Okv

Reading this thread with great interest, as I will upgrade the suspension of my Shark some time in the future to make it less of a boat and more like a car, it strikes me that focus has been aimed at springs and shocks for so long, when it is the bushings and suspension arms that locates and guides the wheels in the right direction and provides the 928 with its no less than excellent high-speed stability; actually; I have never driven anything even close.

Is it possible that the bushings on the lower rear arms, the ones providing the weissach-flex, could be bad? These bushings are made flexible so that the rear wheels will actively 'steer' when loaded (..but you all know a lot more about this than me!), if these bushings are too loose, I would belive it is likely that stability will be very bad.

Bill Ball

Yeah, I'll keep them warm for you, Tim.

Bill Ball

All the bushings are suspect even though they all look fresh. They could have softened with age. The Weissach one looks like an SOB to change. I could pin that if I wanted to.

Warren928

Bill,
When I went to school for aircraft engineering, we learned that when a vehicle is pushed to high speeds they endure a tremendous amount of stress and it could result in a catastrophic failure if parts are periodically checked for tolerance and magnafluxed for hairline cracks, or just replaced at particular intervals. Porsche never outlined what the service interval would be for a 928 that was driven at 160 mph+ for XXX number of miles, but before someone suffers a catastophic loss, I would recommend a full teardown and magnaflux inspection, tolerance specification check and replacement of suspension parts like A-arms, control rods, joints and pivot bolts. Also to add to that list should be any part that if failed could cause an immediate loss of steering control of the car.

To give you an idea of what wierd things can happen, the boeing 727 years ago had developed a stress crack in the rear section of the aircraft after seeing 15,000 cycles of use and could suddenly depressurize or lose its tail section from the continued use at high speeds. They had to retire the whole aircraft early because of the fear. That is an exteme case where the whole vehicle became useless, but the "jesus bolt" on the engine had to be changed every couple of years or there was a risk of the turbine engine falling off. One bolt: $760. Oh well, at least porsche parts are cheaper. I think the comparison mainly has to due with the stress speed puts on things and you need to add the stress of a public road environment.

If you start digging from this viewpoint you may find your problem because I guarantee that a car that is driven like you are driving, at very high speeds is going to wear much differently, and in some unusal ways that most 928 owners won't see due to them only seeing an occasional 20 second high speed run.

Bolts and ball joints can become ovaled, parts heated and fatigued or stretched, and things will start to geometrically become out of synch thereby causing the harsh and unpredictable ride you are describing. It may be that its more cost effective instead to buy another 928 for just high speed cruising and retire this one only for street use.

Warren928

It definately sounds like something beyond the typical shocks, springs, sway bar, or what is preceived as the normal definition of joint wear. On old cars, normal ball joint wear is more longitudinal but on a high speed car you may only see latitudinal wear. this may not be easily spotted until the joint is take or broken apart.
Bolts or pins that appear good and tight could have taken on a oval worn shape on the a-arms or other suspension parts, which would throw off normal movement of the suspension. Worn parts in the suspension that are throwing the caster off would also bind up your shocks when they are loaded with different directional forces. Again, this might be the a-arms, connecting bolts to them or hub assemblies.
Also, even the slightest looseness in your wheel bearings will cause alot of steering issues and handling from the rear end too at high speeds. If you changed your bearings you should also check the spec's on the their races too for a tolerance that is good.

pmotts

Bill,

Didn't this problem start when you had some suspension work? It wasn't a gradual decline but a problem right after some work was done? I am sure you hve thought about this for a bazillion hours but what could have been done during the work that could cause this? Did they replace the sway bars at the time? Are the sway bar mounts tight and are the sway bars centered. I am just grasping at things that you have probably already checked and thought through... ten times. What a bummer.

Bill Ball

Warren : Total teardown and magnaflux?! I may get to that. As to unusual wear that may be hard to detect... My plan at the moment is to go after the soft parts. If that doesn't tighten things up, then the rack, balljoints and tierods are headed to the recycler. Many people have pried on the joints, put heavy pliers on them, and they just don't wiggle in any direction. The wheel bearings are recent, and I just checked the fronts and did snug them up before the last race.

pmotts: Yes, the befuddling thing is that there was a big change 3 years ago when I swapped out the old original Boge gas shocks for the new Konis, particularly involving softness in the front end. The rear wasn't much better. The car lost its previous composer in the twisties. Not understanding what could be the problem, and getting no real assistance from the vendor, I just adjusted the Konis harder. That just made the ride harsher, but no more controlled.

