complete fork restoration and improvement

[pioppiny_81]

hello everyone
I recently disassembled the fork to completely restore the internal parts on my '83 r100,
unfortunately I noticed that some parts are missing and even the two springs are different
Missing parts are numbers 15 and18

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Item n.15 in particular can be purchased in three different thicknesses... why?

part n.18 instead is no available anymore, do you think it's true? anyone knows the dimensions?
and also: Is it better to unscrew the head of the flute or remove the force-mounted lower part?

and now the second issue:

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I'm considering to change the oring over the slider base,but that kind of plug doesn't want to come off, ahhrgh, have you ever removed that part?

thank you

[barryh]

What the part fiche shows is two versions of the the valve body assembly. The original version with the long valve body #14 was secured in the bottom of the stanchion by a circlip and shim to eliminate any longitudinal movement. At some point the shim was replaced by a sort of spring washer. The 2nd iteration shown on the right incorporated a much shorter valve body #19 with a spring below it, then the spring retainer and circlip. This is the version shown in Mike Fishwick's drawing above.

You say #15 is missing but which version of the valve body do you have. If it's the earlier long valve body then #15 may have been optional. If the valve body fits with no longitudinal play, I wouldn't worry about it, although it is important to eliminate any play otherwise the forks my clonk. If your forks already clonk it might be advisable to replace the whole valve body assembly with the shorter sprung type. I know for certain that #15 was omitted from early R65 forks which are essentially of the same type and I had to fabricate a shim to fit. #18 that is missing is simply a spacer below the top hat but I don't have the dimensions for it.

[PAS]

I have an 81 R100RT and converted the internal valve body parts to the later design (on the right )
Part # 15 are shims used to take any play out of the assembly during installation of the old style valve body. Do you have the parts that are circled in red?

#18 "intermediate ring" was used from 08/80 to 5/81 as a form of preload.

[pioppiny_81]

Thanks a lot guys, your information are very useful for me, I have the first version with the long valve but there is no spacer under the spring, in fact I disassembled the fork precisely for the reason that I clearly heard a noise on the rebound. On this occasion I also decided to mount the Hyperpro springs and I did well because even the springs were of different sizes and types.
Hyperpros are a little longer so I think that spacer n.18 will not be needed
At this point I also ask you if the modification of the valve to the second version is worth the expense or not and if instead I can eliminate the flapping noise simply by shimming under the spring

[barryh]

At this point I also ask you if the modification of the valve to the second version is worth the expense or not and if instead I can eliminate the flapping noise simply by shimming under the spring

Just shimming the valve body improved mine but it's not perfect. I expect PAS will be able to comment on the revised assembly.

Bear in mind that this was a recently introduced new type of fork design for BMW first introduced on the R65 in 78 and then with some changes to type 247 airheads around 80/81. They were chasing a reduction in noise through several modifications and you have to assume they made the changes for a reason i.e the previous change wasn't completely satisfactory:

1. No shim

2. Shim selection at factory assembly which was time consuming

3. A type of spring washer to speed up factory assembly

4. Shorter sprung valve body which was the final mod to my knowledge

[PAS]

I have done every modification and service bulletin to my forks and I still have clunking. I have tried different fork oil grades, different springs plus the improved valve body with no change in noise. I wish I had an answer.
Bike has 178k so I guess that's how its' going to be. I have owned it 20 plus years.

[barryh]

I have done every modification and service bulletin to my forks and I still have clunking. I have tried different fork oil grades, different springs plus the improved valve body with no change in noise. I wish I had an answer.
Bike has 178k so I guess that's how its' going to be. I have owned it 20 plus years.

This perhaps explains why BMW kept coming up with modifications - they weren't working.

When I examined my forks carefully I came to the conclusion that rebound damping was inadequate and that the forks were topping out and clunking on the rebound when traversing sharp edges like a kerb or pot hole. I discovered the lack of rebound damping was due to leakage around the valve washer. The clearance between the damper piston rod and the bore of the valve washer provided an area for the oil to flow that substantially exceeded the area of the proper rebound orifice. I could feel the lack of rebound damping by stroking a fork leg by hand with the spring removed. I made my own valve washers with better tolerances. Using the hand stroking test I could now feel the rebound damping and the necessary sudden increase towards the end of the travel as the rebound orifice was closed off. Previously the leakage was so great that the increase over the last inch of travel didn't occur which I think caused the topping out clonk. The forks now work reasonably well with thin oil close to the original BMW spec. If I use thicker oil the forks have too much rebound damping will tend to hydraulically lock so you have to be careful and experiment. I became very practised at stripping the forks to try different valve washers !

[PAS]

barryh, sounds like you got it figured out! Did you make any extra washers?

[barryh]

barryh, sounds like you got it figured out! Did you make any extra washers?

I didn't but I can give you some dimensions:

The original steel valve washer was 1.48mm thick. I increased the thickness to 1.95mm in order to reduce it's travel and made the washer from a hard nylon plastic with a beveled top edge.

The original washer had an internal diameter of 16.3 mm. I reduced this to 16.2mm. This may sound like a very small change but the reduction in clearance around the 16 mm damper rod will have reduced leakage by approx. 50%. I made some washers with even less clearance but they didn't work well so concluded some leakage was necessary to prevent hydraulic shock as the valve closed.

Notes:
When the shorter valve body was introduced they increased the the thickness of the valve washer and made it from plastic instead of steel. They also beveled the top edge. This effectively decreased the travel of the valve washer within the damper valve recess from approx. 1mm to 0.5mm in order to reduce the dead band in the transition from compression to rebound before rebound damping took effect. I effectively copied those changes by making my valve washer approx. 0.5mm thicker while retaining the older long valve body.

[PAS]

I updated to the spring loaded valve body and wasnt told the plastic washer was different, There is only one valve washer listed on MAXBMW'S parts fiche and now it's not chamfered. Thanks for your time.