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Thread: Gearing on R80RT

  1. #1
    Brian
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    Gearing on R80RT

    Has anyone had any experience changing the final drive on an R80RT? I have an '86 that I resurrected from 20 years in some guy's garage. I'm a little surprised how fast it spins at speeds I need to do (80 MPH is 5,000 RPM). In So. Cal. you do this or get run over. Thinking of trying to fit an R100 final drive to bring the revs down. I'm OK with the power so that's not an issue.

  2. #2
    Sir Darby Darryl Cainey's Avatar
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    Changing the rear drive is not hard at all 4 nuts + 1 on the shock and a new gasket.

    You do realize that your speedometer will be off unless you change that as well!

    What ratio do you have now? Small numbers stamped on the rear drive housing like 33/11 = 3:00 ratio.
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  3. #3
    Brian
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    Quote Originally Posted by Darryl Cainey View Post
    Changing the rear drive is not hard at all 4 nuts + 1 on the shock and a new gasket.

    You do realize that your speedometer will be off unless you change that as well!

    What ratio do you have now? Small numbers stamped on the rear drive housing like 33/11 = 3:00 ratio.

    Thanks for the speedometer info - I didn't know that. The current ratio is the stock 37/11.

  4. #4
    Registered User lmo1131's Avatar
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    You do realize that your speedometer will be off unless you change that as well!
    The old story goes that Dusenberg salesmen used to boast that their cars would top 120mph, and they had a box of substitute gears to ensure that they would.

    Small numbers stamped on the rear drive housing like 33/11 = 3:00 ratio.


    BTW, what's wrong with 5,000 rpm?
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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    Registered User jad01's Avatar
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    I replaced the original 37/11 final drive on my '78 R80/7 with a used 34/11 some years ago. It is a relatively modest change, but I really like it compared to the lower gearing of the original. It was just enough to bring the revs down to nearer 4250 rpm during highway travel (~ 65-70mph), without making the bike feel sluggish or geared too tall in town. There used to be a nice technical article on the IBMWR site comparing different FD ratios and their appropriateness for different engines, but I don't see it there any longer. I'm not sure how the outright power of your model compares to mine (or the effect of the full fairing), but I suspect it would be reasonably similar. I did not bother replacing the speedo. I just benchmarked it compared to my GPS at different speeds (for reference- I rarely ride with the GPS in place) and subsequently calculate the approximate odometer error when figuring maintenance intervals and fuel mileage.
    Jim
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  6. #6
    Administrator 20774's Avatar
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    Anton has a gearing comparison chart on his website:

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

    It used to work...click on your bike and gearing then see what other final drives will do for speeds. Doesn't seem to work for me right now...
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  7. #7
    Registered User lkchris's Avatar
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    BMW knows what it's doing and fits the best rear drive ratio.

    So, if you change to an R100 ratio, you should change your cylinders, heads, and carbs to R100 items as well.

    Or, you could just get an R100RT. You might even get better fuel mileage if you do.
    Kent Christensen
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  8. #8
    Beemerphan Radar41's Avatar
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    Cool Rear Drive Change

    Quote Originally Posted by bwing View Post
    Has anyone had any experience changing the final drive on an R80RT? I have an '86 that I resurrected from 20 years in some guy's garage. I'm a little surprised how fast it spins at speeds I need to do (80 MPH is 5,000 RPM). In So. Cal. you do this or get run over. Thinking of trying to fit an R100 final drive to bring the revs down. I'm OK with the power so that's not an issue.
    I just changed the rear drive on my R75/6, from the 37/11, same as you have, to a 32/10.

    This is a common swap and a well tuned, and in good order, 750 cc or 800 cc engine won't know the difference! The 32/10 drops the engine rpm by 213 rpm @ 70 mph. This will of course throw the speedo off a bit but who cares? GPS's are cheap and easy to mount.

    Just one thought.
    Don "Radar" Wreyford
    00 K1200LT, 98 R1100GS AE, 84 R100RS, 76 R75/6 (rebuild complete)
    MOA # 91738, RA #27032 , ABC #7915 - "Insanity is doing the same thing over and over again, but expecting different results" - Albert Einstein

  9. #9
    Brian
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    Quote Originally Posted by Radar41 View Post
    I just changed the rear drive on my R75/6, from the 37/11, same as you have, to a 32/10.

    This is a common swap and a well tuned, and in good order, 750 cc or 800 cc engine won't know the difference! The 32/10 drops the engine rpm by 213 rpm @ 70 mph. This will of course throw the speedo off a bit but who cares? GPS's are cheap and easy to mount.

    Just one thought.

    Thanks for all the input. It's more of a 'I'd like to do it' rather than a 'I have to do it.' I'm just more used to slower spinning R100's. This R80RT just feels tapped out (redline is 7,500 RPM). Also, the gas mileage drops a lot when it's spinning like this.

  10. #10
    Registered User lmo1131's Avatar
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    Well, it's literally a four-nut, half-hour swap, and you can always unload a final drive without loosing your shirt on eBay if it doesn't feel right to you.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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