Drive shaft alignment

[smootz]

I have been taught to assemble drive shafts with the yokes "in line" with each other. There is no way to do that on my 2000 RT. The closest I can get is still one spline off. Neither Haynes or the BMW factory manual address the alignment. BMW certified mechanics don't seem to know either. I know it can cause MAJOR vibration in a fire pump or automobile. Is it a non-issue in the bike?

 

SCOTTIE

[smiller]

I'm not sure how much of an issue it really is but you can get the splines to match perfectly when the two sections are in phase, but in only one position though. Try both possible positions with the yokes in phase, one should line up perfectly.

[smootz]

Trust me - I matched every spline, every way, many times. This drive shaft cannot be "phased" correctly. I just wondered if that is common or if my bike was built by the "Friday afternoon" crew. :-)

 

SCOTTIE

[smiller]

I dunno... it works OK for me...

[T__]

I have been taught to assemble drive shafts with the yokes "in line" with each other. There is no way to do that on my 2000 RT. The closest I can get is still one spline off. Neither Haynes or the BMW factory manual address the alignment. BMW certified mechanics don't seem to know either. I know it can cause MAJOR vibration in a fire pump or automobile. Is it a non-issue in the bike?

 

SCOTTIE

 

Scottie, make sure the rubber isolator in the drive shaft hasn’t spun a little.. There have been cases of that rubber failing & the shaft coming completely uncoupled.. The prop shafts I have worked with have lined back up OK.. If the ends are out of phase then one end can’t evenly speed up as the other ends slows down as the U-joints articulate while spinning.. The only way you would ever want a drive shaft yoke out of phase with the other end is IF the shaft has both a vertical & horizontal offset angle & even then it would be slight..

 

Twisty

[smootz]

Crap! I listened to the dealership mechanic and reassembled it as close as possible. Now I have that nagging worry that it isn't right.

 

The shaft was tight and all the joints were good. Is it possible for the rubber to "slip" without actually destroying the bond? It is definitely 1/2 of a spline off.

 

I hate to tear it down again.

 

SCOTTIE

[smiller]

I wouldn't tear it down again just because the driveshaft yokes aren't perfectly in phase (many skip this step and BMW didn't even bother to specify it in the shop manual.) If you're woried about an impending driveshaft failure because of the apparant slight axial misalignment, that could simply be a result of a glitch during manufacture and not necessarily due to a failing damper. I guess no way to know for sure though.

[T__]

Crap! I listened to the dealership mechanic and reassembled it as close as possible. Now I have that nagging worry that it isn't right.

 

The shaft was tight and all the joints were good. Is it possible for the rubber to "slip" without actually destroying the bond? It is definitely 1/2 of a spline off.

 

I hate to tear it down again.

 

SCOTTIE

 

Scottie, I can’t answer your question on if it can move without it readily showing on the bonded rubber as I have only seen them in complete failure mode..

 

I agree with Seth in that you might as well try it now.. If it is off enough to become a problem the fact that it being off a few degrees won’t in itself cause a failure but it will probably shudder & vibrate when at or near full articulation as the U-joints become more angular & their gain angles won’t couple up..

 

Personally I would call a few (OTHER) BMW dealers & ask them IF they have ever seen one out of phase where yours is & if so you’re good-to-go.. If not then I would worry a little that your rubber isolator has slipped..

 

Twisty

[smiller]

FWIW while driveshaft phasing can be critical in certain apllications I don't think that it is so much in this particular case. As I mentioned, (normnally very anal) BMW doesn't specify that it be done, and in fact I didn't bother the first couple of times I had the swingarm apart. I eventually started to check this just in the interest of 'doing it right', but I can't say that I was ever able to notice a difference either way. Long way of saying that I wouldn't sweat it too much if you were off by a spline.

 

As to the damper being weak, hard to say. My guess would be that if it was bad enough to allow the shaft to rotate at all then it would probably fail completely fairly soon after that... can't imagine it surviving in a compromised condition very long. If it works for a while then you are probably OK. If it does let go the shaft will still be captive so the worst effect will be loss of drive power. That could really suck of course if you were far from home, but my guess is that it will fail soon or it won't fail at all so if it lives for a few thousand miles in additional service then you are probably out of the woods.

[cgdR]

BMW doesn't specify that it be done

Funny thing is that it does specify it for the GS: "CAUTION: The universal joints for universal shaft and rear wheel drive must be installed in the same position." pg. 33.21

[smiller]

Funny thing is that it does specify it for the GS: "CAUTION: The universal joints for universal shaft and rear wheel drive must be installed in the same position." pg. 33.21

Yes, you're right, they do mention it for the GS, I had forgotten that factoid. The reason for the difference could be that the GS swingarm articulates more than the RT so phasing is more critical, or simply that they somehow accidentally omitted the procedure from the RS/RT manual... I don't know which.

