Why do struts leak?

[svx_commuter]

Look at that thing. It is a can with a big wide triple lips seal at the top beteen the can and the shaft. How does the fluid get out of there? It can't flow out because the seal is above the oil level. There is no way to buid pressure in the can so it can't force it way out. Still these things leak once in while.

I have my own idea as to why they leak but I want to see if any one else has the same thought.

[lee]

I don't know, but fools jump in so...

First guess, all seals just wear out (KISS principle).

Second more technical guess: A Macpherson strut is both a shock absorber and a means of locating the front wheels, and therefore is subject to side loads as a part of its function. The side loads pull/push on the seal and eventually wear it out.

OK, once the seal is worn: The up and down shock absorber movement is lubricated by the oil in the strut, so some will be carried up as the strut extends, then wiped off as the strut retracts.

Third part that may (or not) play in both of the above. Aren't about all modern struts gas pressurized - would that help force oil out?

Standing by to see how bad my guesses were

[Beav]

You pretty much hit them all. Gas will force fluid out if the leak is substantial enough. I watched a brand new pair of KYBs puke all over the parking lot one day. The driver hit the driiveway too hard and the wheels left the ground. As he as getting out of the car we heard a sloppy hissing noise and then saw the puddles grow.

Don't forget that the shock stem collects dirt during extension and drags it down to the seal during compression. When you consider how much motion and the loading/abuse that they take it's a wonder they last as long as they do.

[svx_commuter]

Well the front struts on the SVX are not gas filled. he rears are but I think the gas leaks out pretty fast.

I think that another way they leak is from vibration. Either road induced or out of balance tire. This makes the oil foam up and increasese the volume in the shock with foam. The foam works it way up to the seal and leaks. I think it is also possible for the fluid to cavitate or vaporize under this rapid motion of the shock. This is what causes the foaming.

[huck369]

To my knowledge all struts are pressurized, and even if they weren't when extended, every time they were compressed (absorbing a bump) they would be under pressure.
Just like the forks on a motorcycle, if you strap them down for hauling, you run the risk of blowing the seals because they aren't designed to sustain the extra pressure of being constantly loaded (or compressed)
My thoughts on the subject.

[CigarJohnny]

But aren't they constantly loaded with the car sitting on them just parked? This mean they will go bad (apart from seals rotting) by just sitting there?

[huck369]

Well yes, they have some pressure on them with the cars weight, but the springs hold most of the static weight, and this pressure is within the design specs of the strut.

If you were to sit a 5 hundred pound weight on the hood, (besides putting a big dent in the hood ) this would keep a constant pressure above the design specs and cause premature seal failure, as I’m sure lowering springs cause the base pressure to be higher, which will cause the seals to start leaking earlier than the stock springs would, not as drastically as the extra weight, but noticeably anyway.

But I’m no expert in the field, just my experience with bikes and blown fork seals as my guide lines.

[svx_commuter]

Well I have taken an SVX strut apart to see how it works. I try to explain what I see.

The shaft attaches to the car body.
At the bottom of the shaft is a piston with a check valve.
The check valve allows fluid to pass on the compression stroke.
The check valve is a series of bellville washers.
The piston on the shaft slides up and down inside a chamber.
It is a long cylinder inside the cylinder that is seen on the outside of the strut. Call it the working chamber.
At the bottom of the working chamber is another check valve.
This check allows the fluid to enter the working chamber only.
So basically when the strut extends it pulls fluid from the bottom of the shock from outside of the working chamber.
When the strut compresses the fluid moves past the piston on the bottom of the shaft.
On the next stroke the fluid on top of the piston on the shaft is pulled up with the shaft. After a few strokes the fluid above the piston over flows the top of the working cylinder and enters the space between the outsie of the working chamber and the inside of the strut outer cylinder that is seen.

The stut will extend very easily as the check at the bottom of the working cylinder allows the fluid to enter quickly. This is so that the wheels stay on the ground when the SVX goes over a bump. When the car comes down it compresses the fluid between the piston on the shaft attached to the car body and the closed check valve at the bottom of the strut. This is where the pressure is generated by the compression of the fluid. However the pressure will drop as the fluid passes thru the piston valve.

