Rear shock fluid

[JKING]

Having successfully, I hope, repaired the broken brake caliper mounting lug on my S1, I am in the process of reassembling the shock tower/bearing carrier assembly. I am reinstalling the original shock, as it was working satisfactorily prior to disassembly.
My question is:
Does anyone have any advice on what fluid to use in the shock. I figure that since it is a hydraulic device, hydraulic brake fluid should work ok. Or perhaps ATF. Viscosity is important but I have no idea what the viscosity of the original stuff was.
Also, I am assuming that the small plug in the side of the carrier tube is a fluid level hole. I have presently filled the shock to this level with brake fluid and on pumping it by hand it feels like it's doing the job. However, if the car is obviously unbalanced during subsequent road testing I might have to replace the fluid in the other side with the same brake fluid.
Does anyone have any negative comments on using brake fluid?

[ppnelan]

In the UK it is usual to replace the original damper parts with an insert, as these are readily available here. I would have thought it safest to service or replace both rear dampers as a pair, to avoid any 'imbalance'.
But I'm ready (as always!) to be corrected...

Matthew

[rgh0]

If you still have the original rear shocks with the shock components directly running in the tube then they are almost certainly worn out and should be replaced by an insert now after 45 years.

regards
Rohan

[reb53]

In the days when I couldn't get inserts, and used to repair the shocks instead, I used to fill them with a light fork oil bought from a motorbike shop.

The best advice is above.....get some new inserts.
After years of use, Konis are still working fine in my car.

Cheers
Ralph.

[CBUEB1771]

Does anyone have any advice on what fluid to use in the shock. I figure that since it is a hydraulic device, hydraulic brake fluid should work ok. Or perhaps ATF. Viscosity is important but I have no idea what the viscosity of the original stuff was.

I assume that your rear dampers are completely enclosed inserts. The function of the oil between the insert and the strut tube is heat transfer, to take heat away from the insert and conduct it to the strut tube. This oil does not directly influence damping rates (except to prevent the damper from overheating) and its thermal conductivity is more important than viscosity. The workshop manual mentions this oil and I think engine oil is specified.

[types26/36]

[quote="CBUEB1771"][ This oil does not directly influence damping rates quote]


Russ,
I think you have mis-understood the post (or I have) originally the rear struts did not have inserts in them, they had the oil and damping valves enclosed in the strut hence a small plug to top up the oil. I think this is what is being discussed.
My S2 still has the damping inside the tube with no inserts and from memory I topped them up with brake fluid but it a long time ago that I last did it.
As regards being "worn out" also from memory a shock test rated them at about 86 or 87% efficiency.
I will fit inserts one day when I have the time and inclination.

[JKING]

You are correct, Brian. These are original S1 shocks and not inserts. The fluid that's in there is the damping medium not merely for heat transfer. The damping action provided by these shocks was adequate prior to disassembly so I am going to continue using them for now. I appreciate your note that you used brake fluid and so I shall do that also. As I said, a road test should indicate whether I should replace the fluid in the other shock as well to provide balanced damping. As with you, I also shall one day replace the rear shocks with inserts, I have already replaced the fronts with Konis, when "I have time and inclination", and the funds.

[Petter Hval]

John.
Go for the fork oil, it is not so hydroscopic as the brake
fluid, pupose made for dampers and coms in 3 grades.
Start with the light oil and if you need more damping (worn
dampers) go for a havier fork oil.
Regards
Petter

[bill308]

Peter is correct in that most brake fluid is hydroscopic, meaning it has an affinity for or wants to absorb water. I would think oil, of the proper viscosity would be a better damping fluid as it would discourage rust/corrosion rather than promote it.

Bill

[JKING]

Thanks Petter and Bill
I didn't realize that brake fluid is hydroscopic. I'm using Valvoline synthetic. I wonder if it is also hydroscopic. Anyway I think I will take your advice even though I have already assembled the hub/shock assembly into the car. I will once again disassemble it, luckily I haven't yet released the straps that are holding the compressed spring, and dump the brake fluid out and try the medium viscosity fork oil.
Thanks again.

[elj221c]

I don't wish to be pedantic, but.....!
It's hyGroscopic.
Do use propert damper fluid, not brake fluid.
Koni inserts for me. Apart from anything else they have adjustment.

Roy
'65S2

[john.p.clegg]

John

....and don't forget to drop some down the tube before inserting the damper...

John

[hatman]

[quote="JKING
I didn't realize that brake fluid is hydroscopic. I'm using Valvoline synthetic. I wonder if it is also hydroscopic. [/quote]

Dot 5 isn't hygroscopic, Dot 4 is.

[CBUEB1771]

Sorry to continue the hijacking of this thread but brake fluid is hygroscopic by design, at least in the old days. The intent is to keep water in solution in the brake fluid rather than allowing it to pool. If water pools in a braking system it can freeze, resulting in no brakes. The trade off for freeze protection is regular replacement of the brake fluid to minimize corrosion. Modern braking systems are much better at preventing water intrusion. This was an early lesson for me in spannering from my father who spent WWII as a aircraft hydraulics mechanic in the US Navy.

[JKING]

Right Roy. Hygroscopic. As a retired mechanical engineer I should have remembered that. But I rarely came across it.
I have wondered: why do I care about water ingress in a closed and sealed system like this or the brake system. Is there water in solution with the fluid as it comes out of the can? I have lived in some mighty cold places like eastern Idaho and have never had nor heard of brake fluid freezing problems as suggested by Russ. Anyway, I am following the collective advice and going to fork fluid on both sides.