Rear dampers worn?

[ndevans]

Evening all. My rear dampers are covered in a thin coating of oil, which since evrywhere else nearby is dry must be from the dampers themselves. They are about 15yrs old, but have probably not done more than 25k miles. Are they shot? Any expert opinions out there please? Phots are under the link below. Cheers, Neil.
http://minor946.fotopic.net/c1198820.html

[rayofleamington]

if they still have oil in them and they seem to work, then I'd just top up and keep using them.
If they develop a big leak and/or loose all their fluid then that would be a good time to change them.

[bmcecosse]

Almost looks as if someone has painted them with old oil to stop them going rusty! As Ray says - top up and use - and as all on here know by now - I use 20W50 in the dampers for that little bit extra damping! Of course - leaking dampers are an MOT FAIL - so wipe them clean before any future MOT. Worry more about the rusty petrol tank!

[ndevans]

Thanks gents. I need to have the rear suspension apart to replace the springs as they are a bit soft. I'll clean the dampers up & put 20W50 in 'em.
In case you were wondering, the exhaust pipe on the floor is the old one which fractured at the flange where it meets the manifold. A strange failure, perhaps the metal was unusually brittle and couldn't stand the heat?

cheers,
Neil

[Kevin]

I'll clean the dampers up & put 20W50 in 'em.

Neil you should really use the recommended oil or jack oil not engine oil.

[bmcecosse]

And wallow and bounce off into the sunset!

[Kevin]

So are you saying that when buying replacement shockers you should remove the recommended manufactures oil and to replace it with engine oil.
Sorry but I beg to differ especially as the correct oil is widely available

[bmcecosse]

I would - but I know you and several others wouldn't Kevin. It's a personal choice really.

[JimK]

I believe the standard damper oil is SAE20. You can mix oils of different weights to get to the damping you require, but if you do this then you should probably stick to one oil manufacturer. I've done this for years on my bikes.

I personally wouldn't put SAE50 oil in the standard dampers, as I don't believe the valves and damper mountings are designed for it. With more recent damper valves such as on my bike the valve has a stack of shims of varying widths and thicknesses, which allow tuning for different amounts of damping at different rates of suspension movement. On my bike, the suspension doesn't move much under low-speed movement such as that resulting from pedalling, but when I hit a big rock the forks are allowed to compress faster.

I don't think the Minor dampers are this complex, and have a single damping rate no matter how fast the suspension is trying to compress (or extend). I'm certain there will be more stress on the damper and mounting during compression and that the suspension will be slower to extend into dips in the road surface.

No doubt BMC will be along in a moment to say "I;ve been doing this for years with no problems", but I would say that unless you are a suspension expert and have in-depth familiarity with the specification of the standard dampers, you should think more than twice before using them so far out of their intended range.

Another point with using 20W50 multigrade: do you want damping that changes as the dampers warm up? Remember that the damper will dissipate some suspension energy as heat, and as the damper warms up the oil will get thinner. I don't think that's a great idea.

You can change the damping by changing the oil, but you can only really improve the quality of the all-round suspension performance by taking advantage of 50 years of development and fitting modern telescopic dmapers.

[minor_hickup]

Another point with using 20W50 multigrade: do you want damping that changes as the dampers warm up? Remember that the damper will dissipate some suspension energy as heat, and as the damper warms up the oil will get thinner. I don't think that's a great idea

Yes but so will SAE 20, more so. 20w50 will get less thin (if you pardon my bad english) as it gets warmer.

[JimK]

Yes but so will SAE 20, more so. 20w50 will get less thin (if you pardon my bad english) as it gets warmer.

Monograde SAE20 will not change viscosity as much as multigrade 20W50 that is designed to change viscosity rating by a factor of 2.5.

The workshop manual says that mineral oil to SAE20 is only an alternative to the more desired damper oil. Damper oil is designed to have consistent performance over a wide range of operating conditions - specifically not changing its viscosity appreciably over the operating temperature range.

I'd never put anything other than purpose-made damper oil in the dampers. Just as I'd never put damper oil in the engine.

[rayofleamington]

which fractured at the flange where it meets the manifold. A strange failure,

Actually that's a fairly common failure. Either caused by bad engine vibrations / heavy use of the clutch / not enough exhaust mounts.

the damper warms up the oil will get thinner. I don't think that's a great idea.

Whilst I would strongly suggest not to use the wrong oil in shocks that were not designed for high loads...
The comment on 20/50 getting thinner when hot is the wrong way round. All oils get thinner when hot - a multigrade gets LESS thin than a monograde. This is why Austin et al dropped the use of monogrades ASAP when multigrade oils became available - they change LESS with temperature.

[JimK]

So the oil goes from 20W cold to 50W hot? I think I can see that. 50W hot is still thinner than 20W cold, but it's not as thin as 20W hot. Right.

When I filled my dashpot with engine oil, somebody on here told me to remember that while the manual says use SAE20 for the dashpot, cold engine oil is 50W. They were wrong, it seems.

[rayofleamington]

So the oil goes from 20W cold to 50W hot? I think I can see that. 50W hot is still thinner than 20W cold, but it's not as thin as 20W hot. Right.

Yes

When I filled my dashpot with engine oil, somebody on here told me to remember that while the manual says use SAE20 for the dashpot, cold engine oil is 50W. They were wrong, it seems.

The carb gets hot when in use, but shouldn't get red hot, so maybe somewhere half in between the 20 and the 50 is what you'd get when using a 20/50 multigrade... approx 35. This is of course thicker than 20, but it does allow extra damping of the throttle body.
If however you used 15/40 oil, in the dashpot, you would probably get very similar results to the 20 grade oil.

Personally I am happy to use 20/50 or 15/40 in the dashpot as all it does is 'cushion the effect of a quick press on the accellerator'.

I find the Minor throttle to be 'far more responsive than modern cars' anyway regardless of dashpot oil grade, where the throttle response is damped by the electronic gizmos to make the driver feel even more remote from the car

[bmcecosse]

No - the 'W' is to designate winter rating for the '20' part. So - 20W50 is actually the same as SAE20 oil when cold - it just doesn't thin out as much as straight 20 oil at temperature. And yes - the dampers convert ALL the suspension into heat - but i have never found the Minor dampers to be any more than slightly warm. I have seen Mini dampers steaming !! But I believe the Minor dampers were originally filled with SAE 10 oil (maybe even SAE 5) and so the 20w50 does give a moderate increase in damping without any hassle. The little valves on the Minor dampers are adjustable - both with a little nut that tightens up a spring - and with shims. But to be really effective the sizes of the little holes in the valves would need to be changed. I have played about with the valves in the past - but the real results come from using higher viscosity oil. I have a book somewhere detailing all this- it was very common in 750 Motor Club racing circles - they used up to EP 140 oil. I have used EP 90 in the dampers - on my Rally Minor - they worked very well indeed and never gave me any problems or breakages. But I appreciate that amount of damping may not be suitable for ordinary motoring - hence my suggestion to use 20W50. It costs nothing and it works.