[oab_au]

That explanation was so good John, I almost believed it myself.
To pull a strut apart to see how it works is not easy, because once it is apart, it won’t work.
.
The way the strut works is. The piston is on the end of the shaft and the shaft works the piston up and down the ‘working cylinder’. The piston has two valves in it, a check valve and a rebound valve, as does the foot valve at the end of the ‘working cylinder’ it has a check valve and a bump valve.

Remembering that the diameter of the shaft above the piston takes up about 60% of the diameter of the working cylinder. It can be seen that when the piston moves down (bump), not all the oil under the piston can flow through the piston’s check valve section, to fill the expanding space on the top. This forces most of the oil through the bump valve section, of the foot valve into the oil reservoir in the outer strut tube.

When the piston is moving up (rebound) the ‘working cylinder’ it forces the oil trapped between, the ‘big wide triple lipped seal’ and the pistons, rebound valve section. Because it will take more oil to fill the expanding space below the piston than there is on the top, because of the thick shaft. The rest flows in through the foot valves, check valve section.

On bump it is the shaft entering the ‘working cylinder’ that forces the oil out, so pressure is contained between the top seal and the foot valve. On rebound the pressure is between the top seal and the piston valves.

The valves with the spring are the check valves, the rebound and bump valves are made up out of shim washers of different diameters and thickness, altering these changes the stages and amount of damping the shock has. A bit like the leaves in a leaf spring, the diameter controls the progression and the thickness controls the amount of damping.

The whole ‘working cylinder’ is full of oil all the time, the air is in the outer cylinder on top of the oil. Gas shocks may have gas loose in there, the better ones have the gas in plastic bags or separated from the oil by a piston, so it can’t mix and foam.

They leak for all the reasons everyone has said, except for the pressure, that they always have and because the oil is applying full pressure to the top seal.

Harvey.

[Ron Mummert]

Do the little struts that hold the hood (bonnet), & trunk (boot) lids open, work in a similar manner? If so, why do they tend to fail with much less movement over a fairly short time?

Ron.

[svx_commuter]

Originally posted by Ron Mummert
Do the little struts that hold the hood (bonnet), & trunk (boot) lids open, work in a similar manner? If so, why do they tend to fail with much less movement over a fairly short time?

Ron.

No Ron they do not work the same way I haven't taken one them apart.

Yet

[svx_commuter]

Originally posted by oab_au
That explanation was so good John, I almost believed it myself.
To pull a strut apart to see how it works is not easy, because once it is apart, it won’t work.
Harvey.

Yes Harvey when it's apart it does not work the same because it does not have fluid in it. However, when it is empty it will use the air for the working fluid which does allow me to look at it more closely and feel how it works. The air can move thru the check valves easier. With air as the working fluid it is easy to pull out and a lot harder to push in.

Would you agree that a strut can leak some oil and still work? I am not talking about racing here. Just your average everyday driving.

[oab_au]

Originally posted by svx_commuter


Yes Harvey when it's apart it does not work the same because it does not have fluid in it. However, when it is empty it will use the air for the working fluid which does allow me to look at it more closely and feel how it works. The air can move thru the check valves easier. With air as the working fluid it is easy to pull out and a lot harder to push in.

Would you agree that a strut can leak some oil and still work? I am not talking about racing here. Just your average everyday driving.

Yes John while the oil lubes the shaft, it will leak some oil out.
It was not that long ago, (or it may have been), that the struts had a oil filler plug in the side to replentish the oil.

This was very usefull for changing the viscosity of the oil to increase the damping rate. Most, then, had the top of the strut screwed on. You could pull them down to alter the control valving shims to produce the progression and damping that suited the car.

Some places still machine off the top of the tube remove the internals and replace them with a insert unit.

Harvey.

[svx_commuter]

Hi Harvey,

I am trying to get a picture up, maybe tomorrow. I want to show you the guts o fthe SVX shock. I want your comments. I have had a few shocks leak some fluid out. Not much just a little. I also can't find anything wrong with the ones I took out.

What do you thing about drilling a fill hole in the top of the shock? Might save us all some money down the road. Eh?

[oab_au]

I've done that to some before.
You would need to drill and tap the hole in strut so that the oil washes the pieces of drilled metal out. Measure the amount of oil that comes out, so the same amount is replaced. Check the viscosity of the oil, so that it can be replaced with the same or increased, to stiffen the damping.
The strut would have to come out to do it, so I would drill the hole at the bottom of the outer tube, as it makes it easy to pump out the old oil.

Harvey.