New shocks at 30k?

jonathanwalters · 22 Feb 2008

Hi - new to the forum, so hello!

2 questions for you ...

Bought a 30k mile, 04 reg 997 CS and the ride feels a bit vauge. After a bit of testing the OPC diagnosed that the shocks were just getting worn. Now I'm not sure what I would have expected, but worn shocks at 30k miles seems a bit excessive.

Worst part is - because they're not actually faulty or dangerous, just not "as good as new" I'm footing the bill. Anyway, just interested in thoughts.

Secondly, the tell tale light for PSM - should this come on whenever PSM intervenes? I've never seen it come on, even when I'm sure it must have kicked in (a bit enthusiastic coming off a damp roundabout!).

Any thoughts gratefully received!

Cheers, Jon

Jeff · 22 Feb 2008

hmmmm, well to be honest, yes while they would be starting to lose their newness to them, they shouldn't have actually failed. I'm not convinced that a new set of factory shocks will feel any better. I think what your saying is you don't like the standard ride - maybe you'd be better going for some KW aftermarkets which will be far better for similar money I believe.

Can't comment on psm sorry.

Wattie · 22 Feb 2008

What absolute rubbish about your shocks "being worn"...

Shock absorbers either work, leak, or are broken: The active components of a shock absorber are either gas or liquid being forced through a hole by a plunger. If the plunger gets worn, they leak, then they don't work and need to be changed. If they don't leak, and they aren't physically broken, they are fine!

If the shock absorber isn't working, you will know about it - the car will literally bounce all over the place as there is nothing to stop the spring continually bouncing until the oscillations stop.

As Jeff says, if you are not happy with the ride / handling quality, it's more likely that the car needs a geo. or that you have got used to the relatively soft OEM suspension, and want something a bit stiffer.

Sorry to rant, but I am a chassis engineer by qualification, and I get sick of people saying that shocks need changing so soon - it's a very useful revenue earner for garages as nobody has a clue how to check them properly, so just go along with it and hand over the plastic :wall:

And yes, the PSM light will come on, but only if at least one driven wheel has actually started to 'slip' - and if you have Sports Chrono, I believe I'm right in thinking you can actually have a few degrees over oversteer angle before it will come on

Jeff · 22 Feb 2008

Wattie, as I understand it, it's not just that they can leak, it's that after time the shock fluid loses it's viscosity enough to vary the shock performance. So while the pistons, valves etc are all still working correctly (system still sealed) the fluid passes through the orifices easier, hence less bump / rebound.

Wattie · 22 Feb 2008

Jeff said:
Wattie, as I understand it, it's not just that they can leak, it's that after time the shock fluid loses it's viscosity enough to vary the shock performance. So while the pistons, valves etc are all still working correctly (system still sealed) the fluid passes through the orifices easier, hence less bump / rebound.

That's news to me Jeff if that is the case (although I have to confess that I haven't worked as an engineer for 25 years!)

I don't see how the viscosity of a fluid can change permanently without contamination, excessive heat, or dilution though? Shocks should never get hot enough to cause permanent change, and dilution or contamination would also be accompanied by leakage?

I had my 94,000 mile shock absorbers tested properly recently (at the Cologne OPC) and they are all still within manufacturer's tolerance for both bump and rebound. And you should see the state of the roads around my house! :twisted:

What does wear on suspension systems are rubber bushes and the spring rate changes. Fitting new shock absorbers cures the bushes wearing, and if the shocks are uprated, they take some of the load off the springs and so the whole thing feels much better.

I have been told (by a Porsche engineer) that Porsche OEM shocks for the 996 have a design life of 200,000 kms - but they still won't change them under warranty :wink:

Jeff · 22 Feb 2008

It's funny really. No one would ever change the springs and not the shocks would they? They would do it the other way around though.....
You might be right. The suspension manufacturers say their valves wear out. I can't see the valves wearing out - they're just holes. The piston seal could wear allowing fluid to pass but one would expect that would become catastrophic causing full loss of damping, not progressive.

Of interest I'm having my old billies from the 3 put onto a shock dyno to see what state they're in .... I'll let you know the results.

Research time - spring oscillations and change of spring rate over time...

Wattie · 22 Feb 2008

I'll be very interested to hear the results of that test, as I've been thinking about this a lot lately - my suspension is definitely changing and like everyone else I just assumed shocks, until they were tested.

My suspension is much harder than it was when I got the car 50,000 miles ago, and I now believe it is a combination of hardened bushes (less compliance) and worn springs. The worn springs supports the fact that when a full geo was done recently, there was not enough adjustment left to bring the rear camber inside the manufacturer's settings. Also, the suspension feels like a rose-jointed race setup - supporting the hardened bushes theory.

