Results 1 to 9 of 9

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
    Join Date
    Mar 2008
    Belton Tx
    Brake dragging?

    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
    Join Date
    Sep 2009
    Ojo Caliente, New Mexico
    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,

    1992 R100 GS

    Big Bend Ride video at
    More riding videos @

  5. #5
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    ^^^ Definitely worth checking out.

    I managed to find an old thread on the general subject:
    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
    Join Date
    Aug 2006
    Close to Fort Worth Texas
    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
    Registered User
    Join Date
    Aug 2018
    Fargo, ND
    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
    Join Date
    Aug 2006
    Close to Fort Worth Texas
    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

Similar Threads

  1. Replies: 5
    Last Post: 07-05-2017, 09:43 PM
  2. Swap a K75RT for K75S Final Drive& Drive Shaft?
    By 128465 in forum Flying Brick K-bikes
    Replies: 1
    Last Post: 05-19-2012, 05:32 AM
  3. Drive Shaft to Final drive Lube- 99 R1100S
    By roberts735 in forum Oilheads
    Replies: 5
    Last Post: 07-30-2011, 06:18 PM
  4. corrosion on shaft drive and final drive cover
    By usna92swo in forum Oilheads
    Replies: 2
    Last Post: 12-01-2008, 12:01 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts