ErnestSw

Roger has noticed a relatively sudden increase in rear camber and a fall in ride hight of about 30mm (1 1/4") over the last few months and we're trying to figure out what could cause it. The only thing we can come up with is that the shocks are bad causing the entire weight of the car to be transferred to the springs and that the spring rate has increased because of the increased stress.
Anybody have a better or alternate explanation?

FlyingDog

Is it even on both sides?

Garth S

A 30mm decrease is quite dramatic - especially over a few weeks; however, the shocks do not act to either suspend or establish the ride height of the car: They control the oscillations of the spring - unsprung mass on compression and sprung mass on rebound.
It may be possible that the rear upper spring mounts are no more .... but mere rubber dust. Otherwise, I dunno ....

ROG100

I have this afternoon measured the ride height as Ernie has said.

The measurement prior to filling the tank with gas (forgot about that) are :-

RF=130mm LF=132mm RR= 129mm LR= 128mm
After the gas was added:
RR= 119mm LR= 117mm

ErnestSw

Garth,
My point about the shocks being bad is not that they establish ride hight, but that they essentially "protect" the springs from having to bear the full weight of the car on sudden compression. It seems to me that bad shocks might cause excessive spring wear and increased spring rate. I can't imagine that the 10+mm compression caused by a tank of gas is normal.

ROG100

To clarify, we are talking about my 93GTS.
The rear tires are toast after 4k miles and they were Continental with a 400 wear rate.
The front Kumho's are nearly toast after 1500 miles (SITM to DFW)

SharkSkin

Unless they are gas charged, maybe?

ErnestSw

Gas or oil the shocks don't establish ride height. The springs and their mountings do in a coilover suspension.

SharkSkin

Certainly that's the biggest factor. I had a 510 raise up almost 1/2 inch with the addition of gas shocks... and no, the suspension had no height adjusters to goof up. I'd be surprised if a pressurized gas shock on a 928 had no influence whatsoever.

ErnestSw

Interesting. Wouldn't the RESTING height of the shock remain the same? I assumed that the shock would end up whatever length the spring height dictated in a coilover.

Ispeed

A pic of ROG in the GTS low rider at SITM. It sure looks low.
Good luck Roger, and of course get the height right before alignment.

ew928

Rog,
Sorry to hear that the suspension isn't happy.
Anyway to post pics of the worn front tires.
Hope the Texas contingent had a great time at SITM.

My 88 needed a set of shocks from day 1 of my ownership.
The old shocks would not dampen the shark on bumps.
How old is the suspension system?

Ernest (NYC) 195mm front/ 190mm rear 928 Cayenne

jpitman2

The book notes that adding Bilstein gas shocks where non-gas were fitted before can (repeat CAN) raise ride height by up to 10mm, so gas shocks can change height. Usually the the spring is different for gas than not. If those measured heights above are at the specified point I would be pretty worried about the state of the low lying equipment - like the pan! Mine scared me at 145mm. Only way I can think you could loose lots of height like that is for major suspension bushes to collapse.....os somebody fiited wheels too small?
jp 83 Euro S AT 49k

AO

I know a thing or two about shock absorbing systems (I used to be CEO for a company that made ultra-compact shock absorbers). The spring is what absorbs the shock, it is also what establishes the ride height in our cars in combination with the spring perch. The shock/strut (unless air/gas charged) does nothing to set the ride height unless you are completely bottomed out - e.g. broken spring or overweight car. The spring would ocilate uncontrolably if not for the strut. Ever seen an old car driving down the road with one or both back wheels dribbling like basketballs? It's because the strut is dead, but spring keeps springing. The strut is teh damper that absorbs the ocilations of the spring. This allows the wheels and tires to remain in contact witht he ground as much as possible. If you were driving on just springs, you'd end up in the ditch pretty fast in a 928.

On that GTS there are adjustable rears. I would inspect the ride height with everything unloaded from the car, but with a half to full tank of gas. It could be that he has a set of softer springs and the additional load from his luggage was too much for them, but I suspect ride height is just too low.

Nicole

<sarcastic>
Shocks can do very interesting things. In fact, if you have a problem with your suspension, it must be the shocks by default...
</sarcastic>

When I had an unexplainable vibration from the rear wheels on a prevous car, a salesperson at a tire dealer told me that I needed new shocks - because just as in an engine, the oil in the shocks gets old and worn after 60k miles.

In 1993, that job would have been about $500 for that car and surely would NOT have fixed the vibration (it later turned out to be a warped brake drum).

Are you guys shure that Roger did not gain lots of weight recently?

Hope you guys figure it out!