complete fork restoration and improvement

[pioppiny_81]

Wow, This forum is packed of knowledgeable people.
Are you talking about modifying part 12 or 13? I guess item n.12? If so, external diameter is the same as the OEM one?

what can you tell me about the bottom plug of fork sleeve? Have I to change the oring after 40 years or can I keep the same? I didn't find anything about on the workshop manual..

Thant you again for your support!

[barryh]

Are you talking about modifying part 12 or 13? I guess item n.12? If so, external diameter is the same as the OEM one?

what can you tell me about the bottom plug of fork sleeve? Have I to change the oring after 40 years or can I keep the same? I didn't find anything about on the workshop manual..

Thant you again for your support!

I'm talking about #13 which is the valve washer. ~ #12 is the perforated valve plate. There is nothing particularly clever about modifying the valve washer. It's just a simple washer that moves up and down within the recess at the top of the valve body. The OD of the washer was kept the same as oil has to flow around it when the valve opens.

The key to all this is to understand how damper rod forks function. There are much better articles with diagrams on the internet that will explain better than I can here but if you read this together with Mike Fishwicks excellent drawing above it should give you an idea.

Sorry can't help with the bottom bit. If it's not leaking I'd leave it be.



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 rebound damping was insufficient and there was very little effective hydraulic bump stop. In making my own valve washer I reduce the washer to piston rod clearance to cut the leakage in half and this all but eliminated the topping out noise.

[pioppiny_81]

thanks for all this info Barry, I understand better how it works now.
Now I just have to make the washers and close everything,
for sure the fork will be better than before

[Giles B]

Hi Barry,
I have recently joined this forum and found your posts about fork damping really detailed and informative , giving me the incentive to try and sort out( again)the topping out noise on my 1982 80g\s . I have had the bike since 1983 and its now covered 143000 mls. Just need to ask some question if you could please give me advice and thoughts . The bike didn’t have the noise for many years so what could of changed or worn to cause it ? Do you think there is a possibility the plastic valve washer could of worn increasing the internal dia. I cannot accurately measure at the moment as my old vernier calipers are inaccurate .I suppose i could order a new set of the plastic rings from MW for comparison to check. Do you think the ptfe slider rings can affect the damping if they were worn thinner? . I have yet to re assemble them without the main springs to check the damping by stroking them as you suggest to check the current terminal damping close to top out .Do you do this with the smaller spring still on the damping rod .Lastly did you make the reduced size plastic valve washers on a lathe . Thanks .

071D29F1-7F0E-4B92-B755-DFE8F849839D.jpeg

[barryh]

Hi Barry,
I have recently joined this forum and found your posts about fork damping really detailed and informative , giving me the incentive to try and sort out( again)the topping out noise on my 1982 80g\s . I have had the bike since 1983 and its now covered 143000 mls. Just need to ask some question if you could please give me advice and thoughts . The bike didn’t have the noise for many years so what could of changed or worn to cause it ? Do you think there is a possibility the plastic valve washer could of worn increasing the internal dia. I cannot accurately measure at the moment as my old vernier calipers are inaccurate .I suppose i could order a new set of the plastic rings from MW for comparison to check. Do you think the ptfe slider rings can affect the damping if they were worn thinner? . I have yet to re assemble them without the main springs to check the damping by stroking them as you suggest to check the current terminal damping close to top out .Do you do this with the smaller spring still on the damping rod .Lastly did you make the reduced size plastic valve washers on a lathe . Thanks .



071D29F1-7F0E-4B92-B755-DFE8F849839D.jpeg

Although very small changes in valve washer clearance do make a difference, I still doubt that the onset of a topping out noise is due to wear on the valve washer but I suppose it's possible. What matters is the total leakage from all sources so any leakage past the slider rings would be a likely cause not least because when new those slider ringers are much better at sealing than the previous metal piston rings so any change in leakage would be noticeable. I can't recommend the hand stroking of the forks enough. I only removed the main spring because my forks don't have a topping out spring, only what was originally a rubber bush to act as a bump stop. If you want to distinguish the hydraulic topping effect independently of the topping out spring it may have to be removed but you would have to drain the forks and dismantle to do that. It might be worth a try first with the topping out spring in pIace. If you can't feel substantially more rebound damping than compression and can't feel the hydraulic bump stop effect at approx. 1" from full extension then there is too much fluid leakage somewhere.

I was very fortunate in already having some hard nylon washers that were close to the correct size in terms of thickness and OD so they only needed reaming of the ID to fit.

The other possible source of noise is any slack in the valve body but by the time plastic sliders were used I think the sprung valve body had been adopted which should have resolved any concern on that score.


Edit:

Giles,

I've just noticed that you are in Macclesfield which is a bit of a coincidence. I'm in Prestbury so not far off. If you need a 2nd opinion when you get to the hand stroking test, I'd be happy to pop around if I'm free at the time.

[Giles B]

Hi Barry,

Thanks for the offer of a second opinion , I would appreciate that. I had already stripped the forks down and cleaned the internals . Just need to re assemble them without the main springs fitted . Will assemble the right one with the top out spring fitted , and the left fork without it in for a comparison. Might also fit in left fork a thin shim strip behind the ptfe sealing ring to tighten it up , again for comparison.
I have found some new plastic shims which are fitted to the later cylinder head rockers to eliminate the rocker float noise . I think they could be modified into the plastic valve sealing rings as the internal dia is slightly smaller than the originals , have to see.
I will drop you a message in the near future .

Regards , Giles .