944Time

My rear shocks are about 19 years old. Changed in 2001.

I had an exciting moment on the highway when changing lanes. Almost felt like my rear shocks weren't working properly.
But they pass the bounce test.


I am wondering if rear shocks wear and loose their effectiveness. As far as I know, a shock is just a piston with a hole in it, and oil is forced thru the hole.
Other then leaking, what could go wrong? Hole get bigger? I don't think so.

I would guess they have 50k miles on them.

A replacement pair would be about $70, which on a cost-per-year basis is pretty low, but don't want to replace them if it isn't necessary.



Shocks, Year 2001

aussie944cab

The oil thins, seals wear, so yes, they do wear

CyCloNe!

They wear... shocks and struts are supposed to be changed roughly at 50k miles by most manufactuers, they do last longer generally but thats the the number that is thrown out there.

Tiger03447

Any signs of oil leakage? If you see a line of oil coming from under the top shield, it's time to kiss them bye-bye.

MAGK944

No they don’t, I really cannot understand why the parts stores even stock replacements

Seriously though they can leak internally and externally both oil and gas if they are the charged type, plus they can also corrode and the rubber bushes can wear out.

Perry 951

I had a stack of valving washers a KYB engineer gave me once and wish I had a pic to show you. They were about the size of a nickel and very thin, in the thousandths of an inch range. The holes and slots in them were quite small. The selection of washers, number, and arrangement made up the compression and rebound character of the shock or strut. The body, piston, shaft, oil, and gas were not as important as the makeup of these valving washers in regards to how it performed.

90% of shocks working life is within a 1" or less area of travel from the static area of where the shock sits normally on the car. So 1" up or down. As the valving washers wear, both in deflection and in actual wear, they will allow more fluid to bypass causing a "harsher" ride and more spring movement in this range.

In cases of large compression or rebound events, when the valves are worn, you will get more than the desired amount of spring movement and that can correlate into wheel hop or undesirable handling.

The bounce test will pretty much tell you when you've had an outright failure, but not much more. Oil and gas leakage would be present and a better determining factor.

Hope this info helps.

Dan Martinic

Interesting!

Say.. anyone ever actually witnessed a failed bounce test? I've had bad shocks yet bouncing the car has never indicated anything. In fact, right now a shock mount broke on our van yet you can jump up and down bouncing away and as soon as you stop, it stops. Seriously.. I think the suspension resists bouncing far more than we think..

jimbo1111

My recently acquired 944 turbo with factory installed Koni yellows failed the bounce test on the rear. I replaced the rears and all is good. I also plan on sending out the fronts to Koni and have them rebuilt even though they seem to be doing ok.

Dan Martinic

Really? You bounced the rear, stopped, and it kept going? Or did it just feel soft? Maybe I'm misunderstanding this test

jimbo1111

Yes. I bounceed the rear and it bounced many times before stopping. Any kind of bounce is not good. After pushing down on the corner it should just raise back.

944Time

> No they don’t, I really cannot understand why the parts stores even stock replacements

There are things that don't really wear -- like sway bars. Or coils. A coil can fail, but it can't really wear out.

It looks like my shocks have been afflicted by the pox!


Left Shock

Right Shock

Perry 951

Coil springs and torsion bars, and to some extent stabilizer/anti-sway bars absolutely wear out. The metal fatigues, looses its elasticity, and will eventually have a loss of ability to store and release a consistent amount of energy.

This is the reason our cars will lose rear ride height so drastically over time. The torsion bars get weak and can no longer hold the car up, or more correctly, push the wheels down.

MAGK944

No, absolutely not the case. Every old sway bar I’ve looked at off the car has developed a “twist” that’s probably because we do many more rh than lh turns. The coil springs on my car when I replaced them had lost a good inch over the years, so did the bars at the rear. They sure do wear, everything mechanical wears, it’s just coil springs and torsion bars generally don’t catastrophically fail so folk live with them longer in their weakened state.

thomasmryan

most shocks have multiple plate valves orifices of different sizes for different size bumps.

944Time

Actually I was thinking of this kind of coil
Not the spring coil, ignition coil.