Another fork question!

[stubmw]

Hi all. I recently have had a clunk in the forks on rebound. I drained the oil and it is full of grains of a black rubber like substance. So, i strip the forks down and there is no sign of the buffer item no. 12 in Haynes manual. I found what i think is the part from a bmw parts supplier,  it has the same part no. ending as in the REALOEM parts fiche. This buffer is red (i see in chris Harris' youtube vid his are red also). So tonight i fit it all back together but the buffers virtually  have the consistency  of chewing gum and a very tight fit inside the stantions. Anyway after a few compressions  to make sure it's moving i had to strip one again just to see what had happened .....and the buffer had moulded itself to the perforated washer holes, i just can't  see how it can work. Has anyone else used these or have any advice.
The only other topic i can find relating to this is     ' fork springs etc.' started by jeremy R65 which shows a black buffer.
Hope this isn't  too long winded. Stu

[montmil]

Scroll down to the FAQs section and check this topic:

"Does my damper rod look odd to you?"

May be helpful for you.

[stubmw]

Thanks montmil for that guidance,  mine is the single piece valve housing but the principle is the same with the buffer. I know you like pictures, here are a couple. After assembly i could feel they weren't quite right, so another tear down. As you can see one side faired better than the other, even the "good one has gummed the rings up!
I could just put up with the clunk or try and get a valve washer made up as suggested by Barry ( i think).
What are you all doing??

[Barry]

I could just put up with the clunk or try and get a valve washer made up as suggested by Barry ( i think).
What are you all doing??  

Stu,  

The red rubbers are the correct part but it sounds like they went much too far in trying to make them soft enough to actually work as a bump stop.  There is a much tougher white bump stop available from Motobins which I have fitted. They go too far in the other direction being being too hard to eliminate a topping out clunk but at least they are durable..

If you have read previous posts on fork clunks the real solution is to increase rebound damping particularly over the last 25mm of travel.

Here’s how the forks work.

Compression Damping

During fork compression oil is displaced from the lower fork leg into the stanchion. It passes through the compression damping holes in the bottom of the damper rod and also through the  annular gap between the damper rod and valve body lifting the valve washer up against the valve plate. The degree of compression damping is determined by the size of the compression damping holes and the oil viscosity

Rebound Damping

As the forks extend the damper piston forces oil back into the lower fork leg.  The valve washer drops and seals against the valve body and the oil can now only pass through a small 3mm hole near the top of the piston damper rod. This is the rebound orifice. The area of this hole is smaller than the area of the compression damping holes by a factor of 3 resulting in approx 3 times stiffer rebound damping relative to compression. That figure is typical as rebound damping is always substantially stronger than compression damping. During the last 25mm or so of extension this hole drops below the valve washer so there is nowhere for the oil to go except by leakage past the washer and to a much lesser extent past the piston rings. This is meant to be the rebound hydraulic bump stop and its efficiency will depend on the leakages mentioned. Variability here may explain why some owners have problems with topping out clunk and others do not. In my case the area of leakage past the valve washer was twice as much as through the rebound hole proper so there was very little effective hydraulic bump stop effect. In making my own valve washer I reduce the washer to piston rod clearance to cut the leakage in half and this eliminated the topping out noise.

If you have the one piece valve body then the valve washer may be a steel one. BMW later introduced a thicker chamfered plastic valve washer with the washer to piston rod clearance reduced in an attempt to reduce leakage and therefore improve the rebound hydraulic bump stop.

The one essential test I strongly recommend is to remove the wheel, front mud guard and the springs, then stroke each fork leg individually by hand.  What you learn from doing this how the damping feels.  Compression damping should be almost non existent and rebound damping much stronger but the important bit from a topping out clunk point of view is you feel a significant increase in rebound damping over the last 25mm of travel. If you don't feel that then the leakage issue has to be addresed.

[stubmw]

Barry, thanks alot, i understand completely,  really well explained.
 I do have the metal valve washer with the one piece body, do you know if i order that part no. if the plastic washer is automatically supplied, or is it from a different fork set up?
 But first the test with the springs out and wheel and mudguard off.
Thanks again. Stu.

[Barry]

The steel and plastic washers have separate part numbers.  Steel washer is 1.45mm thick and plastic washer 3.94mm. The plastic washer used a modified damper valve body with a 4.55mm deep recess and it wouldn't work in the earlier valve body which had a 2.45mm deep recess .  You could replace the whole damper valve assembly or it would be a very simple job on a lathe to modify your existing valve body.  

[nhmaf]

As Barry points out for some reason there are a number of red bumpers that become very, very softy & gooey and thus useless in short order.   There are also some which seem to hold up OK - it may be dependent on fork oil used.    I put the red ones into my R65 a couple years back and they still seem to be doing OK with BMW fork oil in there.

[montmil]

...  I put the red ones into my R65 a couple years back and they still seem to be doing OK with BMW fork oil in there.

Golden Spectro, relabeled as BMW fork oil. Not sure what fork oil is not the official BMW stuff.