Unknown rattle/clunk near headlight/handlebar

[quixotic]

When I go over bumps, I can hear a slight clunk.  Vaguely metallic.  Seems to be something that at least weighs a few pounds or that can inflict a few pounds of force on something else.  Seems to be roughly in the direction of the throttle, but who knows?  I can hear it with my helmet on up until about 30 mph/ 50 kph.  I've spent quite a bit of time hitting stuff with a rubber mallet, while probing with a stethoscope, but it's still a mystery wrapped in an enigma.  Nothing hiding in the headlight.  

Any suggestions?

Thanks in advance.

[Tony Smith]

Forks topping out on rebound will sound just like that. If so you have 2 options - go heavier on your fork fluid or/and replace top-out rubbers that have turned to mush in the bottom of your fork sliders.
 
For what it is worth  MIL-H-5606E is the correct hydraulic fluid to use in BMW forks - the R65 bible says 190mm on a refill and 195 on a dry fill - I use 200mm wet or dry.
 
The actual oil I use is an Aero Shell fluid - I went to the local oil supplier and they had a 20lt drum of it that was past its storage date - they gave it to me.

The correct oil does make a huge difference, you will find that some fork fluids (notably Bel-Ray in my experience) attack the rubber bumpers in BMW forks and rapidly destroy them.

[Barry]

If the heavier oil doesn't eliminate the topping out clunk then the only option is to use some other way to increase rebound damping ideally independently of compression damping. One method is to reduce the clearance between the valve washer and damper rod.  

If you get to that stage I have some dimensions for making a new washer.

[Bob_Roller]

Have you done anything to the forks recently ??

Like replace springs

[quixotic]

Wow!  Thanks folks!  And thanks bmwr65!  It seems every time I get in a pinch and I'm frustrated enough to sell the old beast, this forum pulls through with vital info that I never would've thought of.  Makes me wonder how I ever survived before the internet.

Anyways, I'll have a look-see at those bump stops and report back.  Maybe before that, I'll replace the fork oil and see if it makes a difference.

No Bob, I haven't replaced springs or done anything to the forks since I bought the bike last year.  And I don't recall the clank when I bought it, so I think it must've snuck up on me and gradually became noticeable.  

[quixotic]

I ordered the "buffer" (as listed in Haynes).  Also listed as the "stop ring" (Clymer).  Is that the magic item that I need?

[Bob_Roller]

Here's a page from RealOEM.com, an online parts manual .

Item # 24 is what you need .

http://www.realoem.com/bmw/showparts.do?model=0363&mospid=51897&btnr=31_0297&hg=31&fg=42

I've used nothing but BMW fork oil and it dissolved the rubber stop as well, on my '81 R65, just a poor material was chosen for this part .

[Tony Smith]

Bob

I am desperately disappointed to hear that as ever since I shifted to my free can of Aero Shell hydraulic oil I have not had rubber bits "eaten".

Mind you using a rubber bumper as a stopper is poor engineering, they could have just as easily designed a system to progressively increase damping rate as the fork reached the limit of its travel, reaching full hydraulic lock just before mechanical end of travel.

BMW are a funny mob in my opinion, sometimes they do things in an amazingly complex way to achieve very marginal increases in reliability and on other occasions they take quick-fix solutions that condemn the owner to feed the machine "consumables". A good example of complexity is the throttle arrangements, and a good example of "quick-fix" is the stamped steel top bridge they used right up to the R65 when they finally designed and fitted a proper top triple clamp.
 
The general R65 fork design continued into the early K series and they just fiddled with it, never actually fixing the problems, then threw their corporate hands up and went to the Telelever. Go figure.

[Bob_Roller]

When I got y '84 LS in 2010, I disassembled the forks to check them out, as the bike had sat for some time unridden .

I purchased new stops and the rubber stops / bumpers, were of a different material than the originals, don't know if they will survive any better than the old ones .

They are red .

[quixotic]

Here's a page from RealOEM.com, an online parts manual .

Item # 24 is what you need .

Thanks Bob.  Actually, I found out about realoem just this morning, when I phoned up Shails (from Langley, BC).

I have to put another plug in for them.  They're incredible.  (I've more or less given up on bmw2valve and Anderwerks).  Anyways, some guy answers the phone, and to make sure that I've got precisely the right part, he walks through the realoem site with me to arrive at the #24 part.  Then a couple of other fork consumables that he thought could possibly need changing (eg crush washers).  It took maybe 5 or 10 minutes total.  And if the last time I ordered from them is anything to go by, I should have the items in less than a week.

Oh, and I went to the local bike barn and got Pro Honda 5W fork oil.

[Tony Smith]

Oh, and I went to the local bike barn and got Pro Honda 5W fork oil.

I'll bet you that it is way too light.

[Barry]

It does seem to be on the thin side with 17 cSt @40C and 3.8 cSt @100C
That's very close to Motul 5W I tried and found a little too thin. Better too thin than too thick though and easy to blend in some thicker oil if needed.

[quixotic]

Should I try Pro Honda 10W, then?  (I haven't broken the seal yet, so I can easily return the 5W).  I guess I'd prefer to feel the pavement a bit more versus wallowing.  

[Barry]

The problem with fork oils is that the nominal rating of 5W or 10W doesn't tell you anything very useful. First of all it's the nominal rating at 100 Deg C which is nonsense for normal use as fork oil doesn't get that hot.  40 Deg C is much closer to reality so any comparison between grades and different makes should be made with a knowledge of the viscosity at that temperature. There are several charts available on the net that list viscosity figures at 40 C. What you'll find is there is a wide disparity of viscosities between makes of oil even when the nominal weight is the same.

For example

Honda 5W is 17 cSt @ 40 C
Silkolene PRO RSF 5wt is 26.7 cSt @ 40 C

The best advice I can give is choose a fork oil that is readily available to you in several grades and that you can find the viscosity figures for at 40 C. Then be prepared to experiment by for example mixing 5W with 10W. You can switch makes provided you know the viscosity at 40C.  I found it easier to home in on the correct viscosity by starting too light.

For what it's worth I've found that viscosities in the range 15 - 18 Cst to be too light and anything over 26- 28 cSt to be too heavy. On that basis Honda 10W may be too heavy but it's a personal thing that you need to set a benchmark for through experimentation.

http://transmoto.com.au/comparative-oil-weights-table/

[quixotic]

For what it's worth I've found that viscosities in the range 15 - 18 Cst to be too light and anything over 26- 28 cSt to be too heavy. On that basis Honda 10W may be too heavy but it's a personal thing that you need to set a benchmark for through experimentation.

http://transmoto.com.au/comparative-oil-weights-table/

Hmm.  Thanks for that.  Based on the link and your past experience, I think I'll try mixing the 5w and 10w half and half and see how that goes.