What's this, where should it connect?

[frankenduck]

+1 Gryph

But I wanna throw something into the mix that I read about monolever bricks way back just before I got one. Because the drive shaft universal joint and the swing arm pivot points are concentric, there's no sliding going on, which is one of the advantages of the compact drive system, and probably still very unusual. Hopefully I've said that right.

It's the circlip that keeps them concentric. Without that the U-joint could slide back and forth and not be concentric at all times.

[The Mighty Gryphon]

Exactly.  The center of the u-joint must be in the same place as the swing arm pivot or much unhappiness will ensue.  That end has to be fixed in position.

The problem is that in the monolever the action of the u-joint results in a very small change in the length of the drive shaft.  That change in length is taken up by the splines sliding back and forth at the final drive.  The motion while very small is constant when the shaft is turning with one cycle for every revolution of the drive shaft.  So it's easy to see that those splines rub back and forth literally millions of times between spline lubes.

With a properly phased Paralever drive shaft, the u-joints push and pull in unison so much of the sliding contact is eliminated.  I suspect that was the real reason for the design change, that is to eliminate the spline wear found in the Monolever design.

BMW, rather than admit the Monolever was a flawed design, chose to sell the Paralever as a solution to acceleration "jacking" which because of weight transfer on acceleration was only visible when a bike was run on a dyno, where rear wheel acceleration did not result in weight transfer. 

If anything, "jacking" actually helps traction when accelerating because it helps stiffen the rear suspension to handle the weight transfer giving better ground contact.

[Ingo]

Understood. The R90 was extreme in the "jacking" and looked real funny under short bursts of gas in first gear! Is this drive shaft work done with the bike assembled? I imagine it might be easier with the gear box installed, before all the other stuff (brake, foot-rest, battery box...) gets installed.
Another q: Anyone with a good idea on carbon crud removal off piston? Experience says "can't use sandpaper" since the crud is harder than the piston itself...

[Ingo]

Anyways, at 44 k miles it's time for the spline service as I understand it. Sure it was never done before, I got the moto with about 28K on the clock.
As far as piston cleaning: I'm soaking it in carb cleaner, crape some off and soak it some more, like overnight, seems to kind of work...
Will post some pics of rod bearings and pistons later. Probably just the worst looking parts.

[frankenduck]

Exactly.  The center of the u-joint must be in the same place as the swing arm pivot or much unhappiness will ensue.  That end has to be fixed in position.

The problem is that in the monolever the action of the u-joint results in a very small change in the length of the drive shaft.  That change in length is taken up by the splines sliding back and forth at the final drive.  The motion while very small is constant when the shaft is turning with one cycle for every revolution of the drive shaft.  So it's easy to see that those splines rub back and forth literally millions of times between spline lubes.

With a properly phased Paralever drive shaft, the u-joints push and pull in unison so much of the sliding contact is eliminated.  I suspect that was the real reason for the design change, that is to eliminate the spline wear found in the Monolever design.

BMW, rather than admit the Monolever was a flawed design, chose to sell the Paralever as a solution to acceleration "jacking" which because of weight transfer on acceleration was only visible when a bike was run on a dyno, where rear wheel acceleration did not result in weight transfer. 

If anything, "jacking" actually helps traction when accelerating because it helps stiffen the rear suspension to handle the weight transfer giving better ground contact.

It's not the splines sliding back and forth that causes the wear on monolever splines, it's sliding UNDER LOAD. That's why only one side of the splines exhibits wear - the side of the splines that is sliding under load when accelerating.

Whether or not monolever spline wear is a "design flaw" is a matter of opinion. Well-maintained (lubricated) monolever drive shaft splines will last over 100,000 miles. Your brake rotors will wear out faster than that. Is that a design flaw? You can similarly look at drive shafts as a wear item - like brake rotors.

Also of interest is that the Z16 splines in the 85 and earlier Ks hardly wear at all compared to the later Z20 splines. I've never figured out why BMW switched over to the Z20 drive shafts. (If you're a cynic you might posit that it is planned obsolescence to sell more parts.)

A downside of the monolever jacking/unjacking effect is that it shifts the back of the bike up and down which changes handling geometry and weight distribution. The paralever does that much less.

[Ingo]

So here's pic of rod bearing and pistond. The rod bearings all look pretty much identical in wear. Was hard to find one worse over others.

[Ingo]

Turns out that pictured rod bearing is cyl4, the heaviest of the pistons, by ~0.61 gram. Does that difference justify getting the file out?

[Laitch]

Does that difference justify getting the file out?

You're the maestro. It takes a grand jury to convince me to clean an air filter. 

[frankenduck]

Turns out that pictured rod bearing is cyl4, the heaviest of the pistons, by ~0.61 gram. Does that difference justify getting the file out?

Out of how many grams total? If the other pistons weighed a gram then it would matter. If they weigh 100 grams then not so much.

[Laitch]

Also of interest is that the Z16 splines in the 85 and earlier Ks hardly wear at all compared to the later Z20 splines. I've never figured out why BMW switched over to the Z20 drive shafts.

Engineering is rarely free of some design flaws. It was probably too late to go back to the drawing board with that one. 

[frankenduck]

Engineering is rarely free of some design flaws. It was probably too late to go back to the drawing board with that one. 

They had the Z16 shafts that they'd designed, manufactured and used in production already. Switching back wouldn't be that hard.

[Laitch]

They had the Z16 shafts that they'd designed, manufactured and used in production already. Switching back wouldn't be that hard.

It might be difficult it they or their contractors had tooled up and produced thousands of Z20s with an eye toward using them in future models under development, plus they probably thought everything was ok until, years later, units had accumulated multiple tens of thousands of miles in real-world conditions and were howling like banshees then stalling on the Autobahn. 

[Ingo]

Glad I did the spline job. Splines have been serviced sometime when, but the boot between gear box and swing arm was not correctly installed. It sat ok on the gear box but was half in and half out of the opening in the swing arm. Looked for the clip that's supposed to be inside the rubber in the swing arm hole, eventually found it in the larger diameter in the rubber boot. Almost missed it.... Got the boot installed correctly on swing arm, still have to install swing arm. Getting the boot correctly onto the gear box might be interesting. Pulling the drive shaft: it came apart unexpectedly easy, no idea if it was aligned or not, but it is now. Today I got the new valve stem seals...