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Thread: How warm should the final drive become

  1. #1

    How warm should the final drive become

    I'm experiencing some light shudder under load after running the bike at interstate speeds for a while. It's not easy to tell if its engine rpm related but it only happens after the bike is thoroughly warm. I'm experimenting with a 4:1 ratio 89rug/100LL blend so it might just be the issue. However, the final drive gets pretty warm to the touch and I just wanted to confirm if that was normal. Thanks!

  2. #2
    Registered User
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    Brake dragging?
    Ron

    91 K75RT ABS

  3. #3
    Spins freely, there is oil. And the temps may be normal. I just donít know.

  4. #4
    '92 R100GS brittrunyon's Avatar
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    I thought my FD was too warm as well.
    '92 GS
    After 30 miles, 75 degrees and no use of rear brake, this is what I found.
    I've been told that's a normal running temp.
    Hope This Helps,
    BR

    DSCN4034.jpg
    1992 R100 GS

    Big Bend Ride video at http://brittrunyon.com/
    More riding videos @ http://vimeo.com/user2721333/videos

  5. #5
    Liaison 20774's Avatar
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    ^^^ Definitely worth checking out.

    I managed to find an old thread on the general subject:

    https://forums.bmwmoa.org/showthread...1-Hot-rear-hub
    Kurt -- Forum Liaison ---> 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!

  6. #6
    Hmm. Iíll go get a cheap ir gun and check. Be back shortly..,

  7. #7
    John D'oh
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    Early on in the 70's I noticed the final drives would get quite warm over a day's ride. I theorized that engine operating temperatures of 180 - 200 dF migrate to the rear drive through the drive shaft from the transmission connection, (the transmission quickly reaching engine temperature), and oil-bath in the shaft housing. The drive housing temperature starts at the ambient air temperature. Braking transfers heat to the drum and housing, more-so with hard frequent usage but, it dissipates more through the surface of the wheel hub. After a day of riding in 90 degree ambient I would expect a final drive to be at or near +/_ 140 dF I seem to recall a BMW service manager confirmed that was the case too.
    John D'oh

  8. #8
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    IMO, I don't think the engine temp transferring back has much to do with it as much as the slight amount of friction from the wheel bearings and gears in the drive plus the brakes. I would expect the drive to get too hot to touch after a long day in warmer temps.

  9. #9
    John D'oh
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    I decided to read up on automotive differentials and the only actual external factors influencing the temperature in a rear wheel drive vehicle differential is ambient temperature, load, speed and proximity of exhaust system. The main contributor to differential or final drive heat is lubrication, friction between gears and bearing loading. The inefficient transfer of power through the gear sets consumes power which translates to heat due to friction. In the case of our airhead BMW's the drive shaft because of its direct connection to the heat source (engine/transmission) is effectively a heat sink and conducts the engine temperature to the final drive in the enclosed housing. Weather it is more or less effective at heat transfer than exhaust proximity is another question.

    On differential failure: The "light shudder under load (after running at interstate speeds for a while)" comment in the first post is mentioned as a symptom of bearing failure in several articles on the subject. Differential failure is commonly due to low or no lubricants, the wrong lubricant and water incursion. High temperature is not noted as a reason for failure, only as a symptom. Diagnosing the shudder is difficult in automotive applications simply because of the location of various components that also create a shudder eg. dirve shaft u-joint, carrier bearing, transmission output bearing, pinion support bearing and ring support bearing. You should be able to isolate any bearing noise using a stethoscope and rotating the rear wheel. You would hear a rumble.

    A 4:1 blend of 89ron to 100ll is a bit hot. Assuming a late 70's model, you probably don't need more than a quart per tank. My 77RS and R90/6 in sidecar application will not run worth a crap on contemporary fuels without a de-tuning OR, a quart of LL per tank.
    John D'oh

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