Factory Service Manual "Cautions"

[Global_Rider]

Two "Cautions" in the BMW Service CD that have me wondering "why", preferably substantiated by a "reason" out of a Machinery Handbook or similar, if possible.

 

Caution: Contact faces of the rear wheel drive and hub must be clean and free of grease.

 

Caution: Do not oil or grease the wheel studs!

 

I can understand the necessity for mating parts to be "clean", but the paranoia for grease, where does that come from?

 

I've been greasing the hubs and wheel bolts/nuts on my cars for over thirty years and on my BMW motorcycles for over fifteen years without the slightest hint of a problem. The last thing I want forming is rust.

 

Are they afraid that the wheel bolts will work themselves loose?

 

And yes, I know about "dry" torques.

[rondob4]

Are they afraid that the wheel bolts will work themselves loose?

 

As the wise old sage Mr. Rogers might have phrased it, "Can you say 'lawyers'? I knew you could..."

[Joe Frickin' Friday]

Caution: Contact faces of the rear wheel drive and hub must be clean and free of grease.

 

You want solid, full-friction contact between the hub and the wheel in order to deal with power transmission. In first gear at WOT, the rear wheel sees about 800 pound-feet of torque); you don't want that torque to be transmitted by putting the bolts under shear load, you want it transmitted directly by friction between hub and wheel. If the surfaces are lubricated, they may allow slight, relative movement between the hub and wheel, despite the preload produced by the bolts. Over the long haul you could conceivably end up with fatigue issues (due to the cyclic loading), or weird stresses at the hub due to side loading of the bolts and the conical holes in the rim (as the rim slips rotationally under heavy acceleration).

 

Caution: Do not oil or grease the wheel studs!

 

Lubrication affects the preload produced by a given torque on the bolts; with lube, the preload can be as much as 30% higher than without lube. There may be problems with overloading the threads in the hub, overstressing the bolts, or cracking the cast aluminum wheel.

 

The last thing I want forming is rust.

 

I don't think you need to worry about rust on your bike, since it's rarely driven on salty roads.

 

And yes, I know about "dry" torques.

 

Whoops, sorry for the unnecessary review.

[Global_Rider]

Caution: Contact faces of the rear wheel drive and hub must be clean and free of grease.

 

You want solid, full-friction contact between the hub and the wheel in order to deal with power transmission. In first gear at WOT, the rear wheel sees about 800 pound-feet of torque); you don't want that torque to be transmitted by putting the bolts under shear load, you want it transmitted directly by friction between hub and wheel. If the surfaces are lubricated, they may allow slight, relative movement between the hub and wheel, despite the preload produced by the bolts.

 

Yes, I've considered that. But since nothing has failed over all the years that I've done that and with my relatively lighter right hand, I won't worry about it. Movement would be so slight that all you'd see is some contact point shine on the mating surfaces, which you'd get in any case.

 

Caution: Do not oil or grease the wheel studs!

 

Lubrication affects the preload produced by a given torque on the bolts; with lube, the preload can be as much as 30% higher than without lube. There may be problems with overloading the threads in the hub, overstressing the bolts, or cracking the cast aluminum wheel.

 

That is why I've always backed off the torque by 10 to 15 lbf.ft. Next time I'll preload the dry wheel bolt by say 15 lbf.ft., mark it with marker and then torque it in two stages to 77 lbf.ft. and note how many degrees I've gone through. Then I'll do the same to 60 lbf.ft. with greased wheel bolts and see how many degrees I've gone through.

 

Thanks for a detailed response.

[Pilgrim]

Or you could just do it like the book says, and realize that rust is really not an issue you need worry about as long as you are riding enough to have to change the tires once in a while.

 

Pilgrim