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1991 BMW R100RT Fork Oil Capacity...not what you think.

B

banzaibob

New member
Aforementioned bike....for the last 10 years of ownership I've had fork seal leakage. I've replaced the fork seals 3 times now. Every time I have used the OEM BMW seals....the blue ones. ALWAYS checking for damage and unusual wear. It always works just fine and then after about 3 to 6 months or so, the new fork seals always start leaking. I generally use ATF (the equivalent of 7.5w) in all my bikes and do NOT have any problems in the other bikes.

Every source states that the fork oil capacity is 320ccs. Basically the fork oil has all slobbered out and it is not leaking (as bad) anymore. So yesterday I decided to try a little experiment: I drained each fork leg to determine just how much oil was left in each fork. Each fork had about 200ccs in it. So what I did was to compress the forks all the way by ratcheting down the front suspension all the way bottoming it out. I then measured out 320cc to see just how much can actually be poured into each leg when the forks are fully compressed.

The result? Each fork leg would only accept about 250cc of oil at the most.

My question: Where there different forks offered during those years? Is this an anomaly?

My tentative solution: Run 175cc-200cc in each fork leg...what problems can I expect by using the minimum amount of oil possible?
 
My Haynes says that for the R100RT 1987-on it's 320cc. Doesn't look like there were other options for the 1000cc bikes. It was even more for the R100R and R100GS of the era.

I don't see any reason to not try different levels. You'll feel it at some point where the front end won't respond or it responds too much.
 
I also tried using a very thin pick and lifting the lip of the seal and running a light cloth under it to clean the surface. The rubber material of the OEM "blue" seal appears to me (totally subjective) to be a little harder and less flexible than most of the Asian bikes in my garage that I have that I have done fork seal replacement on. Could it be just my bad luck to have gotten different batches of hard, inflexible OEM seals?
 
How is the surface of the forks? I presume you don't have the rubber gaiters over them. Any chance they've been hit by rocks over the years?
 
20774 said:
How is the surface of the forks? I presume you don't have the rubber gaiters over them. Any chance they've been hit by rocks over the years?
Click to expand...

Surface of the fork tubes appear to be normal. They actually visually appear to be in the middle of their service life. No pitting of any kind, no flaking of any plating, no rust and no indication of any damage. Handling-wise, the bike has always been a winner....and still is. Front suspension appears a little softer once 1/3 of the 7.5 wt. ATF has slobbered out but it is still a very rideable machine.
 
This is a picture of the offending forks....it's the only one I could find.
 

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Many years ago when Voni and I were riding our /5 Airheads the Fork oil specification was for Aeroshell something and several other 3 wt oils. And ATF was considered a good substitute for the 3 weight stuff. I do not believe that ATF is anywhere near 7.5 wt fork oil.

I don't know if this diverence is the cause of the leaks, but it can't be helping in my opinion.
 
A wild guess, wilder as I take it you are the original owner -- would an anti dive kit, which used to add extra springs in the bottom of the forks reduce oil capacity? At any rate, the only thing that makes sense is that there is something in there decreasing the interior volume.
 
A heavier oil (as Paul G points out) would slow the flow rate thru the "valve washer", and may be the cause of the oil looking for somewhere else to go. Is it possible the valve washer is clogged by some debris?
 
FYI, my 88 R100 RT has the same forks (to the best of my knowledge) and they always taken 320 cc. As others have said, it's time for some investigation.
 
Fork Oil Quantity

I have been through the proper fork oil quantity game, and I think I can add some useful information. My introduction did not come from "why are my new fork seals leaking AGAIN", but "why are my new fork springs so STIFF".
Scot Marburger, AKA Gunsmoke, has posted an excellent article on replacing fork springs and changing the oil on his 1995 R100RT, http://www.gunsmoke.com/motorcycling/r100rt/forkSprings/index.html. Rather than adding the specified quantity of new fork oil, he measured the height of the oil in the tube before installing the springs. His explanation of why this is a better procedure is the secret trick. 320 cc of oil is probably correct for a dry, freshly cleaned damper assembly, but too much for a just drained damper. Too much oil means not enough air, and higher oil/air pressure in the fork tube. In some cases this can cause oil seal leakage, in my case it caused too high an effective spring rate and and very stiff forks.
Removing the fork tube plugs and springs adds a lot of work to the job of just changing the fork oil, but it only has to be done once if you make some additional measurements after replacing the fork springs and tube caps. Make a dipstick like Scot's to find the oil level as measured through the M10x1 plug hole in the tube caps. This now becomes your BMW Special Tool for changing fork oil.
This process did solve my overly stiff front forks, and I bet it will solve the new fork seal leakage.


Andy
1995 R100RT
BMWMOA# 225424
ABC# 15601
 
Welcome to the forum, Andy! Thanks for the added info and link to Scot's site. I have a number of links to his site in the link in my signature line...although it doesn't look like I have this one specifically...I'll have to correct that. :thumb
 
Leaking seal

On my 1980 RT I had a leak around the outside of the seal were it presses into the fork.I put gasket sealer around the outside and pressed it in ,it seems to be working so far
 
Oil Level

Race Tech does not specify an oil quantity, for any of their fork kits always a level. Might be something to what was just mentioned.
Gator
 
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