U-joints on driveshaft design?

[JamesW]

Maybe I'm just bored (not) but I've long wondered why BMW designed the driveshaft on my bike with a U-joint at each end of the driveshaft.  Seems to me it would have been simple to eliminate the joint at the final drive end by just casting the FD housing so that the FD could be rotated slightly CCW to eliminate the rear U-joint thus simplifying things.  Kind of like the driveshaft on my double swing arm FJR but I think it would have worked with the single swing arm arrangement of the BMW.  I like the FJR arrangement where you just remove 4 bolts that attach the FD to the swing arm and off comes the FD with the complete drive shaft as the male splined end just slips out from the  splined coupler at the transmission input.  I'm also a big believer in double sided swing arms never mind it's harder to remove the rear wheel better that than risk final drive failure in my ever so humble opinion.  German engineering?? Sometimes you gotta wonder, again imo.

[szurszewski]

No comment on the u-joints, but man - have to say, single swing arms LOOK so MUCH better! 

 

I know they mucked up the left side of your poor RS with that giant exhaust, but look how nice the rear wheel looks on the S! (And it looks even better when it’s spinning.)

 

[AviP]

Agree on the German engineering statement. It seems overrated at times. That said, perhaps the second U-joint reduces torsional stress at the FD under full load, at full compression and lots of power. It's hard to say without modeling it. 

[dirtrider]

1 hour ago, JamesW said:

Maybe I'm just bored (not) but I've long wondered why BMW designed the driveshaft on my bike with a U-joint at each end of the driveshaft.  Seems to me it would have been simple to eliminate the joint at the final drive end by just casting the FD housing so that the FD could be rotated slightly CCW to eliminate the rear U-joint thus simplifying things.  Kind of like the driveshaft on my double swing arm FJR but I think it would have worked with the single swing arm arrangement of the BMW.  I like the FJR arrangement where you just remove 4 bolts that attach the FD to the swing arm and off comes the FD with the complete drive shaft as the male splined end just slips out from the  splined coupler at the transmission input.  I'm also a big believer in double sided swing arms never mind it's harder to remove the rear wheel better that than risk final drive failure in my ever so humble opinion.  German engineering?? Sometimes you gotta wonder, again imo.

Afternoon James

 

That can't be done with the torque link system as the rear drive continually changes angle in relation to the swing arm. (not as much as the front but enough to require a U joint)

 

The whole idea of the rear link system with front & rear U joint is to prevent rear jacking or rear squatting on heavy acceleration or dropped throttle deceleration.   

 

Plus,  a single conventional U joint with at a constant RPM input doesn't allow the output RPM to remain constant if the U joint is operating at anything other than 0°. With constant input RPM the output gains & loses RPM twice per revolution if operated at an angle, the larger the operating angle the more the output speed gains & loses as the U joint  spins).

 

Constant velocity joints were developed to reduce the above RPM variations as the shaft spins.  (they are more expensive, take a more operating real-estate, & add length to the joint packaging area) 

 

With 2 U joints, lets say one operating  at 2° & the other operating at 1°, that equals out to only 1° net operating angle. (as long as the U joints are phased correctly) 

 

Or even more dramatic, lets say one U joint is operating at 3° & the other end operating at 3°, that gives a 0° U joint net operating angle (as long as the U joints are phased correctly) 

 

 

 

[JamesW]

Hi D.R.,  Jacking up and down with deceleration and acceleration you mean kind of like the old airheads did?  My FJR with its single u-joint doesn't seem to suffer without a u-joint at the FD end.  Maybe because the distance from the FD to the transmission input coupler is farther than on the BMW.

 

Hi Szurzewski,  That's exactly the reason I recycled that boat anchor of an OEM muffler/cat enclosure in favor of the Dominator muffler without cat enclosure since my bike's ECU no longer sees an O2 sensor.  Trashed the O2 sensor right along with the original muffler.

 

 

[RPG]

Anton has a great write up of the Engineering principles involved. http://largiader.com/paralever/

 

Owning both a R1150RT and a '74 R90s, there is quite a bit of "jacking" on the old bike under hard acceleration but still not objectionable IMHO.

[JamesW]

I agree with that.  I've owned 4 airheads and I never had an issue with "jacking" and in fact I thought it was kind of cool.  Anton's explanation was good reading and it's easy to understand how correct phasing between U-joints is important.  I still prefer the double sided swing arm and the old final drives would last forever.