Another fork clunk thread

[quixotic]

I already took my forks apart last year, so I know it's not the disintegrating bushing culprit.  But I came across the following thread, which notes that a lower fork oil level solved the problem...at least in that poster's case (on a 1980 R80).  I think I'll try it, but was just wondering if anyone else already had tried it, and if so, was it successful?  And if successful, is there a particular oil volume or level which seems to work?  
http://www.advrider.com/forums/showthread.php?t=921191

[Tony Smith]

The R65 fork damper components were changed on a regular basis to try and get rid of "clunking" afaik they never succeeded. I do know someone who fitted R80ST forks and howled with rage when he discovered that they clunked too.

In my experience the best way to reduce clunk is to use the right fluid. It is fluid viscosity that does the damping, not fluid "weight". For example Motul 7.5wt fork fluid will give you less damping than Castrol 5wt, in fact as far as I can determine you may as well use water as Motul 10wt.

For my wife's bike Castrol forkfluid 5 is too light and fork 10 is too heavy - a 50-50 mix is about right. I weigh a bit mor ethan her an dI run straight castrol fork 10. Our forks still clunk, but less often and less annoyingly.

I once did the exercise of workign out what the hell the now obsoleted aviation hydraulic fluid originally specified was - for all intents and purposes it was Dexron I trans/hydraulic fluid, which kind of explains why castrol fork fluid works better than motul. (Hint, shake the bottles)

[Tony Smith]

And one more thing.

Worn-out or missing piston rings on the damper rods will do it too. Remember to take them out of the BOTTOM of the forks and put them back in the same way to avoid damaging them.

[Barry]

This clunk you are experiencing, does it occur when riding off a kerb or across a pot hole or sharp edges. If it does then it's a topping out clunk as the fork reach full extension.

The cause is too much spring pressure or insufficient rebound damping or both. One solution is thicker oil but that increases compression damping as well which will produce a hard ride. You can help a little on the spring pressure side of the equation by running the minimum level of fork oil (20mm) which will reduce the air spring effect.

In that ADVrider thread mention was made of manually stroking the forks without the springs fitted so you get to feel the damping effect. That is a very valuable exercise. You should feel practically nothing on the compression stroke but rebound damping should be quite noticeable and most important of all as far as eliminating the clunk is concerned you should feel a substantial increase in rebound damping over the last inch or so of travel. If you don't feel that increase then the problem lies with leakage of oil past the valve washer and possibly damper piston rings although they would have to be very bad to make a big contribution to the problem.

The solution is a valve washer that has a little less clearance between it and the damper rod which reduces the leakage and allows the rebound damper orifice to do it's job.

[quixotic]

This clunk you are experiencing, does it occur when riding off a kerb or across a pot hole or sharp edges. If it does then it's a topping out clunk as the fork reach full extension.

No.  I get the clunk -- actually more of a clack -- when I go over any little imperfection in the road surface.  Very similar to what the poster in advrider was experiencing, so I thought I'd try his solution.  

I did just take out 10 ml's from each fork to see if there was any change.  Nothing too discernible, so I may try it with another 10 ml's taken out.  

[quixotic]

In my experience the best way to reduce clunk is to use the right fluid. It is fluid viscosity that does the damping, not fluid "weight". For example Motul 7.5wt fork fluid will give you less damping than Castrol 5wt, in fact as far as I can determine you may as well use water as Motul 10wt.

I'm pretty sure I've got 7.5 wt in there.  I see I have a bottle of 5 wt and a bottle of 10 wt (both Honda Pro brand stuff), so I'm pretty sure that I mixed the two last fall, when I had one of the forks apart.

[Barry]

I did just take out 10 ml's from each fork to see if there was any change.Nothing too discernible, so I may try it with another 10 ml's taken out.  

Measure the level with a dipstick lowered until it hits the top of the damper piston. You can go down to 20mm of oil  on the dipstick.


Tony mentioned that the damper valve assembly was modified over the years. The first thing they did was to shim the damper valve body to eliminate any vertical play where it is secured in the stanchion. This tends to eliminate rattling noises on coarse surfaces and small bumps.You will see these shims listed in the online parts fiche  Next they replaced the thin steel valve washer with a much thicker plastic washer and increased the depth of the recess in the damper valve body but not by all of the additional thickness so the net result was to reduce the valve washer travel by half. This reduced the dead band in the transition from compression to rebound damping and tighter tolerances reduced the leakage past the valve washer.  The final mod was to spring load a shortened version of the damper valve body. this was done to cushion the shock loading on the damper valve.  All of these mods were done to reduce noise and although the fact that there were several iterations suggests at best they were not immediately successful, it may still be worth while retro fitting the later damper valve components.

[quixotic]

Tony mentioned that the damper valve assembly was modified over the years. The first thing they did was to shim the damper valve body to eliminate any vertical play where it is secured in the stanchion. This tends to eliminate rattling noises on coarse surfaces and small bumps.You will see these shims listed in the online parts fiche  Next they replaced the thin steel valve washer with a much thicker plastic washer and increased the depth of the recess in the damper valve body but not by all of the additional thickness so the net result was to reduce the valve washer travel by half. This reduced the dead band in the transition from compression to rebound damping and tighter tolerances reduced the leakage past the valve washer.  The final mod was to spring load a shortened version of the damper valve body. this was done to cushion the shock loading on the damper valve.  All of these mods were done to reduce noise and although the fact that there were several iterations suggests at best they were not immediately successful, it may still be worth while retro fitting the later damper valve components.