[AndyS]

I am sure it only relavant when a shaft articulates through a large arc and the knuckles can try to lock out. On the RT it is very little movement in the scale of things for UV joints.

Andy

[T__]

I am sure it only relavant when a shaft articulates through a large arc and the knuckles can try to lock out. On the RT it is very little movement in the scale of things for UV joints.

Andy

 

 

Andy, it’s always relevant.. If you look at how a prop shaft with U-joint works when spinning The output shaft,, prop shaft,, & rear pinion shaft are supposed to spin at the same speed.. Well if the U-joints are not aligned to be cancelling they don’t spin at the same speed through the entire 360° rotation.. Mismatched angles of as little as 1° can cause vibration,, & shaft harmonics..

 

As an example (not true numbers but close enough to show the problem).. You have a constant spinning trans output shaft spinning at 100 RPM’s,, as long as the front U-joint has NO angle in relation to the trans output shaft the prop shaft is also spinning at a constant 100 RPM’s..

As soon as you introduce an angle to the front U-joint the prop shaft starts spinning at a NON constant RPM.. It will gain & lose RPM based on the phase of the U-joint at any particular point in it’s rotation.. So now you still have a constant spinning trans shaft at 100 RPM but the prop shaft is now spinning at 75 RPM & 125 RPM & incremental RPMs in between those speeds as it goes through a 360° revolution.. Not a big deal but disruptive to the smooth operation of the shaft..

 

To offset that non linier spinning of the prop shaft most vehicles do the inverse on the rear U-joint.. That is phase it & angle it the same at as the front U-joint so when the front U-joint is at the 125 RPM stage the rear U-joint is at the 75 RPM stage & when the front U-joint is at the 75 RPM stage the rear U-joint is that 125 RPM stage.. What that does is keep the pinion shaft spinning at a constant RPM compared to the trans shaft..

 

OK given the above what do you think happens if the U-joints are phased incorrectly so they are BOTH at the 125 or 75 RPM stage at the same time..

 

Now the above is just for reference,, in real life there are other things that also figure in,, like deflection under load,, rear end squat under load, lateral & well as vertical angularity differences,, non linier angle changes front to rear as the suspension moves,, etc..

 

Now using CV (constant velocity) joints that is a whole different ball game.. The constant velocity part is the key to those..

 

Twisty

[smootz]

Twisty is correct. Years ago I went to a Darley fire pump school in Eau Claire, WI. There was a static display with a drive line that we could physically change and observe the results. I argued with the Instructor that 90 deg. should be as good as "in line". He proved me very wrong by placing large striped cardboard wheels around each joint. We could crank the shaft by hand and clearly see one joint speeding and slowing in relation to the other. Even one tooth is dramatic.

 

That is why I am paranoid about my shaft. I will probably buy and install a new one so I can sleep at night.

 

SCOTTIE

[smootz]

Well --- It's coming out. I just had a lengthy discussion with Guy from hendersenprecision.com. He has extensive experience with airhead drive shaft repair. He says the drive has slipped and the joints need to be in phase. He has been looking for a donor shaft for an oilhead prototype. It looks like my bike may provide a cadaver.

 

The scoresheet doesn't look too good. My final drive was shimmed too tight at the factory, the needles in the swingarm pivot cut grooves into the race, and the driveshaft has slipped.

 

Chain drives sure are starting to look good again.

 

SCOTTIE

[smootz]

I may have to eat a little crow. My RT driveshaft is back on the bench. The yoke alignment is not perfect but it is not as bad as I remembered it when I put it together yesterday. I put the front of the shaft in a vice and applied most of my 210 lbs. on a 16" strap wrench. The rubber flexed slightly in an appropriate manner and I saw no "slippage". The yokes are really within a few degrees, not half a tooth as I said earlier. I think Hans just eyeballs these units when they are pressed together at the factory. I am going to put the bike back together and plan another 20K mile summer for it. Wish me luck!

 

SCOTTIE

[SAAB93driver]

The rubber coupling in the driveshaft may have taken a permanent set, I am not familiar with the details of the particular design on the BMW but the company I run makes rubber couplings for rotating machinery and we expect to see permanent set in the rubber - a certain degree is normal. That means after set takes place a perfect alignment of the yokes is not possible.

 

Whether the misalignment of the yokes will cause a noticeable vibration depends on many factors like the speed of the shaft and the angles but the fact there is a rubber damper there that is designed to deflect under torque anyway could indicate that the expected vibration from slightly out of phase yokes is not a major issue on this particular design.

 

When I replaced the FD on my R1150RS I tried to get the yokes lined up perfectly, I had a hard time telling given the poor view of the front joint but I must've gotten it close enough because vibration has not been an issue, and the FD I put in was a lower gear so the shaft is spinning faster than it was prior.