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Miles of Smiles
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Discussion Starter #1
Marked the rear U-joint/Shaft assembly prior to disassemble while doing the clutch drill. Looking at them while apart, much better alignment/phase when almost around 180 degrees. Just wondered if the rubber cushion in the drive shaft can "creep" a little radially, or did the original tech not quite get it right? Also look after those needle bearings in the rear drive, expensive little things. DAMHIK
 
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Driveshaft Phasing...

Driveshaft Phasing is not really even mentioned in the BMW shop manual for the K1200LT. My guess would be that they are not "phased" from the factory (unless anyone knows different???).

On the five or so I have disssembled, they were not phased 180 degrees so I never marked them before disassembly. I marked them at 180 degrees ON THE WORKBENCH before reassembly.

It can be a chore getting the driveshaft re-installed and getting the marks aligned at the same time, but with someone helping you can check it with a light. Oh yeah...for me, it seemed to be easier to install the driveshaft with the transmission IN gear (prevents the Transmission output shaft from rotating). Hope this helps.
 

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Miles of Smiles
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Discussion Starter #3
Maybe those BMW Tec's are on the Ball.

Thanks Jack. Having had a closer look at things I have a theory. Assuming the cush drive has some angular take-up ?? under power (when the u-joints would have the most load ) the shaft would rotate a couple of degrees and thereby improve the Phase Angle. Just a thought. Good enough for me to assemble back as original. I mark and measure most stuff prior to pulling it a part just as a check. Oh, likewise with the transmission in gear 1st
 
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OK - for those of us weekend wrenchers, who understand 'phase angle' as being more pertinent to wave relationships or carrier modulation than with driveshafts and U-joints, would one of you all care to explain shaft phasing in a bit more detail?

Sounds like a few degrees can make a difference?!

Thanks!
 

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Miles of Smiles
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Discussion Starter #5
Rough Stab at Explaining

Tate, I'm no expert, but reading up on the subject it has to do with acceleration and velocity at both ends of a shaft connected by universal joints. With Input and Output shafts not in line (but parallel) joined by a drive shaft, the universal joints at each end are "mirrored" i.e. Phased so the differences in angular velocity cancel each other out. It helps to minimise wear on the joints. HTH
 

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I don't know if BMW even phases them at assembly. Strangely, nothing is mentioned in the service manual about it.

Some BMW techs know they should be phased, but many probably don't. Any decent mechanic knows that this is something that should be done, and you can read about it in many places.

Vibration is the result if they are not phased correctly, and it gets worse as the angle between the parts increases. Even phased correctly a standard universal joint coupled shaft will have a slight speed variation between the ends as it goes through a complete turn. That is why Constant Velocity joints were developed, and are on most short drive shafts that move up and down a lot, such as to wheels on independent suspension systems. The LT drive shaft fits this scenario, but CV joints are large, heavy, and expensive.
 
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It is difficult for people to know what we are talking about from description, so here is a picture from the BMW parts system. Strangely, they show it phased correctly there, but no mention of phasing in the service literature.


Note that you line up the "inside" ends to be in line with each other, and the two moveable "outside" ends are also lined up. When the angles on the input end cause the center shaft to "speed up" during a rotation, that is mostly canceled on the other end as the relative angles are similar, slowing down the output joint.

It will never be perfect, as the angles between the transmission output shaft and final drive input shaft are not constant, but it will be better than if the drive shaft is not phased.
 

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Phasing Made Simple....

Here is a simpler picture and explanation of Phasing a Driveshaft. This is for those of us (primarily myself) who do not have an engineering degree. Wow...some of you guys are VERY intelligent!

PHASING MADE SIMPLE --- See picture below:
 

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