Front Fluid Quantity and Suspension Response
by Brian Curry and Tom Bowman
Thu, 19 Nov 1998 18:33:15
My latest K75RT, a 92, had Showa forks. This is the first I had Showas. The front of the bike was scary under braking. It dove like crazy. I thought I needed new Progressive Springs.
As part of the prep for running it to the West, I drained and refilled the forks. For some reason, I checked the amount of fluid that came out of the first fork, ~360cc. Odd number it seemed. The Non-Showa RT’s fork took 280cc, and the non-sport suspension was supposed to take 330cc, and the drained amount was more than this. %0
I checked the owners manual. It said 280 and 330cc. Hmmm. I checked the manual version date. It was old… and pre Showa… Time to call the dealer.
Lee Kundrat, of Otto’s BMW Cycles, told me Showa forks were supposed to take 410cc.
OK, I tried that and put 410cc of BMW 7.5W oil in each leg.
Then I rode the bike. Even I, the suspension idiot, could tell the difference. :):) It was a night and day difference. The dive was much reduced!! The front end suspension felt much more composed.
Then I thought about it a bit. The new, proper, larger fluid volume, reduces the air volume above it, compared to the way I got the bike. This means that when the fork compresses, the air PRESSURE increases much faster than when the volume is larger. Increasing air pressure acts like a rising rate spring. Effectively, putting more fluid in the fork increased the spring rate a lot when the fork tried to dive. (Some time back, HD had a brake dive reduction scheme, by isolating an air chamber that connected to above the fork oil. It gave a “higher” spring rate, when you were on the brakes and the fork air volume was cut off from the chamber.)
Having the proper amount of fluid in the forks IMO, eliminated the need for a spring transplant and made the CC jaunt much more pleasurable.
So, if you want more “cushiness” and “plushness” although with more dive, put in less fluid. If you want more “control” and “firmness” and “less dive” put in more fluid. Remember there is a limit on how low or high you can go. If you go too low, you can hear the transition from air to fluid as the fork valving compresses. Shock “absorption” will vary a lot depending if there is air or oil going through the restriction orifices. If you go too high, the fork tube pressure will go REAL HIGH. The suspension may appear to be “solid” or “locked”. You might be able to blow the fork seals or O-rings out. Either are not good things. Remember, when you deviate from the BMW factory values, you are on your own. Travel with care.
BWM specified fork oil quantities/capacities have varied over the years. Here is a table of values:
Fork Oil Quantities/Capacities Model Leg Change Disassemble K75 Showa L 410cc 420cc K75 Showa R 410cc 420cc K75 BMW L 330cc K75 BMW R 330cc K75 Sport L 280cc K75 Sport R 280cc (Has an "S" stamped on the aluminum plug on top of the leg.) *16V* K1100LT L 350cc 400cc K1100LT R 400cc 400cc K1100RS L 350cc 400cc K1100RS R 400cc 400cc K1, K100RS L 380cc 400cc K1, K100RS R 380cc 400cc *8V* K100 L 330cc K100 R 330cc K100 RS/T/LT L 360cc RS/T/LT R 360cc (Sport) L 280cc (Sport) R 280cc (Has an "S" stamped on the aluminum plug on top of the leg.) R1100 Oil Change only if disassembled L 450cc R 450cc R100R L 410cc 420cc R 410cc 420cc R100GS/PD L 410cc 470cc R 440cc 470cc >88-R100RT/RS L 320cc (see note) R 320cc >85-R65/R80 L 300cc (see note) R 300cc >85-R80RT L 320cc (see note) R 320cc R80GS L 220cc R 220cc R80ST L 190cc R 190cc >84-R65/LS L 190cc (see note) R 190cc >81-R100 R100CS/RT/RS L 220cc (see note) R 220cc >81 R60/7, R75/7, R80/7, R100/7, R100S, R100RS, R100RT R60/6, R75/6, R90/6, R90S R50/5, R60/5, R75/5 (see note) L 265cc 280cc R 265cc 280cc R50US, R60US, R69US (1968-1969) L 265cc 280cc R 265cc 280cc PRE 1968 Model Twins 150-170cc Both Legs
There is a Service Instruction 31 026 92 (2547) which is supposed to have additional information. Note: A “>” indicates that the data is correct for models up to that model year from either start of production, or an earlier date when the value was different.
This data is/was correct as of Oct 4, 1993. After that, you are on your own. But it may be some help. Why does the amount vary from the Left to Right leg? Don’t know. BMW says it does, but not why.
K75RTs both coasts
Chester Springs, PA
Tom Bowman added some additional info, and excellent concerns that should be considered if you start changing the fork fluid levels:
From: Tom Bowman <firstname.lastname@example.org>
Date: Thu, 19 Nov 1998 20:45:43 -0500
Brian points out a useful “quick-fix” and nails the reason it worked. But, a word to the wise:
Air-spring rate is temperature-dependent, increasing as temp rises, lowering as it falls. One reason few MFRs use air in non-race applications is that you have to constantly fiddle it to keep the ride correct, and few people have the patience or time. Also, most OEM engineers spec a very soft spring, compression damping that is initially soft but ramps-up quickly to a too-high rate, and a light oil to get that soft highway ride most people want, which is why when you pick up the pace the fork feels harsh and still dives under braking. Adding oil to increase the ending spring rate with a high air pressure doesn’t solve the damping or total spring rate issues, and there’s a better way.
Were I to be tuning on a bike with a dive problem, I’d start by installing a straight-rate spring that has the proper “end” rate, adjust preload for optimum static ride height and initial rate, then tune compression damping for best average bump performance (ideally, with the fork’s integral compression adjusters, but since those don’t exist on BMWs, I’d try different oil weights first, then fiddle the damping orifices). FWIW.