I have enountered a strange problem with a fixed gear build: when the wheel/crankset spin, the crankset's rotation creates a periodically pulsating sideways wobble on whole the bike. It can be felt when riding, and upsets the balance (riding is possible, but not pleasant). See image below for details.
At first, I assumed the wheel's rotation was somehow creating that force (angular momentum of a wheel spinning at a crooked angle, or untrue wheel). However, the wheel looked true. So I took the chain off to decouple the crank and wheel rotations. Holding the bike up by the saddle, front wheel on the ground, I spun the wheel and cranks separately. Spinning the wheel did not produce fluctuation, but spinning the cranks produced a lot of wobble.
The strength of the wobble got higher the faster the cranks spin. I assume it might be caused by one of these:
- Bent crank makes one side much heavier than the other? These are 90's Shimano Dura Ace cranks, so such a bend appeaning unnoticed is unlikely
- Issues with bottom bracket axis or bearings, which makes the angle of the crank spin not parallel to rest of bike? The angle seems good enough from looking (chainring distance from frame stays the same when rotating), so also unlikely
- Some other mystery reason?
Has anyone encoutered this? Any advice on finding the issue and how to fix it?