I have a (non-Thorn) Rohloff bike.
At certain cadences--roughly between 80-100--and high force I sense a short flutter or vibration through the frame/cranks during the power downstroke of each leg. A sort of "thrr,thrr,thrr". Not a noise, more like a resonance. And only on the quarter of the revolution when each leg is pressing hard.
It is independent of bike speed--I can get it at five mph up steep hills or 25 mph if I'm really cranking hard. It will happen in all 14 gears. The only factors are cadence and pedal force. If I slow the cadence, or spin with less force the flutter subsides.
So it would seem that the only things that are directly linked to cadence and force are the bottom bracket/crank, the chainring, the pedals, and the chain. As far as I can tell the bracket/crank, chainring, and pedals are fine. I've replaced the chain and have adjusted its tension from quite loose to quite tight with no effect. I still get the flutter.
My only theory is that the the upper run of the chain--which is under tension--has a resonant frequency that causes a mild flutter at certain combinations of tension and frequency of pedalling. I've found suggestions on the net that this can occur; I know that some power meters use chain vibration frequency to measure tension (and hence power); and the effect _feels_ like it could be a chain resonance.
I don't recall this effect on any other bikes, only on my Rohloff. (The fact that the Rohloff has no chain tensioner shouldn't matter, one would think, if the _upper_ part of the chain is what resonates, and only the lower part is affected by a chain tensioner.)
Has anyone else experienced this on his bike, Rohloff or not? Is there anything to do about it? (In theory the resonant frequency should depend on the length of the upper chain which is fixed; the tension, which is what it is when you pedal; and the square root of the mass of the chain. I suppose I could get a heavier chain--I use an 8 speed 3/32"--but that would only lower the frequency a little, I would think.)
