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Thread: Anyone swapped rear drives on K100RS?

  1. #1
    Registered User beemerguru's Avatar
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    Anyone swapped rear drives on K100RS?

    I noticed the K75S has a slightly lower ratio than the '88 K100RS. I've hopped up the K100RS and now the engine wants to really pull. So now at around 80+ the engine is reving faster than I like. Can I knock off a couple hundred RPM on the top end by going with the lower ratio K75S rear drive?

    Anyone done this before?

    Thanks
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

  2. #2
    TESLAESQUE
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    without changing gear ratios, how can you tune an engine so the revs at a certain speed, change? what did you do to it?

    that said, yes, you can swap final drives on 2valve K bikes. the final drive from an "S" will fit...but if its an '88RS SE, you should find one with an ABS ring, right?

  3. #3
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by beemerguru View Post
    I noticed the K75S has a slightly lower ratio than the '88 K100RS. I've hopped up the K100RS and now the engine wants to really pull. So now at around 80+ the engine is reving faster than I like. Can I knock off a couple hundred RPM on the top end by going with the lower ratio K75S rear drive?

    Anyone done this before?

    Thanks
    If I understand what you want is for your K100 to be running at *lower* RPM at 80 - right? If so, then the K75 read end is the wrong direction to go. It's geared lower and will cause your K100 to running at higher RPM's at the same road speed.
    Greg Feeler
    BMW MOA Foundation Secretary, Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  4. #4
    kayseventyfive
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    I always get confused when people mention lower or higher gears. I never know whether they mean a lower numerical ratio (which is a higher ratio) or a lower ratio (which has more reduction but is a higher numerical number)

    I think I understand gear reduction, but I don't always understand what people are saying.

  5. #5
    Registered User Rinty's Avatar
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    revs

    The K 100 RS really likes high revs. It doesn't seem to really come alive until 4,000 r.p.m., where it also smooths out. I wouldn't worry about it; these are tough engines.
    Rinty

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    I did a quick search and I'm finding rear drive ratio's of 3.20 or 3.09 listed for a K75S. You should currently have a 2.81 on your K100RS. I looked in to this myself a while back for my K11RS, which also has a 2.81 final drive. The only thing better I could find listed was a 2.75 for a 90's k1100LT, however they normally have the 2.81 also so I don't know if it was for Euro bikes or what. Regardless, putting the numbers in to a ratio calculator, the difference between the 2.75 and 2.81 was a wopping 100 rpm @ 60. You would have even more issues since you don't have a paralever bike, somehow you would have to switch the gears out but keep your housing. It seems like a lot of work to me for next to nothing of a gain. I say ride it as is, if you want lower RPM's at speed, buy something with a 6 speed. Hmmm would a 6 speed from a k1200RS fit
    1993 K1100 RS
    Jeff

  7. #7
    Registered User beemerguru's Avatar
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    Quote Originally Posted by rinty View Post
    The K 100 RS really likes high revs. It doesn't seem to really come alive until 4,000 r.p.m., where it also smooths out. I wouldn't worry about it; these are tough engines.
    With my tweaks..it comes on cam around 3500 now and is more linear..and has a more vertical curve. Have plenty of power lower down.

    Since most of my riding is 80+ out here, I just wanted to take the RPMs down a little for that band and expand the range on the top end.
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

  8. #8
    Registered User Rinty's Avatar
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    Have plenty of power down low...beemerguru
    Good idea; I found the K 100 RS quite peaky, as compared to an R 1150.