The issue is, I actually quite like it :lol: And Andy, from Promax Motorsport, loved it when he collected and delivered it back. It feels more and more like a proper track car: Very little compliance and too much rear camber

The only downside is that it is getting very uncomfortable on our roads around here (anyone driven from Silverstone to Buckingham lately will know what I mean).

tazsura · 22 Feb 2008

Interesting thread. Really good. My suspension is feeling really firm at the mo. I don't know if it is becasue i am jumping from car to car or what, but it does "seem" to be bumping and thumping more than i though it did.

I remeber reading the thread regarding the shocks ages ago where Wattie told about the OPC Colognes comments RE: shock absorber life, so thought that it could be that my bushes are worn? I've looked back through the receipts and can't see any new ones being fitted. How can i tell if they need changing? Is it an expensive (in Prosche terms!!) fix?

Cheers

Taran :?:

Jeff · 22 Feb 2008

This makes no sense at all. Take my dear mothers old Subaru. Had it for 100,000 miles from new. Unquestionable by the end it was like being in a boat. The shock weren't officially gone (ie they stopped bounce within the require cycle) but it was so wallowy for want of a better word. From new it was taught and firm. Now obviously I put it down to shocks not springs but in reality it was probably a combination of both. I would have said the shocks were working at about half rate though compared to new. They were not leaking etc.

Now in your case, I am confused. you would expect the spring rate to drop not increase. I'm not finding much evidence to backup your car though.........

Wattie · 22 Feb 2008

Wallowy suspension is more likely to be a combination of weakening springs and anti-roll bars, than failure of the shock absorbers. My understanding is that shock absorbers either work or they don't.

However, it is possible that there has been an internal failure of the shock absorbers in one direction (are they twin tube shocks on a Scooby?) If the shocks are working in one direction, then they will still dampen out the spring oscillations and may still remain sealed as a unit, but I would think this is unlikely that more than one has failed at the same time?

Robertb · 22 Feb 2008

Surely 997S has whizzy semi-active PASM shocks. I'd be amazed if they'd become worn already in 30k miles of normal use.

I'd certainly suggest a full geo first.

Re psm, I've never managed to get the psm light to come on in my 4S either except for when I booted it, out of interest, when I was driving over a very icy stretch of road. I think it can differentiate between needing to reign things in when you are pushing it on purpose (i.e. no light) and 'unexpected' slippage, e.g. on ice, when you do get a light.

smj996c · 22 Feb 2008

+1 for the Geo
-Wheel balancing
-tyres and tyre pressures

before you shell out for new suspension, 30K miles seems a bit of a joke :shock: get it to a renowned indy to check this out :thumb:

LOJO · 22 Feb 2008

OK, so I used to work for a shockabsorber company and was told :

- the fluid shold not break down unless you have a rally car and it is heated to boiling point
- if the piston seals go that will be obvious because fluid will run down the side of the SA and collect dust etc.
- constant pumping of the shock causes the fluid to enlarge the holes in the end plate 'sieves' and as that happens the shock loses it's firmness
- most people just replace shocks, but springs are very cheap to replace and harder springs 'protect' the shock from excess work, so why not do them all at the same time.

My 10p worth -

LJ

jonathanwalters · 22 Feb 2008

Wow - great response - good to see so many with a similar work / life balance mentality!

Just to complete the diagnostic - they did indeed say it was the fluid which gets contanimated with air which over time change the viscosity and therefore the damping effect.

I'll go ahead and get them changed and report back (hoping this fixes it - otherwise and expensive way to eliminate the problem).

Is the general consensus that you wouldn't expect this to be replace FOC by the OPC as the shocks were clearly in this state when I bought the car in Janaury!!

Also - any more views on the PSM tell tale light?

Thanks!

LOJO · 22 Feb 2008

OK, so I used to work for a shockabsorber company and was told :

- the fluid shold not break down unless you have a rally car and it is heated to boiling point
- if the piston seals go that will be obvious because fluid will run down the side of the SA and collect dust etc.
- constant pumping of the shock causes the fluid to enlarge the holes in the end plate 'sieves' and as that happens the shock loses it's firmness
- most people just replace shocks, but springs are very cheap to replace and harder springs 'protect' the shock from excess work, so why not do them all at the same time.

My 10p worth -

LJ

New shocks at 30k?

jonathanwalters

Trainee

Jeff

Indianapolis

Wattie

Zandvoort

Jeff

Indianapolis

Wattie

Zandvoort

Jeff

Indianapolis

Wattie

Zandvoort

tazsura

Imola

Jeff

Indianapolis

Wattie

Zandvoort

Robertb

Yas Marina

smj996c

Montreal

LOJO

Sao Paulo

jonathanwalters

Trainee

LOJO

Sao Paulo

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