Originally Posted by
Andrew R Stewart
Sure the problem could be only one issue and that issue could be an out of true chainwheel. There are simple tests to try to figure this out. One is to grab one crank arm at it's pedal end and try to rock it in and out against the frame. I find it easiest to rotate the arm so it nearly lines up with the frame's chainstay. Placing one's fingers between the frame and the arm and the other hand squeezes the arm against the frame while the fingers try to pry the arm back. If the BB bearings or the arm's attachment to the BB are the problem you'll feel slop. By removing the chain from the rings then spinng the crank you can watch the rings as they rotate past the ft der cage. Do they run past without any side to side run out? Remember that if the arms are loose on the spindle or if the BB is sloppy these will cause run out.
For an experienced wrench these steps take less then one minute. From these two steps a lot of info can be had. Andy.
The crank arm seems pretty sturdy. Rotating the pedal backwards with the chain on I can still see the chain moving laterally the same amount with each rotation, even without the force of me pedaling with my full weight. With the chain off, the chainrings are apparently oriented at a slant, so that the teeth move toward and away from the center of the derailleur cage as they turn forward or backward. It seems obvious that the crank itself isn't loose or shifting, but that the bracket (or at least the chain rings) is crooked in relation to the frame. Which I wish were not the case since I'd rather just have to adjust something myself than have to go to the shop and get a piece replaced, but cest la vie I reckon