Thanks Barry.  I'll look into this further, since when I disassembled the fork last year, I didn't recall seeing any parts which weren't totally stock from 1979.

[quixotic]

I had a look on Max BMW.  Looks like there's a valve body that got updated in September, 1980, as well as a variety of shims ranging from 0.1mm to 0.5mm thicknesses.  Am I barking up the right tree so far?

As well, there is an updated gasket ring (or fork seal) that fits in the top of the slider body, but I suspect that that's not what I'm looking for...or am I?

[Barry]

As well, there is an updated gasket ring (or fork seal) that fits in the top of the slider body, but I suspect that that's not what I'm looking for...or am I?  

That's the fork seal and not related to noise problems.  What you will need though unless already replaced is the red topping out bush 31 42 1 237 215.

I had a look on Max BMW.Looks like there's a valve body that got updated in September, 1980, as well as a variety of shims ranging from 0.1mm to 0.5mm thicknesses.Am I barking up the right tree so far?
 

Shimming the valve body is the first step but only if it actually needs shimming.  The valve body is retained in the bottom of the stanchion by a large circlip. You have to check if there is any up and down play in that arrangement. I had 0.020" and fitted a shim to eliminate that.  The cost is negligible except for time and for me it cost nothing because I made the shim. It's worth pointing out though that this is only step one in improving  the damper valve and if you intended to go to the latest version of the sprung valve body then shimming becomes redundant. Part of the reason they introduced the sprung valve body was to eliminate the need for the factory to select shims during the assembly process.

This PDF service bulletin attached below describes what was done and provides the part numbers. The diagram is hopeless unless you already know what you are looking for but you'll see the text describes the thicker valve washer I mentioned earlier and the deeper recess in the valve body to accommodate it. The seeger circlip has spring tabs attached which effectively spring loads the valve body and eliminates the need for shims. They mention a hardened valve plate (the large washer with lots of holes in it) of the same dimensions. I've never understood the need for that to be replaced and have seen no signs of wear on my original plate.

I should point out that I haven't fitted the new parts because they were not readily available in the UK. If only we had a MaxBMW.  I made my own although not to the same dimensions as the upgraded parts because at the time I didn't have those dimensions.  I just adopted the concepts and the changes shown in the drawing below cured the problem.

[quixotic]

Thanks Barry.  Looks like Max BMW no longer has the valve housing, but Engeland Moto does: http://www.engelandmoto.com/bmw-valve-housing-r80-gs-part-31422301885.html

It says it's for an r80, but the Max BmW site seems to indicate that the above unit will work for the r65 and r45's (part # 31 42 2 301 885).  I haven't checked all of the parts yet, but Engeland Moto has the improved circlip also.

Is the improved "damper tube mount" necessary?  It seems to be the only part which is significantly more expensive than the other parts.

[Barry]

That damper valve body is not the correct one. It's a later type that use a coil spring to pre-load it in the stanchion. It could be made to work but you would also need the spring and it's carrier. I can find a diagram to explain that if you need it. It would be very simple to modify the existing damper valve body by machining the recess a little deeper. Somewhere I have the dimensions needed which were kindly provided by another member here. I'll try and find it.


The damper tube mount is the hydraulic bump stop for full compression. Although the diagram doesn't show it clearly the bump stop is tapered so that as it enters bottom of the damper valve towards full compression it gradually produces a hydraulic lock to prevent metal to metal contact and shock loading. In practice it's very difficult to get the forks to full compression so I wouldn't worry too much about it.

The explanation of the comment must be that the deeper recess in the top of the damper valve would allow the top of the bump stop to interfere with the valve washer. I'm a little surprised that it would but It's very simple to check this when you have the forks apart and if it is a concern you could cut a few mm (the minimum amount needed) off the top of the bump stop  - ideally in a lathe but it could also be hacksawed of and trued up with a file.

[quixotic]

Thanks again.  My hunch is that, given the difficulties of fiddling with the valve body, my best option might be to initially try shimming.

In the diagram above, what would be the optimal travel distance?

[Barry]

In my drawing above I have retained the original valve body but shimmed and replaced the original steel valve washer with a plastic nylon washer of my own manufacture that had the different dimensions as shown. The hole in the washer is smaller by 0.1 mm or .004" to reduce leakage and increase rebound damping. The washer is thicker by 0.47 mm to reduce the travel from 0.97 mm to 0.5mm or approx. half.

I was subsequently given accurate dimensions of the the BMW supplied modified washer which was 3.937 mm or 0.155" thick and the recess in the valve body was 4.5466 mm or 0.17 mm deep giving a travel of 0.6096 mm.

So I had a little less travel but close enough. My washer bore size at 16.2 mm is slightly bigger than the BMW modified washer at 16.129 mm but that's also a very small difference.

[quixotic]

I may also start putting 10wt oil in there.  I know it'll be a bit harsher, but my daily driver is a Miata with tires at 40 psi, so I might not notice the difference.