Over the 3 years, the Konis deteriorated from there such that in the last 2 years the car began to bounce like an old Cadillac. Finally this year it got even worse, to the point where it was evident the shocks were shot when I could not drive the car even in a straight line at over 120 Mile per hour without it wallowing all over the road. The vendor agreed something was wrong. I replaced them with new Konis on the same springs. Although the new shocks were quite an improvement over the old in terms of dampening, far better than the previous set ever was, the overall ride softness was still there. The car was driveable in a straight line (I hit 170 and cruised at 150-170 for miles) and modest turns, but lost all composer when side-loaded in long, high-speed sweeper, wallowing in scary, lazy oscillations. Thatt was new - previously the car could be driven into turns with just minor handling issues. This was more life-threatening. So, now I have new Eibach/Bilsteins. The ride is jittery, jiggling and not very composed when cruising slow or fast, although I haven't tried and 140+ MPH sweepers yet.

I know this seems like a saga, but I really haven't done much but change shocks a couple of times, and recently springs. I have a lot more to try. It is rare that anyone pushes a 170 K mile car this hard, so, worn bushings should be presumed. If the old rack and balljoints would just give up, I'd have something else to do. If the bushings don't help, they are next.

Ron_H

Warren brings up some interesting points. I have been crying the blues all this time, but have not even thought to check the rear suspension parts. Thinking in the box I guess. If it doesn't steer properly, check the steering. The rear affects the steering too. I just want it to stop wandering all over the road. My shocks are Konis on stock springs, and maybe you should try my car Bill. At any rate, this thread is getting interesting. Now and then it's good to change the head doing the thinking for a fresh look at things.

drnick

this sounds like a really frustrating problem... i remember trying to chase down a handling problem that mainly resolved after the new rear tyres heat cycled, obviously not the case here but i remember racking my brain trying to think what it could be. the answer seems to be to go through and replace everything in stepwise fashion, its just expensive and time consuming. wheel alignement shouldnt be overlooked as a starting point, the front upper A arm bushes seem to show the most wear and might contribute to poor hold on an alignement. IMO the rear suspension bushes dont make a huge difference to the low speed handling, after changing mine for polyurethane.

re reading what you are saying about the handling i think there are two seperate issues - at least. one is the 'jittery' feel over uneven surfaces at modest speed and the other is lack of composure in high speed sweepers. my 88 SE on stock worn out suspension was totaly stable at high speed, but couldnt change direction without significant delay and then only slowly. i changed it for a worn out bilstein and eibach combo and it is still stable but more of the road filters back to me as driver, i know more about camber changes etc through the wheel, the stiffer springs alow for more rapid changes in direction at speed but the worn out shocks bounce around.

overall after changing suspension on both cars only a few times in total i would rate the biggest determinants of high speed stability to be 1st tyres and 2nd algnement. 3rd would be shocks and 4th proabably bushes/springs.

GUMBALL

In racing, one tries to eliminate all unknowns.
Just a thought, but, before doing any more changes, may I suggest having the springs tested to see what the spring rate ACTUALLY is, and the same with the shocks - get them run on a shock dyno, and compare the graphs - shock to shock. Another advantage here - the shock engineer can usually look at the graph, and tell YOU what the car is doing.....

If you like, I can make some phone calls to find someone who has the equipment in this area.

Bill Ball

Gumball:

The koni/spring combo is coming apart this weekend. It should be easy to measure the spring rate with, say, 100 pounds on top, measure the compression. I don't know if it is worth the trouble to dyno test the shocks, although I do have some suspicions about one of the shocks, even though it is new like the rest.

Nick:

Alignment was fresh and set mid-range except for caster, just as I have always done it. Tires and wheels were what I have run for a long time. The tires are not the greatest for traction (Yokohama ES100) but traction is not the issue. They may have flabby sidewalls, but have served me well before. Shocks, well, how many sets do I need to try? Granted, I haven't stress tested the Bilsteins yet, although their daily driving road manners with the Eibachs are atrocious (again, that may be due to some other underlying problem). So, I'm going to look at your fourth choice.

sharkmeister85

It's been awhile since I started following this thread, so I don't recall if this has been answered. But, how does car act under hard braking from a higher speed?
Glenn

Bill Ball

Glenn:

I don't know. I don't use the brakes. Track guys do, but in open road racing, they rarely get touched. However, I do remember it pulling right when I tested the brakes once at about 140 last year. I do think I may have a sticky caliper/piston as the inside pad wears more than the outside upfront. There does not appear to be any drag when off the brakes. I have the kits to rebuild the calipers (seals, boots) and plan to do that soon too.

Ron_H

Again, Bill, we have more in common with this suspension/steering thing. My calipers need rebuilding and drag also. Until they're hot, they pull unevenly. I often ride them a bit before using them at speed for this very reason. Not a good idea, but it warms them and they act evenly. Time for a rebuild for sure.

sharkmeister85

I guess what I was getting at was whether or not you had handling issues under a different driving dynamic, ie. any "squirrelly" or erratic steering response under hard braking might give further clues as to the nature of the problem.
Glenn