Forum bmwbike.com

Final drive of my R51/2 is getting warm. Estimated temperature is 55 °C (Can almost keep my hands on it after a long drive). Does someone know whether this is normal?
I use Hypoid 90 gearbox oil.

Regards
Ivo

It is not clear if this is the first time you have felt your final drive after a long ride, or you have felt it before and it has now become much hotter.

The final drive can get quite warm after a long fast ride on a hot day. Hard braking can also contribute heat. Whether your drive is too hot, I cannot say, nor is there much you can do about it. Usually a final drive in distress makes noise and particles, but I don't know if it would make great heat.

I would drain the oil and if it looks OK, refill the drive and don't worry about it. If you want to reduce both the temperature and the wear a little, use synthetic gear oil.

Following on from Allan's comments, ditch the Hypoid 90 oil and use the correct monograde 40 oil. The final drive was never design to churn over in such heavy gear oil, so this might be a contributory factor, especially if there are no signs of wear.

I trust you use monograde 40 in your engine and gearbox too...?

As far as oil is concerned, the standard available 80W-90 gear oil is the same viscosity as 40 weight straight oil. In fact, the 80W designation is actually equivalent to 30 weight straight oil. If you use a 75W gear oil, it is equivalent to 20W crankcase oil. Gear oils and crankcase oils do not use comparable weight designations. The rear unit will feel hot the the touch normally. As Allen says, if you have a problem, it will show up in other ways than excess heat. I use Amsoil synthetic 80W-90 in my tranny , swingarm and final drive. It last longer and lubes better than standard oils.

The rear drive of my R6 rig (the ancestor of yours) gets as warm as you describe, so I assume that is normal. I have used two rear drives, one with solo gears and the other with sidecar gears (recently rebuilt) and they both reach the same approximate temperature.

As others have pointed out, the oil should not make much difference. (I use 40W with a small amount of additive called Militec).

Best regards,

Bruce

As Brain is mentioning, the viscosity of a gearbox 80W-90 gear oil is the same viscosity as 40 weight straight oil.

So based on all the reactions it looks like a temperature increase to about 55 °C is normal.
Thank you all.

Ivo