ms9811

Ok, Ok. Please don't flame me. I know that the subject has been beaten to death in many ways, but I cannot seem to do a search that answers the simple question of how you can tell if your OEM shocks are shot. I know that the old fashioned "bounce test" does not work and that people say that at anything over 30K miles they are done, but the shocks have no visible signs of failure and the car does not SEEM to be doing anything all that strange other than oscillations of the nose in high speed sweepers that have dips, so I am at a loss. My 96 993 Coupe has 63K on it (58 when I bought it), all apparently gently driven. This is my first Porsche and I am not sure what handling traits are just specific to the 993 and what might be a sign of worn shocks. I drive a race car (not the Porsche) and am familiar with the descriptions of handling traits, so feel free to get technical with me. If I go the replacement route I am going to do M033 and Koni FSDs since this car is used for pleasurable road driving, not track use and I am a believer in good suspension compliance with decent shock control as opposed to uber-stiff setup, but I do NOT want to fix something that ain't broke, if you catch my drift (Sorry - old Porsche handling trait pun intended.)

Thanks in advance.

goofballdeluxe

At 63K miles, your original shocks are done, whether you feel it or not. Easiest way to tell the difference is to upgrade to something like PSS-10s or something similar, have the car aligned and corner balanced, and then drive it. Then you'll be able to tell the difference and see how bad your old shocks really were

RocketJohn

Well it sounds like you know what you want you suspect what you have.

Doing suspension improvements and refreshes transform these lil' cars better than anything you can do, in my short experience with them.

Love my PSS10s, but that's because that's what I have...

bruce7

Hi, did a little technical research on this which might be helpful.

Remove the shocks and put them on a shock dyno.

Generate 3 plots: Force/Postion, Force/Velocity, Average Force/Velocity.

The Force/Position plot is very good for showing where in the shock's stroke various things happen. It helps isolate force changes due to shimstacks opening and closing, bleeds topping out, seals sticking, cavitation, and whatnot. It is most helpful when trying to build a shock or diagnose a wonky one.

The Force/Velocity plot is the most useful for determining shock quality. A well constructed shock will produce a trace with very little hysteresis gap between the "bump-accelerating" and "bump-decelerating" (and "rebound-accelerating/rebound-decelerating for that matter) portions of the trace. No shock is ever perfectly on the same force (no gap) but you want it as narrow as possible.

The Average Force/Velocity plot is useful for comparing shocks to each other, looking at the effects of adjusters or valving changes, and matching shocks left to right. As long as the hysteresis gap on the FV plot is narrow, the AFV plot is valid for this use. Watch out though for shocks with crappy FV plots because the FAV plot will hide the shock's faults.

Thanks to Dennis Grant for this explanation.

-bruce

Mike J

The following comments are just from observations:

I have replaced several sets of stock shocks now on 993's, and I have never found an original shock that, once the spring is stripped off, where I can't push/pull the shock shaft with one hand no problem. They also all have zero rebound, ie. I can push the shock down to its bottom and it will just sit there. Higher milesage ones seem to offer virtually no damping, they start to feel like nothing is inside the shock. I predrove many of these cars, they felt OK on the road, but had some characteristics like thumping on railroad tracks, or jiggly feelings when on pitchy roads.

As a point of comparision, I noticed that, on brand new sets, with a shock mounted in a vice:

- Koni FSD's - they can be pushed/pulled fairly easily, but they do rebound, and offer much more resistance than the worn ones if you try to push quickly, but they are the softest by far of the shocks I have installed.
- Bilstien HD's - much stiffer feel than the Koni's - you can actually struggle to pull down the damper, they resist much more than Koni's
- Bilstien PSS-9's/PSS-10's - really stiff, I have a hard time compressing these manually at all, even with my 230lbs pushing on them.

Again, no measurements taken, all by observation...

Cheers,

Mike

kjr914

The issue with this approach is that once you've removed the shocks, you've done all the work required to replace them and will need alignment when done anyways. So "while you are in there"...

We've got a large number of data points here on RL993 that say 63k and the age of these cars points directly to dead original shocks. I don't recall a single person saying, "I replaced my shocks and the ride/control is no better". The only issues I've seen are people who go too stiff for their intended application (road only, daily driver, weekend cruiser, etc) and don't like the resulting ride.


Mike,
FIFY...

SchnellAuto

yup mine were dead at 23k .. and then again the car sat around 17 years. give you a deal on pss 10

Lemon

bruce7

Hi, you're right of course. Most people don't care to know why something doesn't
work or to learn what the failure modes are, they just want to change parts and move on.

-bruce

kjr914

Rest assured, I'm keen on "figure out what is broken and fix the right thing" rather than shotgun it. Like you, I'm also the "how does it work" type. I'm an engineer, there is no cure... I'm guessing from the use of the term "failure modes" you're an engineer also. As you say, not many of want to test a shock or cut a cross-section to figure out how it works. Luckily, we've got some folks here that post such pics and it's awesome!

Rinty

ms:

I thought my shocks were normal until, one day, I drove up my brother's long, undulating, gravel driveway, and the car hobby horsed. Prior to this, on brisk runs on rough, two lane roads in the Rockies, I thought the car handled just fine. But I'm no Jeff Zwart.

The shocks had 50,000 miles on them.

Ed Hughes

As Mike noted in his experience, the car may not drive "bad". Although, your oscillations seem to be some tangible.

I knew my car wasn't crisp, even though I could take it up in the hills, and have a lot of fun. When I put new shocks in, it was pretty dramatic, the improvement. This was also coupled with new springs and control arm bushings, ad a first rate alignment and corner balance.

s4magman

A leaking shock is a sure sign.

pirahna

My car handled very well with the original shocks at 67k miles when I had them replaced with FSD's. I wouldn't have replaced them if I didn't want a lower ride height. I'm glad I did, the FSD/M033 combo looks better, handles better and is even smother than stock.

Ed Hughes

But, not necessary. Many have no evidence of leakage.

chris walrod

They are done when they do this. (Off a 75k mile car)