Originally Posted by bigbear
first off I am NOT a degreed mechanical engineer.
My thoughts on this procedure are that as long as you are measuring the "Hub" movement tha calculations are correct for amount of shims required.
Since the Crown Bearing Outer Race (CB OR) is NOT the stopping point, the Tapered roller bearing is, when the hub assembly is pushed in. This negates or in esence eliminates the CB Axial free play from the equation.
IF the CB OR was hanging up or stopping the hub before the Tapered roller bearing seats then the CB Axial free play "would" be an issue.
Did I say this was going to be fun?
After a good night's rest and a cup of coffee, I'm going to analyze this again.
At first I thought Les was right, but now I think Ray has it.
Having gone round and round visualizing Les' concept, and Roy's comment that "....Crown Bearing Outer Race (CB OR) is NOT the stopping point, the Tapered roller bearing is, when the hub assembly is pushed in...." makes sense to me.
In the technique shown in the video, when the hub is in the "down" position the taper roller bearing is seated. Where the outer race of the crown bearing is with respect to the inner race doesn't matter; this is the "zero" position with the hub firmly pushed or tapped down into the housing. With the cover heated, the outer race will slide down in its seat in the cover and never meet a stopping point. The hub assembly as a whole comes to a stop when the taper bearing is seated.
When the hub is pulled up with leverage (tire irons were used in the video) the crownwheel assembly pushes on the crownbearing inner race in an upward direction, the inner race presses the balls against the outer race, which then has a limit of travel when it hits the bottom of the seat in the final drive cover. When the hub is held in the "up" position the crown bearing is compressed in the same direction that it will be when shims are installed. That is to say, the axial play in the crown bearing is not interfering with the measurement, as the bearing is being loaded with leverage applied by the tire irons in the same direction of force that will be applied by the shims.
Since we are measuring the extent of travel of the hub from its most down position with the taper bearing seated in its race, to the hub's most up position with the crown bearing firmly seated in the cover, and the axial play taken out of the crown bearing with pressures in the same direction that will be applied by the shims, we are measuring the shim space accurately. When the specified preload is added to that space we have the needed shim thickness.
If we were taking our measurement in the down position from the outer race of the crown bearing, Les' observation would be absolutely correct. But since we are measuring the hub, the amount of axial play doesn't matter. The bearing's dimension is only measured when it is loaded in the same direction that the shims will load it.
I think the initial larger measurement shown in the video which changed with subsequent measurements I believe was a result of the dial indicator not being properly zeroed or having been jarred slightly. If I had been more careful in setting up the dial indictor for the first measurement, that difference would not have been seen.
My next project is to make a BMW "special tool" out of a spare final drive cover I have and compare the BMW service manual technique for calculating preload with the technique shown in the video.
Anyone have a final drive they want rebuilt? It will be rebuilt having been measured for preload every possible way I can.
Thanks to Les and everyone for their comments, theories, and insights. My goal is to make the FD rebuild technique as simple and reliable as possible so that anyone considering a DIY job has the best info this collective group can put forth.
I just got my copy of November BMWON and have started the FD article. I did read the FD article that was in MCN (THANKS GRIF!) and it looks very much like the same article, but I will read it again with interest.
PS I had a nice ride on the LT this afternoon. It is getting close to the end of riding season here in the Deep South of Vermont, some of the roads had been salted because of recent snow. I never think about my FD when riding the LT, I only think about final drives when reading and posting here about them.