Another fork clunk thread

[Tony Smith]

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.

Before you get too deeply into the depths of your fork internals I really would look at what fluid you have in them.

Bluntly, fluid weight has F'all to do with the fluid's viscosity and it is viscosity that does the damping bit. For the minimal amount of money involved by a bottle of Castrol fork 5 and Castrol fork 10 and give your forks a 50~50 mix of both. I will be seriously surprised if most of your clunk problems do not go away.

Having said that Honda fork fluid might be exactly the same, but neither I nor you know that and that makes comparing "apples with apples" impossible.


Why do I bang on about Castrol fluid? Because I went through what BMW, Snowbum et. al. recommended and it was the one i could buy locally and having used it, I know that it works well.

BTW Snowbum has some interesting (and mostly accurate) things to say on the relative viscosity of forkfluids and how to calculate VI from the less than helpful figures published by some manufacturers.

[quixotic]

For the minimal amount of money involved by a bottle of Castrol fork 5 and Castrol fork 10 and give your forks a 50~50 mix of both. I will be seriously surprised if most of your clunk problems do not go away.

Thanks Tony.  Weird thing is though, that although we can buy Castrol products in Canada, all they -- apparently -- sell us here is engine oil and chain lube (just going off their website).

[quixotic]

Well, I ordered the shims from Max BMW.  But they no longer had the 0.1mm shim (I assume that's what "0,1" means).  And Engeland Moto doesn't have them either.  Therefore, I'm guessing that they're either exceedingly rare or non-existent. So I'm stuck with the 0.2mm, 0.3mm and 0.5mm shims that I ordered.  

Given those restrictions, could I make my own out of brass sheets?  I can get it in thicknesses ranging from .001" to .01" (.025mm to 0.25mm).  The question then becomes: although the thickness can be precisely obtained, the other dimensions would be rather rough (ie, the inside and outside diameters of the washer-like objects).  Would these dimensions also need to be precise?  

[Barry]

Given those restrictions, could I make my own out of brass sheets?  I can get it in thicknesses ranging from .001" to .01" (.025mm to 0.25mm).  The question then becomes: although the thickness can be precisely obtained, the other dimensions would be rather rough (ie, the inside and outside diameters of the washer-like objects).  Would these dimensions also need to be precise?

I measured the clearance at 0.020" between the bottom of the damper valve body and the circlip with feeler gauges then made my own out of shim steel but brass would be fine and much easier to work with. As far as precision on inside and outside diameter goes, although it's very fiddly to make such a small section shim don't be tempted to make the inside diameter any smaller than the bore of the damper valve body. It's easy to think the shim only has the clear the damper rod diameter but the the hydraulic bump stop enters the valve body at full compression and you don't want it to catch on the shim.

[quixotic]

While I'm waiting for the shims to arrive, I may as well make sure that I understand what I'll have to do when they arrive.  Would they go in between the valve ring and orifice plate (from Snowbum's diagram) so that the resulting free play between the two would be approximately 0.5mm's?

[Barry]

While I'm waiting for the shims to arrive, I may as well make sure that I understand what I'll have to do when they arrive.  Would they go in between the valve ring and orifice plate (from Snowbum's diagram) so that the resulting free play between the two would be approximately 0.5mm's?

That's the wrong end of the valve body. Which Snowbum diagram as I'm not aware he has ever done one for R65 forks and a diagram of any other forks may well be misleading because they introduced a rubber washer in the position you mention which BTW it didn't really work long term anyway.


I'll go over the whole history of mods again because I'm concerned you might have an unrealistic expectation of what will cure the clunk. I don't have dates for when these changes were made in the early 80's but they are in the correct chronological order.


1.The earliest R65 forks had the valve body retained in the stanchion by a simple circlip with no attempt to remove free play due to manufacturing tolerances. In some but not all instances this allowed the valve body to move up and down enough to create complaints of noisy fork action.

2. BMW introduced the shims that you have ordered so the the factory could shim out the valve body free play on assembly.

3. To speed up production they introduced a seeger circlip with spring tabs that would pre-load the valve body on assembly and dispense with the need for shims.

Note: Eliminating valve body free play addresses only one potential  source of noise in the forks. They went on to do further mods:

4. A thicker bevelled valve washer was introduced together with a deeper recess in the top of the valve body as detailed in the service bulletin.  The aim of this mod was to reduce the valve washer travel by approx. half which provided better control of the damping in transition from compression to rebound. At the same time they reduced the clearance between the washer bore and the damper rod in order to reduce leakage during rebound damping. The effect this had was to increase rebound damping independently of compression damping and in particular to make the hydraulic bump stop effect function on full extension.