    Good luck on finding a solution. On the internet, I have seen places that custom make gear sets, but the cost would probably make the project impractical.
    Rinty

  9. #9
    Registered User AntonLargiader's Avatar
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    The parts wouldn't be identical, but you could probably get away with using a gearset from a Paralever bike. Look around at the bottom of this page for some ideas:

    http://www.largiader.com/articles/gearing/

    Might have to machine a few mm off the tip of the pinion.
    Anton Largiader 72724
    Tech articles - YouTube
    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  10. #10
    SweetT
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    Quote Originally Posted by janicho View Post
    I say ride it as is, if you want lower RPM's at speed, buy something with a 6 speed. Hmmm would a 6 speed from a k1200RS fit
    My experience tells me that all the 4 cyl Kbikes have the same block castings so I think it would be possible, except you'd have to find a way to put the old swing arm back on. Maybe that wouldnt' be so hard?

  11. #11
    Registered User beemerguru's Avatar
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    Quote Originally Posted by 58058 View Post
    Greg,
    You've just been away from that green K75 with the Pichler fairing too long, too used to those diesel burning twins.....Just kidding of course, but if you find a solution, come up Mendo way. Ayla and I have a B&B up here now, long time no see.
    Jim Douglas

    That was long ago...the green Pichler turned into a silver blue Sprint front end...and sold a few month's ago to fund this KRS SE restoration.

    Is ANYONE ever going to let me forget about the diesel running G/S...ran fine but killed every bug within a mile of the exhaust pipe ;-) Looked like a minesweeper laying down a smoke screen!

    I've been to Manchester Beach KOA for the Airheads in Sept. and NorCAL in Oct. ...and then for day rides up and over 128.

    I go through Mendicino all the time..especially when Dave owned the Daughtery House. He sold and moved to Canada a few years ago.

    So where's your B&B? Got room for a half dozen bike people

    Greg
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

  12. #12
    Registered User beemerguru's Avatar
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    Quote Originally Posted by SweetT View Post
    My experience tells me that all the 4 cyl Kbikes have the same block castings so I think it would be possible, except you'd have to find a way to put the old swing arm back on. Maybe that wouldnt' be so hard?
    Oh goody...off to see if I can fool the BMW engineers again

    Now to see who has one sitting around on a shelf that I can borrow for a mockup...that's a hint..anyone?
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

  13. #13
    Registered User 58058D's Avatar
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    Greg,
    We are on Albion St by the Mendo Cafe. We have the Sea Gull Inn - I was the only non-Parts or whatever adv. in the latest Anonymous...We have 9 rooms, River City Beemers does a B&B ride in March between Whale Fests. Or see us and our winter specials at http://www.seagullbb.com. Dave's ex works for us part time, he has been by a time or two, I think he is back in the states...

    And I also remember using my '85 KRS to jump that same GS at the beach with two electric vest cords rewired together.....

    So what did you do to the K100 engine to beef it up? I have a Rhinewest cam gear and chip set in my K12RS and love it, but am not sure what all was available for the K100's other than the old Luftmeister Turbos...? I like the idea of the 6 speed tranny with the hydraulic clutch actuation.

    Jim Douglas

  14. #14
    Quote Originally Posted by beemerguru View Post
    I noticed the K75S has a slightly lower ratio than the '88 K100RS. I've hopped up the K100RS and now the engine wants to really pull. So now at around 80+ the engine is reving faster than I like. Can I knock off a couple hundred RPM on the top end by going with the lower ratio K75S rear drive?

    Anyone done this before?

    Thanks
    Wrong way Corrigan. I put a 2.91 to 1 FD from a K100 on my K75 - replacing the 3.2 to 1 standard K75 ration. That reduced engine speed at any RPM by 10%.

    If memory serves, the KRS has a 2.8 to 1 ratio. So putting a K75 FD on a KRS will INCREASE engine RPM by 14% at any speed.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  15. #15
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by beemerguru View Post
    With my tweaks..it comes on cam around 3500 now and is more linear..and has a more vertical curve. Have plenty of power lower down.

    Since most of my riding is 80+ out here, I just wanted to take the RPMs down a little for that band and expand the range on the top end.
    What "tweaks" have you done, Greg? Did you find some Luftmeister cams or a Nigel Patrick head??
    Greg Feeler
    BMW MOA Foundation Secretary, Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

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