Author Topic: 1992 BMW K75S Fork Springs  (Read 208 times)

Offline ExpatAussieBrick

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1992 BMW K75S Fork Springs
« on: September 04, 2019, 02:02:29 PM »
G'Day Brick Riderz.

So I am finding that my K75 has a fair bit of dive on heavy front braking. Also noticing that the front wheel can become quite unsettled by large bumps on the highway. I am thinking it might be time to replace the stock springs and replace the oil in the front forks. As far as I know the previous two owners never did it. I don't know how difficult this is to do? I have never attempted a fork rebuild before. Is it possible to do this using basic tools and keeping the forks on the bike? just jacking them up? I don't have a garage until winter comes so ill be doing this on the sidewalk outside my apartment. Any suggestions for oil weight and for where to buy springs are welcome.

Here are some pics of me and my girl going to the beach on Long Island for Labor Day. She's running like a top otherwise.







  • New York
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Offline Martin

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Re: 1992 BMW K75S Fork Springs
« Reply #1 on: September 04, 2019, 06:03:22 PM »
Try giving it an oil change first I use 7.5 W but you can go up to a 10 W. The action of the front forks can be firmed up by adding  more oil but seals can be blown if too much is added. It is best to add oil in small increments and test on the same road at the same speed between additions. Reducing oil will do the opposite and soften up the action of your forks. I have the same model as you have and when I first got my bike the forks were way to soft so I did an oil change and found the that they had way to little oil in them. There were various forks fitted to the K during their life, and what it was fitted with originally may no longer be true. First establish what forks you actually have fitted before you do anything and measure the amount of oil that is drained.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline mlytle

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Re: 1992 BMW K75S Fork Springs
« Reply #2 on: September 04, 2019, 08:47:38 PM »
change the oil in the forks with 10w honda fork oil.  use the precise amount specified for the forks you have.  K75S?  think it is 410ml.

still not firm enough?  get RaceTech front springs in the correct stiffness for your weight.

changing springs is not hard.  details on this site and in the shop manuals.
  • alexandria, va
  • 92 K75s, 94 K75s, 96 K1100RS SE (custodian), 09 K1300s
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Offline ExpatAussieBrick

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Re: 1992 BMW K75S Fork Springs
« Reply #3 on: September 04, 2019, 08:54:01 PM »
Thanks guys will get some oil and give it a go and post back my results.

bmp
  • New York
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Offline Martin

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Re: 1992 BMW K75S Fork Springs
« Reply #4 on: September 04, 2019, 11:53:35 PM »
The standard forks on a 92 75s should be Showa.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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Re: 1992 BMW K75S Fork Springs
« Reply #5 on: September 05, 2019, 11:11:49 AM »
My forks seem to like the cheapest ATF I can find at AutoZone.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
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Offline natalena

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Re: 1992 BMW K75S Fork Springs
« Reply #6 on: September 05, 2019, 11:21:53 AM »
Thanks guys will get some oil and give it a go and post back my results.
You may already know, but be super delicate with reinstalling the fork bleeder bolt (strips easy). I use a copper washer on it too. Cheers
  • East of Joshua Tree
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Offline Laitch

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Re: 1992 BMW K75S Fork Springs
« Reply #7 on: September 05, 2019, 11:33:59 AM »
You may already know, but be super delicate with reinstalling the fork bleeder bolt (strips easy). I use a copper washer on it too.
+1

Use a little heat and penetrant prior to removal.

  • Along the Ridley in Vermont.
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Offline johnny

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Re: 1992 BMW K75S Fork Springs
« Reply #8 on: September 06, 2019, 08:15:27 AM »
greetings...

if mine and its been too long...

drain hot > fill with cheep atf > ride hundert miles minimum > drain hot > fill with marvel mystery oil > ride hundert miles minimum > drain hot > fill with honda ss8-10 suspension fluid > poast photos of each drain and report handling back here...

https://bmwmotorcycletech.info/kforkoil.htm

j o
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Offline beemuker

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Re: 1992 BMW K75S Fork Springs
« Reply #9 on: September 06, 2019, 08:55:26 AM »
change the oil in the forks with 10w honda fork oil.  use the precise amount specified for the forks you have.  K75S?  think it is 410ml.

still not firm enough?  get RaceTech front springs in the correct stiffness for your weight.

changing springs is not hard.  details on this site and in the shop manuals.
make sure you know which forks ya got and use the link for the correct amount of oil, the table lists the sport forks as 280cc.

found this:

Here is a cut and paste of a great set of information from the inestimable Brian Curry.

Front Fluid Quantity and Suspension Response
By Brian Curry
November 1998
My latest K75RT, a 92, had Showa forks. This is the first I had Showa's. 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, ~360 cc's. Odd number it seemed. The Non-Showa RT's took 280 cc, and the non-sport suspension was supposed to take 330 cc and the drained amount was more than this.
I checked the owners manual. It said 280 and 330 cc's. 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 410 cc's.
OK, I tried that and put 410 cc's of BMW 7.5W oil in each leg.
Then I rode the bike. Even I, the suspension idiot, could tell the difference. SmileSmile 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 CCjaunt 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 be 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 410 cc 420 cc
K75 Showa R 410 cc 420 cc

K75 BMW L 330 cc
K75 BMW R 330 cc

K75 Sport L 280 cc
K75 Sport R 280 cc
(Has an "S"stamped on the aluminium plug on top of the leg.)

*16V*
K1100LT L 350 cc 400 cc
K1100LT R 400 cc 400 cc

K1100RS L 350 cc 400 cc
K1100RS R 400 cc 400 cc

K1, K100RS L 380 cc 400 cc
K1, K100RS R 380 cc 400 cc




There is a Service Instruction 31 026 92 (2547) which is supposed to have additional information.
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.
   
4Back to top Go down   fork oil capacity confusion Empty Re: fork oil capacity confusion on Sat Nov 27, 2010 10:32 pm
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To be sure you do need to determine the brand of your forks, and the production date will only help, but tread carefully if deciding to add up to 410cc of oil to a fork that might be the version that's intended to accept only 330cc. The resulting harshness of ride and chronically leaking seals will be reason enough. I have played with different levels of +/- 20cc in Airhead and K-bike forks and clearly felt the difference.
  • Panama City, FL
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