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Thread: Final Drive

  1. #1
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    Final Drive

    I am a little confused and could use some help:

    I want to buy a spare final drive for my rear drum brake 1978 R100S.
    The ratio of the unit on the bike is a 32:11
    I have a line on a great condition 32:10.
    What will the effect be?

    Will the bike be faster or slower at the same rpm??

  2. #2
    Registered User mneblett's Avatar
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    The 32/11 FD is a 2.91:1 ratio. The 32/10 is a 3.20:1 ratio. That is a 10% difference in gearing

    If you were at 4000 rpm with the 32/11, you'll be at 4400 rpm with the 32/10.
    Mark Neblett
    Fairfax, VA
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  3. #3
    Administrator 20774's Avatar
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    Anton has a nice little chart that shows some of this visually:

    http://www.largiader.com/articles/gearing/
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Unregistered User dduelin's Avatar
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    Acceleration will be a little quicker and gas mileage might be little lower with the 32/10. That is the trade off for running slightly higher rpm per given road speed.
    Dave
    R100
    ST1300

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    And, unless you swap out your speedometer for one with the proper ratio (or get your existing one converted to the new ratio) then your odometer will also be 10% in error (too high) and your speedometer reading will be in error by similar amount.
    BMWs in my garage: 1982 R65LS, 1978 R100/7

  6. #6
    Grammarian no, Rider yes ISAMEMON's Avatar
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    Quote Originally Posted by boxermaf View Post
    And, unless you swap out your speedometer for one with the proper ratio (or get your existing one converted to the new ratio) then your odometer will also be 10% in error (too high) and your speedometer reading will be in error by similar amount.
    does that mean 10% more than the error that it seems airhead speedos already have

  7. #7
    Sir Darby Darryl Cainey's Avatar
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    that could work in your favor, or not.
    Ambassador BMW MOA Ontario Canada
    President Niagara BMW Riders #298
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS (just getting started)

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    Thank you all. I am familiar with the ratio calculation and all. I just wasn't sure which one would result in higher vehicle speed at the same rpm or lower rpm at the same vehicle speed. I am totally amazed about the number of different ratios that BMW offered on the bikes.

  9. #9
    Unregistered User dduelin's Avatar
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    Quote Originally Posted by Isamemon View Post
    does that mean 10% more than the error that it seems airhead speedos already have
    My R100 has the most accurate speedo of any motorcycle I have owned. It is less than 1 mph off at an indicated 60 mph.
    Dave
    R100
    ST1300

  10. #10
    Administrator 20774's Avatar
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    Quote Originally Posted by dduelin View Post
    My R100 has the most accurate speedo of any motorcycle I have owned. It is less than 1 mph off at an indicated 60 mph.
    Is that the '81 R100? Doesn't that come with the 85mph speedo? If so, those were touted as being very accurate.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  11. #11
    Registered User Bob_M's Avatar
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    Quote Originally Posted by 20774 View Post
    Is that the '81 R100? Doesn't that come with the 85mph speedo? If so, those were touted as being very accurate up to 85 MPH .
    There fixed

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    Administrator 20774's Avatar
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    Yup...no disputing that! Thanks, Bob!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    It really must be an exception! There was a study a few years ago about the deviation of speedos and BMW was one of the worst. Ducati and Harley Davidson instruments were the most accurate.
    One of the reasons is, in Germany it is governed by law that a speedo can not be showing less than the actual speed. That's why all manufacturers calibrate to the safe side.
    An officially verified and calibrated speedo by a qualified calibration service will have a max of 4% deviation. +4% that is.

  14. #14
    Administrator 20774's Avatar
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    BMW released a bulletin showing that the maximum permissable speedo error should be 10% of the actual speed +2.4 mph. The reasons given were to compensate for the negative tolerances in tires sizes between manufacturers, electric speedometer indications, and other factors. In the end, as you say, the displayed speed must never be less than the actual speed.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  15. #15
    Registered User stanley83's Avatar
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    Quote Originally Posted by EMSimon View Post
    There was a study a few years ago about the deviation of speedos ....
    One of the reasons is, in Germany it is governed by law that a speedo can not be showing less than the actual speed. That's why all manufacturers calibrate to the safe side.
    I wonder if mine is a real anomaly? I recently purchased a GPS and discovered my speedo read quite low, 55 indicated is 61 on the GPS and it gets worse as you go faster. Can these be re-calibrated?
    Justin in Somerville, MA
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