Clanking in forks

[Teo]

The bike: 1982 R65LS with a known 21,000 miles on it.  Possibly there is another 10K - 20K from the PO but the speedo was kaput when I bought it.  The rest of the bike (tx and rear end splines looked very low mileage). I have had it for 5 years.

Anyway, the problem: when I drive off the driveway onto the street there is an audible clank in the forks.  When I hit noticeable expansion joints at lower speeds there is a clank.  The clank, not a click, is like loose springs rather than a head bearing.

I have banged and shook everything I can touch - fairing, headlight, intruments and covers, control levers, brake rotors (CBC floaters), calipers, cables, and head bearings - no movement of noise whatsoever - sidestand, centerstand, tank, heat sink isolation from the tank.  The headlight, turn signal, and instrument bracket is in good shape (the turn signal stalks were repaired with extra weld spots a couple of years ago and are solid).

The head bearings are smooth and properly preloaded (placed my finger on the upper triple tree and bearing cover while hitting bumps - no movement or sensation).

I know the interior of the forks are complete as I rebuilt and replaced all the consumables (nylon topout stop donuts) two years ago.  

The only thing I haven't done is drain and replace the fluid to verify that no leaks have occurred (nothing shows on the fork legs under the gaiters).

I have a 1974 R75/6 that I bought new and do all my own work on it and the LS - there is not a part I haven't fixed or screwed up.

Any thoughts or suggestions? What have I missed or is this a normal sound from LS forks? Very frustrating.

Thanks,
Teo

[montmil]

...  The only thing I haven't done is drain and replace the fluid to verify that no leaks have occurred (nothing shows on the fork legs under the gaiters).  Teo

That said, I'd suggest you drain and refill the fork reservoirs. What brand, weight and volume fluid are you using? What is your ATGATT riding weight? ...If you'll share.

BTW, the clunking is not so unusual but can be altered a bit with fork juice experimentation.

Monte

[Bob_Roller]

If by chance, the previous owner had installed progressive springs, there needs to be a spacer, usually schedule 10 (thick wall) pvc tube about an inch or a little longer to take up the reduced spring length .

Only thing that comes to mind right now, but it is New Year's eve and I've had a few homebrew beers !!!!!!!!!!!!!!!!! [smiley=1drink.gif] [smiley=1drink.gif] [smiley=beerchug.gif] [smiley=beerchug.gif] [smiley=beerchug.gif]

[Barry]

Teo

Forks that clunk on sharp edges like a pot hole or riding off the edge of a curb are not uncommon. It's a lack of rebound damping causing the forks to top out at full extension. More correctly I think it's a lack of an efficient hydraulic bump stop at full extension. There is one it's just not efficient enough because of leaks past the damper valve washer. The lack of a topping out spring in our forks doesn't help either.

As has been said fork oil must be at the right level. Rather than using volume measurements it's more accurate to use a dipstick and set the level to 35mm ⁺_- 15mm above the damper piston. I actually use the dip stick from my car with a line scribed at the correct level. Lower than min stock oil level may allow air into the damper valve which will reduce rebound damping and higher than max stock oil level will also make the noise worse as the reduced air space above the oil level will make the forks extend even faster. Thicker oil will increase rebound damping and help reduce or eliminate the noise at the expense of the ride as compression damping will also be increased. You have to experiment with oil grades to reach a compromise.

Unless you are above average weight anything stiffer than stock springs are in my opinion potentially a bad idea as they make the deficiency of rebound damping  even worse as will having excessive pre-load on the springs. If you do have stiffer springs fitted or pre-load spacers then damping should be increased to match by using thicker oil.  

The damper valve was modified over the years to try and resolve the problem with some if not complete success. When you dismantled the forks what did the damper valve washer and valve body look like? I modified my own forks by making a thicker damper valve washer which by reducing it's travel brings rebound damping in a little quicker. Reducing the clearance between the valve washer and damper rod also helps to improve the hydraulic bump stop effect at full extension.

Have you seen the service bulletins on fork noise ?

[Teo]

To all,
Happy new year and thanks for your answers.

The springs looked stock when I had them out.  I don't think a spacer was required because the springs needed to be compressed by the top nut when I reinstalled them.

I am about 220 lbs with ATGATT.  I used 10 wt synthetic fork oil (I forgot the brand) but haven't checked the level as you suggest, Monte.  Thanks for the measurement.  I'll drain and do that tomorrow - about time to do it anyway.  

As to the washer, Barry, I'll have to pull them apart to look at that setup.  Everything looked stock and complete when I rebuilt them after I bought the bike.  I have not seen the service bulletins for the forks.  Do you have a copy?

Thanks again, gents.  I'll let you know what I find.
Teo

[Barry]

This service bulletin specifically excludes R65 forks which I rightly or wrongly assume means the mod was already done to R65's by that time.

Anyway it gives an insight into all the mods done to this type of fork which focussed on 2 issues.

1. Eliminating any play in the axial fit of the valve body in the stanchion first done with shims and later with a sprung valve body. My early forks had .020" play which I eliminated with a shim. This produced a definite reduction in noise.

2. The damper valve washer was originally a thin steel washer 24 x 16.3 x 1.48 that had quite a lot of clearance (approx. 0.12") around the valve rod. This was later replaced with a much thicker beveled plastic washer with tighter tolerances around the valve rod. I can't tell you the dimensions of the production item but I made my own at 24 x 16 x 1.85 which had the central hole no more than .002" bigger than the valve rod diameter. I have a Word document illustration of this if you wish. Again this was an improvement and eliminated my clunking on all but the most severe sharp edged bumps.

[Teo]

Barry,
Great info.  I would like to get your word document if it is convenient for you.  I think I will pull the forks and verify the assembly.  

Many thnaks,
Teo

[Barry]

The word document shows only a section of the damper rod, the valve body, valve plate and valve washer shown in yellow. It does not show the stanchion. I only did it as a record of how much reduction in travel the new valve washer achieved.

I found it essential to thoroughly understand the damping mechanisms in compression and rebound. These diagrams which I think are originally from Racetech are the best I've seen to help visualise what's going on inside the forks. If you ignore the topping out spring which our forks don't have the damping mechanisms are identical in principle. In fact All damper rod forks are very similar in principle.

Rebound damping forces are always much higher than compression. If you were to stroke a fork leg without the spring in you can feel the huge difference in damping between the compression and rebound strokes. Without a check valve compression and rebound damping would be much the same. The increase in damping on rebound is achieved when the check valve or valve washer seals off chamber B and forces the oil to exit via the rebound hole which is very much smaller in area than the compression damping holes.  Now imagine if the topping out spring was not there and consider what happens when the rebound hole falls below the level of the valve washer - the oil has no where to go except via leakage and that's the hydraulic bump stop effect in our forks when the forks top out. Again if you were to stroke the fork leg without a spring you can feel the increase in rebound damping as the fork leg reaches full extension. This is why I was keen to tighten up the tolerances on the valve washer hole as a means of reducing leakage and increasing this effect.

[Teo]

Excellent explanation, Barry!  

Thanks again.  

Teo