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Discussion Starter #1
Hi all.
Been riding the legs off the Frankenbike during this surprisingly good summer.
Started to notice that around 90 mph in TOP gear there is quite a harsh vibration. If I drop to 5th, the vibe disappears. New DID chain, new sprockets (my first conclusion) have not made any difference. I've ruled out the output shaft bearing as the vibe goes away in 5th. Creamy smooth everywhere else, roughness diminishes when entering go to jail speeds.
Anyone else had / noticed this and any ideas?

Cheers
 

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Discussion Starter #2
Finally, FINALLY, got to the bottom of this one.

Runs sweet as a nut up to 88mph, then a roughness starts that becomes more annoying with speed. Not the harsh vibes of a loose engine mount or the tingles from an unbalanced engine, more a roughness felt mainly through the pegs but through the frame as well.
Finally had some time to sort out my own bike.
Chain and sprockets replaced.
Wheel bearings replaced.
Thought that would do it... Nope.
I then checked the chain for clearance on the swinging arm, that was fine.

First clue. I was going to fit a 17 tooth front sprocket to ease cruising speed revs, but it already had one fitted.
Second clue. I generally fit a 530 pitch chain for longevity, these are slightly more beefy that the 520 / 525 types usually fitted.
The combination of these two factors was the cause of my problem.
Any thoughts? Answer below.










Closely inspecting the chain after a run, I noticed a clean area in the chain oil, right in the middle of the pins. This was on the outside of the chain run. The answer then fell into place.
There is a 'chain-guard' semi circle of metal just behind the plastic sprocket cover. This is a very close fit with the 17t sprocket, closer than on a standard 16t sprocket, not helped by the fatter 530 chain.
I am surmising that, as speed rose, the chain would fling a little from the sprocket. At 88mph the chain was then was lifting sufficiently to start interfering with this chain guard, each pin rattling against it to give me the rough vibes.
Removing the metal semi circle has completely cured this annoyance, just have to keep a close eye on chain wear as there is now only the plastic sprocket cover as protection if there is a snappage situation.

Cheers.
 
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