Fork Springs etc.

[Jeremy R65]

Well guys, I have had my 1980 R65 "Otto" for a year now and enjoyed riding him very much. My Velo has not moved out of the garage!  I have the front wheel out to change the tyre as it was ancient (fitting BT45s after much thread reading!) and, while I was at it, decided to service the forks as I have the "clonk" and black bits in the oil.  I have dropped the stanchions out of the yokes and am about to start dismantling them.  I have taken the top circlip out of one and found it surprisingly easy to depress the plug against spring pressure.  The bike is some 36 years old but has only covered 13000 miles, so I would be surprised if the springs are knackered, but can anyone tell me what the original length was please? I wondered about fitting progressive springs, but do not want to make the ride harder.  I weigh about 77Kg kitted up - would they be a good idea or a bad idea?  I have just trawled 82 pages backwards through the site looking for nuggets of wisdom on fork strip/rebuilds.  I found lots of good stuff, but can any one point me to  the definitive threads please.  I did once see reference to a special circlip to take out end float in the damper assembly and cure the "click", rather than using shims, but I am buggered if I can find it now!  All advice gratefully received. Thanks, Jeremy

[wilcom]

I found lots of good stuff, but can any one point me tothe definitive threads please.  

Have you checked out these two YouTube vids disassembly and assembly of the R65 forks

https://www.youtube.com/watch?v=ymse3196ORY

https://www.youtube.com/watch?v=WNHC00THAbw

[Barry]

Spec for the stock spring is 490-502mm.  
Wire diameter 4.25mm
No of active coils 66 of which 14 are closely spaced.
Spring rate 26.9/34.1 lb/in  (my calculations)
Stock spring will have an orange /brown paint mark.


1980 forks will be in the transition period where BMW started to address play in the valve body.  They will either have a solid valve body with shims to eliminate play or the sprung circlip you mention. I don't know if the valve body is the same length when the sprung circlip is used but some how I doubt it would fit. For example I had to shim out 20 thou of free play and I can't see that being enough of a gap to fit the spring circlip. Either way eliminating axial play in the valve body is the easy part.

As for choice of spring, the stock item is stiff enough in my opinion but the test would have been to measure sag before removing the forks as that's the only real way to determine correct spring weight. If it was me I wouldn't dream of using anything stiffer as it's likely to make a topping out clunk worse unless damping is also increased with thicker oil - which is a recipe for a stiff ride.  Changing the springs at a later date if needed is so easy, it's not worth worrying about as part of the rebuild. Even if the spring are shorter than spec I would fit a spacer before stiffer springs.

Clunks are not always due to valve body axial play alone.  Because these forks have no topping out spring (only a rubber bush) they are dependent on sufficient rebound damping to control the forks on full extension.  Here's how the damping mechanism works and how I addressed that problem:

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 of all forks 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 now 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 calculated to be twice as much as through the rebound hole proper so there was very little effective hydraulic bump stop effect. You can easily test for this by stroking each stanchion by hand with oil still in but the spring out. Expect not to feel compression damping at all but rebound will be perceptible and the bump stop effect very obvious. If it's not then the forks will clunk. 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 clunk. This manual stroking of the forks is perhaps the single most important diagnostic test to do when trying to eliminate a clunk.

When the later sprung shorter valve body was introduced a thicker chamfered valve washer was used with less washer to piston rod clearance and therefore an improved rebound hydraulic bump stop. So it's obvious that BMW recognised the problem and addressed it.

[Jeremy R65]

Thanks Joe.  I have now looked at the videos and there is some useful stuff in them....amongst the swearing and irrelevant digressions. Thanks Barry, that is very helpful too.  I will measure the springs and not bother with progressives as I certainly do not want it to be a harder ride!  I will also try the rebound damping test before I dismantle any further.  Jeremy

[Tony Smith]

My 2c worth.

I fitted springs designed for the R80ST, a slightly heavier bike than an R65 that uses very similar forks.

I weigh a fair bit (actual amount classified) but I've gone initially for a 3/4" preload shim which gives just on 1.75" drop. I will make new 1" shims and try those, I am wanting no more than 1" static drop which I do realize exposes my forks to the risk of "topping out" - I'll cross that bridge if I have to down the track.

[Jeremy R65]

I have now dismantled the forks and had a good look around.  To my surprise the rubber buffers were both in good nick and there was not much crud inside.  Assuming they are originals, would it be better to leave them or replace with new ones from Motobins?

The fork springs are 495mm so all is ok there.  The valve housing is retained by a proper circlip with "ears" (which have been called Seeger circlips on this forum) and there is about 2.5mm of end float in the assembly - which must surely account for the rattle!!  I intend to turn up a couple of steel or brass washers to take this play out.  Interestingly, my BMW parts book does not show any shims but the diagram in the Haynes manual does and labels them "shims as required".

Barry - I tried the rebound damping test you mentioned with the spring out and there was not a lot of difference in the last 25mm of travel.  I have measured the clearance between the hole in the valve washer and the central damper rod and it is 10 thou (same as yours was?). You mentioned making new valve washers with closer tolerances.  I am not sure about doing this as the BMW ones are hardened.  What did you use to make new ones and what clearances did you settle for?  I presume that I cannot buy a later replacement item that will fit?  

 Tony - Thanks for that. At 75Kg all up I think I will stick with standard springs and no more preload.

Thanks for the help gentlemen.

[Barry]

I have now dismantled the forks and had a good look around.To my surprise the rubber buffers were both in good nick and there was not much crud inside.Assuming they are originals, would it be better to leave them or replace with new ones from Motobins?  

My original rubber buffers were intact after 30 odd years and had not remotely turned to sludge. Quite the opposite they were rock hard which meant they wouldn't be absorbing much shock so I replaced them.

The fork springs are 495mm so all is ok there.The valve housing is retained by a proper circlip with "ears" (which have been called Seeger circlips on this forum) and there is about 2.5mm of end float in the assembly - which must surely account for the rattle!!I intend to turn up a couple of steel or brass washers to take this play out.Interestingly, my BMW parts book does not show any shims but the diagram in the Haynes manual does and labels them "shims as required".  

2.5 mm end clearance would certainly rattle. My experience was that 0.5mm clearance caused a rattle on rough surfaces. But it was a rattle rather than the topping out clunk that was eliminated by shimming it out.

Barry - I tried the rebound damping test you mentioned with the spring out and there was not a lot of difference in the last 25mm of travel.I have measured the clearance between the hole in the valve washer and the central damper rod and it is 10 thou (same as yours was?). You mentioned making new valve washers with closer tolerances.I am not sure about doing this as the BMW ones are hardened.What did you use to make new ones and what clearances did you settle for?I presume that I cannot buy a later replacement item that will fit?

I'd have to search back on my old posts to be certain what my valve washer bore to damper rod clearance was or at least what it ended up as but 10 thou is 0.25 mm and some notes I have suggest that was the clearance I started with and end up reducing that by .1 mm or 4 thou.  To be honest it was a long process that took years of experimentation. One thing I learnt was not to reduce the clearance too much as it caused the forks to lock up  I made new washers out of nylon because that's what BMW did in the later forks when they introduced the shorter sprung valve body.  I made my own washers a little thicker to reduce the travel within the constraints of the existing recess.  BMW had also done that.

[Jeremy R65]

Thanks Barry, that is very helpful.  I will go and have a play! Jeremy