like this, stating they have the results of their suspension upgrade, exactly like I want it:

"The car is amazing at any speed over 40 mph. Zero body roll!. Completely planted and rock solid."
[posted by user RDS928S]

This is just one of many examples, so, I really think it is possible to fix it.

And this is what to fix:

- It still does not feel as "planted", stable, solid, fixed, at higher speeds.
- Ok at low speeds, feels controlled and well damped, but, what doesn't.. even a Volvo feels safe at 50.. (km/h or mph, you choose)
- At speed; unstable and needs steering corrections under heavy breaking, road undulations, turns.
- A floating feeling, which gets worse the faster you go.
(As the car is now, it is the reincarnation of the Nisse governments definition of the ideal car, as the character of the car prevents speeding.)
- Never feels really hard and communicative, even at low speeds the ride feels rather soft.


After reading a lot about this here (an excellent knowledge base), I have come to this conclusion:

- It is the shocks.
- A 928 can be made to handle like I want it to; that is, a stable, rock-solid feel, that gets even better the faster you go.
- Sorry to say, but the Shark will never handle like the Cayman in low speed turns - but that is not the issue here!
I can live with that - it is two completely different animals - that is why I have both now.
- 500/350 springs are rather soft, but that is not the problem.
- The 928 suspension is very critical to shock absorber damping characteristics.

Wonder if had I gone for the Hypercoil-Koni set-up, I may well have been completely satisfied now, for one third of the cost.
But that is not too important, the car is a hobby, and it is supposed to cost me money, I do not expect it to be profitable..

So, the options now:

1. Re-valve the Bilstein BM5 to enw characteristiscs
(More rebound? More slow-motion damping? More overall damping - now maxed out in adjustment range.)
2. New Hypercoil-Koni set-up.

Apart from the performance, I really like the Bilstein set, so I think I would like to try a re-valve.
The colour is nice, the adjustment ***** are nice, and so on.

And now, comments are welcome.

Bill Ball

Most likely you need a lot more slow-motion dampening in the Bilsteins for stability at high speed, although I do not know the characteristics of your custom set. I had a very bad experience with Konis, possibly due to poor quality control in the two sets I got, as other friends of mine have perfect high-speed handling with Konis. I have Bilsteins now with stock springs and they are just OK (although far better than the Konis I had), similar to what you describe - great in twisties, not as stable in very high-speed (140+ MPH) turns (some wallowing instead of quickly taking a set). They need more dampening.

Ispeed

I would do a re-valve. There is every chance your original valving was way off from the beginning, and the adjustment range is too subtle to get it close to right.
Still a guessing game, but I have been happy with my re-valved bilsteins and 600/400 springs.

Bill Ball

Yeah, I need to send my off for some re-valving like you did. I'm very happy with everyday driving and running around the twisties, just need more control going into long sweepers at very high speed.

Okv

Yes, and this makes me think that my springs are fine, 500/350 is not that far off from what you have.
Also, it indicates that properly valved Bilsteins will perform just as good as any other brand.

Okv

And this is where the 928 should be at its best - high speed, long turns, where the weight is not that much of a penalty, and the stability-focused suspension design is just right.

I think I will plan for a re-valve, with more overall damping, and much more low-speed damping, also some more rebound.

May be that the damping in front suspension bushings (the same that makes the car sit high after jacking) requires some special attention on shock characteristics.

Problems may occur if the damping is too stiff as well.
As for comfort, it is the damping that transmits impacts from potholes and so on, not so much the springs.
Too much rebound may cause less grip because the wheels become more unweighted as they leave bumps.

20 years ago I had an Opel, which I supplied with racing springs (intended for rallying) of original height - stiff, but retaining stock ride height.
Bought a set of expensive "super sport GT gas shocks", and the car was almost undriveable, jumping around like a japanese car.
Then I got a set of cheap Biltema yellow's, the were valved for rally racing, especially the rear ones was, you could say, rather on the stiff end of the scale.
At slow speed this car was a jarring and seriously bumpy ride.
but - the faster, the better, providing a ride that never became unsettled, always predictable, at any speed this car was capable of (which was not that much, like a 928 with only 3 gears..).
Only drawback was that you should not eat right before driving it.

To all; thank you for your contributions, very much appreciated. This goes for all other as well, having posted over several years on suspension issues.

Okv

928andRC51:

Do you have any specifications on the valving?

And, is this at all possible to express in numbers, seeing as the characteristic is complex and can really only be presented in charts.

Ispeed

Call Bilstein in CA and ask for Ramone. It is hard to get him on the phone, but he will call back if you leave a message. He is the guy who does the re-valving. He may be able to send you the exact data you need to have someone local do the work (a Bilstein service center).
I asked for 50% more rebound and 40% more low speed compression than original. I was sent shock dyno data that was hard to interpret but I have it. If you do a search under my name you will find it from about 5 months ago.
If you have 500/350 springs, don't get any less than I got for a change. You will like it I'm sure.

Okv

I got it.
I am going to think about it, and then contact the Nisse shock dealer.

Trying to compare Bill *****'s dyno sheets with yours, but it is difficult to get significant information out of this.
(I just marked Bill's set with metric scales to be albe to compare them).

slate blue

I was wondering if you have any updates for us, I have just given up on converting my Ohlins shocks, I think it would be better just to buy them new, not much back up here in Oz. They are brilliant shocks none the less, they have an automatic compensation for temperature change that most shocks don't have, i.e they preform exactly the same regardless of their temperature, a handy feature.

Greg

Okv

Not yet, I have not received a reply from the supplier, I will have to follow this up.
Had the car on stands until today, actually, lowered it to be ready for a test drive for the exhaust re-alignment i did the other day.
Now the weather is too cold with occasional snow or otherwise cold, wet roads here, so the car is parked.
I do not have winter tires for it, and do not wish to drive it in the salt.

If the weather permits, I was hoping to be able to do some driving one day, then perhaps look in to a possible problem with the rear lower arm pivot bolts, mentioned on another thread.