Replacing shock oil? Anyone tried it?

[priapism]

Since my Lemons days I've wanted to try replacing the oil in a set of shocks to restore them to new(er) condition. This is generally difficult because most shocks are solid units, so replacing the fluid requires cutting open the shock, then resealing it with a welder. Among other things, you have no way to know if you put the right oil in until you reseal the shock and reinstall it in the car.

Looking at some pictures I noticed what might look like a twist-off gland nut on the top of some S60 shocks. e.g.: http://www.autopartswarehouse.com/detai ... cid=nextag

Has anyone that's replaced shocks noticed if you can twist off this nut on the top and access the inner fluid? I'd like to know if it's possible before tearing the girl's car down

[MoVolvos]

Since my Lemons days I've wanted to try replacing the oil in a set of shocks to restore them to new(er) condition. This is generally difficult because most shocks are solid units, so replacing the fluid requires cutting open the shock, then resealing it with a welder. Among other things, you have no way to know if you put the right oil in until you reseal the shock and reinstall it in the car.

Looking at some pictures I noticed what might look like a twist-off gland nut on the top of some S60 shocks. e.g.: http://www.autopartswarehouse.com/detai ... cid=nextag

Has anyone that's replaced shocks noticed if you can twist off this nut on the top and access the inner fluid? I'd like to know if it's possible before tearing the girl's car down

Since shocks are not too big of a deal item in terms of product variety and availability it does not hurt to remove it first and try tearing it down. Seems like a futile exercise however but they need to be replace so why not drill it or cut it up!

Blessings,
BKM
-

[fazool]

Well, I used to work, as an engineer, for the company that invented shock absorbers. I'm not a total expert but I did spend a few years designing hydraulic shock absorbers, so I will chime in.

In a lightweight shock absorber, the oil is not usually the failing item. By lightweight I mean automotive style (unlike heavy duty like truck and railroad).

The oil will break down some over time, but its not subjected to the heat and external contamination an engine oil sees.

What typically breaks down in linear shock absorbers are the internal and external seals. So, replacing the oil may help a little, but its certainly not worth any such effort, unless you intend to replace the seals. Those that you are looking at have a gas-assist or gas-adjust pressure side as well.

The loss of oil is the number one failure mode and that comes from a failed external seal.

Its not going to work and not worth the effort.

[priapism]

There was an article in this month's Grassroots Motorsports where they tried this with decent (preliminary) results. I'm mostly interested in this as an academic exercise. If it saves $, bonus.

[jimmy57]

There is not a nut on the P2 struts, There is a gold colored cap pressed down on the top of the strut but under that is a rolled/knurled/welded (Fazool, please explain if you can) seam where the end piece with a seal is maybe induction welded to the shock tube. This process leaves a patterned seam.
I think an extremely small number of high end (high cost) automotive struts and shocks are made with provisions to disassemble. In the 70's and into the 80's there were a lot of McPherson struts that could be disassembled and the replacement was a complete shock insert.

[fazool]

I haven't looked closely at thie one you refer to but basically there are a couple dynamic events that happen:

The oil itself is incompressible and viscous. The harder/faster you press against the fluid the stronger its reaction forces. The fluid is regulated through precision valves and ports that restrict the flow. So, if you push slowly and gently, the fluid flows easily through the ports but slowly. If you slam or pound the oil it reacts rigidly. The trick of a shock design is to optimize the porting so that it gently restricts the spring bouncing at the extremes but without reacting to rigidly. To get this, you want to use different valved porting systems, some are regulated themselves with springs or discs or geometry so when a gentle push happens (loading a bag in the trunk) it gently flows to stop the car from bouncing and when it hits something hard (like a pothole) the "valves" open and allow more fluid to flow so it doesn't feel like a solid unmoveable bar.

This balancing can sometimes be accomplished by including pressurized pneumatic (gas-assist) which *is* compressible. You don't get the same effect with compressible gas - its not a damper its actually a spring, which bounces, but when you add this additional spring to the system, you literally tune by adjusting the frequency of the response. Now, by porting gas, you can get a damping effect (if you remember your screen door damper on your parents' house). So there is some advantage to gas assist - it adds a tuning and a lower strength damping. The problem is its much harder to seal gas, especially under pressure. And the gas has to be pre-pressurized inside the shock. So, they might have a special assembly process to connect to a port, pump in gas to a specific pressure, then seal the port with welding as you suspected.

While going down memory lane, I'll share - one of my favorite memories as that young new engineer (who happened to be a hot rod motorhead) was going for a ride on GM's high speed test track, then visiting their Mesa proving grounds and seeing all the secret stuff get abuse tested.