Originally Posted by Voyager
How would you design the bearings for an FD like this? Two large tapered bearings? For many years cars used opposed tapered bearings on the non-driven wheels and these bearings were not all that large yet supported one corner of some pretty heavy cars for many thousands of miles. I suspect the front wheels on the 1970 Plymouth Fury III that I once owned had at least 1,000 lbs on each of them, yet the front wheel bearings never failed in well over 100,000 miles of service. I did repack them every 30K miles or so, but that is less frequent maintenance than the BMW final drive specifies.
I suspect the issue with the BMW FD is the side thrust from the bevel drive. I'm not an ME (EE and CE), but I would imagine that under high torque the bevel drive will place substantial asymmetric axial force on ring gear. Is possibly a reason to favor the ball bearing over a tapered bearing? I don't know if this side force is any worse than the side force on the front wheel of an auto during a sharp turn though...
Good question about how one would design such drive! I was going to ask Niel the same thing.
My experience with final drives had been those for much different functions than these light-duty drives - namely for battle tanks (built them, from M1A1 through to Bradley FVS), and it seem to me that the BMW drive that I saw is nice and simple. It is obvious to me that the problems of the drive failures are quality related, most likely with improper assembly of the drives. If it was design related failure, you would have a much higher failure rates.
The taper roller bearing is to take the axial load for when you ride the bike and lean it over! The load (which comes to the drive axle from the rear wheel) is virtually radial on the main bearing, when the bike is upright. OK, there is a small cantilever from the bearing support point to the rear wheel, but the axial component of this moment will be fairly small. When you take corners and lean the bike over, the weight component on the rear wheel will be translated into axial load and that will increase the further that you lean.
The bevel gear will tend to push against the crown gear, which will have a force component to push the drive shaft away from the taper roller bearing, and that increases the importance of having the correct pre-load on the taper roller bearing!
I have to give the German engineer the benefits of the doubt that they have done a proper job of designing. After all, back when I got my first degree in engineering, they were considered the best mechanical engineers around!
As a matter of fact, back then, part of the pre-requisite to taking a degree in ME is to take a language course in German so as to be able to read some of the better textx in ME at the time. I have to quickly note that it was a long time ago, and in another country.