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Thread: Gear ratios

  1. #1
    Registered User Bob_M's Avatar
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    Gear ratios

    My silver 1984 R100 turns 4000 rpm at 70 mph and 4500 at 80. The black R100RS turns 4200 at 70 and 5000 at 80. Needless to say I like the former better than the latter. The R100 has a 32:11 rear end, and the 1982 RS has powder coat covering the differential. Were there different transmission ratios? or is the gearing difference in the differential? The RS has a new transmission. How big a deal would it be to swap out the differentials?
    Thanx

  2. #2
    On the Road MAGOO's Avatar
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    It's not that hard of a job. Remove the rear wheel, undo the top shock mount, then undo the four nuts that hold the final drive(FD) to the swing arm. Pull back on the FD, it may be "glued" by the gasket if so give it a few raps with a rubber mallet. It's kinda heavy so don't have your fingers under it when you are working it loose.

    Do both then reverse the procedure, you will need 2 new gaskets.

    Doug Mc Gee

  3. #3
    bmwmick
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    Quote Originally Posted by Bob_M
    My silver 1984 R100 turns 4000 rpm at 70 mph and 4500 at 80. The black R100RS turns 4200 at 70 and 5000 at 80. Needless to say I like the former better than the latter. The R100 has a 32:11 rear end, and the 1982 RS has powder coat covering the differential. Were there different transmission ratios? or is the gearing difference in the differential? The RS has a new transmission. How big a deal would it be to swap out the differentials?
    Thanx
    The R100RS could be ordered with either the 2.91 or 3.00:1 Final drive. I suspect you have the 3.00:1

  4. #4
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    BMW made a number of different gearsets. And most of the "old" airhead rear drives will fit all the different models. The basic difference is whether the swingarm/drive is double sided (1970 - 1994 or so) or single sided. The double sided have the rear drive bolting to the end of the swingarm with four studs.

    Different final drive gear ratios could be ordered, depending on where the bike was to be used, speed limits, etc. When the national speed limit was lowered to 55mph, final drives for US models typically had higher ratios.

    The R100, R100RT, and R100s could have a 33/12 (2,75), 32/11 (2,91), 33/11 (3,00) or 34/8 (4,25)

    The R60 had 25/8 (3,13), 27/7 (3,86) or 26/6 (4,33)

    The R75 had 34/8 (4,25) or 37/11 (3,36)

    The R80 had 32/10 (3,2)

    The R90s had 32/11 (2,91) or 33/11 (3,00)

    So, you have a wide choice of final drive ratios, just by obtaining a good used rear end and swapping out. That also gives you the option of disassembling your old drive and sending the gear out for "new" splines.

    Good news/bad news. Since the speedometer is driven off the transmission, changing the final drive ratio makes your speedo read higher or lower. You can order a new speedo with the appropriate ratio, if it drives you nuts.

    When installing a final drive on the old style, tighten the nuts finger tight, then insert the axle and snug the pinch bolt temporarily to line up the drive. Torque down the nuts, then "unpinch" and remove the axle.

    pmdave

  5. #5
    Registered User lkchris's Avatar
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    Quote Originally Posted by Bob_M
    My silver 1984 R100 turns 4000 rpm at 70 mph and 4500 at 80. The black R100RS turns 4200 at 70 and 5000 at 80. Needless to say I like the former better than the latter. The R100 has a 32:11 rear end, and the 1982 RS has powder coat covering the differential. Were there different transmission ratios? or is the gearing difference in the differential? The RS has a new transmission. How big a deal would it be to swap out the differentials?
    Thanx
    You're looking at differences in speedo and/or possibly tach calibration.

    99.9% of all R100 sold in USA came with 2.91 (32/11) rear axle ratio. Only significant difference was '77-'78 R100/7, which had lower horsepower than S and RS of same year.

    From 1979 on, all R100 drivetrains were identical during a given year, and all were 2.91 save for some rumored special orders.

    Regardless of model, all Airhead transmissions were identical in a given year and mostly for all years. Certainly all '81-'84 were identical, including R80.

    BTW, my '84 R100RS reads 80mph at 4000 rpm. So did my '78 R100S. I believe this is closer to 70 actual, but there's no room for GPS on an RS.

    No telling what a PO has done. There are aftermarket 1st and 5th gears. Yes, there was a "close ratio" trans from the factory. They're about as prevalent as 336-degree camshafts, i.e. you won't find one.

    When BMW reduced R100 horsepower when they "returned" in 1988, the rear axle ratio was changed to 3.0.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

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