This bump stop effect occurs when the rebound damping orifice falls below the valve washer so that the oil has no where to go except to leak past the washer. If the leakage is too great then the bump stop effect will not occur.

With my forks that last two sentences are the absolute key to understanding how to eliminate a topping out clunk. In an earlier post I mentioned how useful it is to stroke the forks manually with the springs out so that you can feel the strength of the rebound damping and most importantly you get to feel the substantial increase over the last 1" of travel. In my experience if you don't feel that increase then there is no chance of eliminating a topping out clunk.

[quixotic]

Sorry, I should have referred to Clymer or Haynes.  

Snowbum terminology: valve ring and orifice plate
Clymer terminology: washer and valve washer
Haynes terminology: valve washer and perforated washer

The Clymer terminology seems to be the most unhelpful, so ignore that.  The orifice plate is the same as the perforated washer.  And the valve ring/washer sits below that.  Do the shims then sit between the two?

I'm not sure what you mean by topping out.  Is that when the front of the bike is totally unloaded and the forks are fully extended?  If so, then that's definitely not the problem.  I tend to hear the click/clunk when running over tiny little imperfections in the pavement.

[Barry]

Haynes makes most sense.

Here's a picture of the valve components minus the shim and circlip.
from left to right:  valve body,  my modified plastic valve washer, the original metal valve washer and the perforated plate.
the shim goes underneath the valve body and then the circlip goes under the shim.

The shim is the same OD as the valve body so In terms of removing any play there are only 3 places it can physically fit:
1. between the circlip and the underneath of the valve body.  
2. between the top of the valve body and the perforated plate - that would restrict oil flow
3. above the perforated plate but that would also cause significant restriction to oil flow

So only 1. makes  any sense at all.  If anyone else is suggesting a different position then they are either talking about a different component or they don't understand how damper rod forks function.

Topping out is the forks reaching fully extension when the damper piston comes into contact with the rubber bush that sits between it and the perforated plate.  If you are sure that the clunk is not due to topping out then maybe shimming the valve body (if it needs shimming) will do the trick but it didn't for me.  

If shimming doesn't help then consider the plastic valve washer. Even if you do not need the additional rebound damping to prevent topping out there are two other reasons to fit it. BMW chose plastic to reduce noise because it would eliminate the constant metal on metal contact and secondly the reduced travel of the thicker valve washer gives better control in the transition between compression and rebound damping.  And come to think of it there is a third reason. In the service bulletin BMW say that they have put a bevelled edge on top of the valve washer to reduces noise. I don't pretend to fully understand why that would help but it must be something to do with the dynamics of oil flow past the washer in that it reduces the masking of the holes in the perforated plate (see my drawing in an earlier post). I duly copied that feature in my valve washer.

[quixotic]

So when I try the different shims between the circlip and the underside of the valve housing, what should I be measuring or looking for?

And I see that Max BMW has the updated plastic valve washers in stock. Would it be worthwhile getting a couple of those?  Or would they be impossible to coax into the old valve housing?  Maybe I could sand off some of the material to make it fit?  (since the critical dimension -- the inner diameter -- should be the same)

[Barry]

So when I try the different shims between the circlip and the underside of the valve housing, what should I be measuring or looking for?
 

I assembled without a shim and inserted feeler gauges between the circlip and valve body to determine what thickness of shim to use.

I see that Max BMW has the updated plastic valve washers in stock. Would it be worthwhile getting a couple of those?Or would they be impossible to coax into the old valve housing?Maybe I could sand off some of the material to make it fit?(since the critical dimension -- the inner diameter -- should be the same)  

It would be impossible to coax in the new washer as it will be thicker than the depth of the recess. You could certainly reduce the thickness of the washer and aim for it to be 0.6mm thinner than the depth of the recess (effectively what I did). Alternatively get someone with a lathe to increase the depth of the recess so that it ends up 0.6mm deeper than the new washer thickness.

[quixotic]

Thanks Barry and Tony for all your help.  I'll update the thread when I get everything assembled and out on the road again.

[quixotic]

Problem solved!  Even though it took me all day and a lot of cussing, I can now go over bumps without hearing the annoying clickity-clack.  

I wound up inserting a 0.5mm shim in one fork and a 0.3mm shim in the other.  And as for that nylon washer, I initially tried sanding it down, but then I saw that I had some spare 1/2" nylon drain plug washers.  I just had to ream them out slightly, since their thicknesses were exactly what I needed (about 1.9mm).  They didn't turn out to be perfectly concentric, but I gambled and put them in there anyway.  Hopefully, they'll be just as robust as the BMW ones.  

[Barry]

Good result.

I bet it will transform your enjoyment of the bike.