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[windmill john]

Just running with Dale’s off topic... ahem.

BMW X series bikes have the engine sprocket in line with the swinging arm pivot, so chain always at same length.



John

[Wheaters]

Even better, some don't have a chain....

[Rattlebattle]

What's wrong with a rigid back end and a sprung saddle? Don't answer that. Actually a friend of mine let me have a ride on his pre-war Speed Twin, the one with girder forks and a rigid rear end. I have to say how impressed I was by the ride quality. Admittedly large potholes were a no-no but generally speaking it wasn't bad at all. I was also struck by how smooth and lively the engine was - definitely the best size for a two main bearing engine without balance shafts. It went really well and must have been a revelation back then. I bet chains lasted well too with relatively smooth power pulses compared to the single cylinder bikes.

[stinkwheel]

Just running with Dale’s off topic... ahem.

BMW X series bikes have the engine sprocket in line with the swinging arm pivot, so chain always at same length.

Even the best aligned setup is only directly in line at one point in ther arc of the swingarm. The only way to remove the chan tensioning effect entirely would be if the front sprocket and swingarm rotated around the same point or for the swingarm to have a carefully calculated elliptical arc.

[Wheaters]

Somewhere in the back of my mind I'm sure I read about an older bike which had a two part chain run. The first chain, from the gearbox ran only to a second sprocket on a "jackshaft" which ran directly on the same axis as the swing arm. The further away from that line is, the more the tension is likely to vary with rear suspension movement.

A good friend of mine is a very clever fabricator who is very well known in the Vintage Sports Car Club for the wide ranging work he's done for car owners. He built his own three wheeler car from scratch; a 1920s/30s "racer" starting from an old Ford beam axle at the front. It uses a highly tuned 850 cc car engine, car clutch and gearbox and a very short propshaft. The propshaft drives a right angled bevel gearbox, sourced from a 1980s Yamaha 750 triple "shaftie" motorbike. The output drive of the latter has been converted to drive a "front" sprocket and a chain drives a second sprocket on the rear wheel. The drive sprocket was made to sit directly in line with the rear suspension pivot. The drive ratio can be altered just like a bike by changing the sprockets.

https://www.youtube.com/watch?v=kIkWBnRqVYg

[windmill john]

I was referring to the 450X:

“On the BMW, the swingarm pivot goes directly through the gearbox sprocket, straight through the entire gearbox and out the other side. Therefore there is no movement between the swingarm or the run of the chain, and therefore no stresses are placed on the drive system. This allows BMW to run the chain with hardly any slack for maximum feel at the throttle, and boy does it look weird. “This chain looks way too tight, can you ask one of your mechanics to slacken it off, please?” I ask project manager Markus Theobald when I see my bike. He just laughs and explains that since the chain is at constant tension, it doesn’t need any slack. There is every reason to suspect that this clever design may well find its way onto high-powered superbikes in the near future.
“

[stinkwheel]

Impressive, I didn't realise that. Be a real saving in weight there too.

[vince]

Wheaters, the model with that dual chain set up was OEC comp model ST3. Vince

[PeteF]

Just looked at the manual.
"Set chain play to 25/30mm with bike on centre stand"

[Rattlebattle]

That's all I do, though on my C5 it's 30-40mm. I've always adjusted mine to the higher limit on all my chain-driven bikes and never had any issues. I assume the engineers have worked out the play needed for adjusting in the way they specify, which is of course the easiest. The bigger issue is usually chain alignment and the inaccuracy of marks on swinging arms, or the stupid eccentric cam set-up on the RE that never quite allows one to set each side exactly where you want it. And, yes, I did buy our host's draw-bolt replacements but they didn't fit: I can't remember why not now but they needed way more fettling than I was prepared to do.