28-02-13, 03:50 PM
I can't imagine that the nut is simply vibrating off (the thread would need to be non existent at that point). I suspect that the socket is "bouncing" back and forth (left to right) and impacting the nut, forcing it off the drive shaft. The only reason the sprocket doesn't follow it immediately is that the chain holds it in line. Also with a high tension on the chain it will act to pull the front sprocket into line with the rear sprocket. It would be interesting to see a breakdown of speed, sprocket alignment and engine mount condition (loose or soft mounts would allow the engine to rotate anti clockwise, when viewed from above, under load changing the alignment of the two sprockets) on bikes that have lost the sprocket.