Bicycle Mechanics - Chain rings not quite true?

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hsjb
03-15-04, 10:29 AM
I noticed that there is a 1 mm difference in the distance between the middle or large chainring and the front deraileur cage during a single revolution. It's not a matter of a sudden difference but a gradual increase and decrease in the distance. Is this normal to allow for ease of shifting? It certainly is not the result of a bent chainring.

madpogue
03-15-04, 10:55 AM
If the variation is a steady "wave", then the chainring is probably not bent. It sounds out of alignment. If the chainring spacing or placement on the crank spider isn't perfect, I could see this happening. It's also possible the right crank arm isn't seated on the BB properly, or perhaps even the BB is bent. Or maybe 1mm is within tolerance...
MichaelW
03-15-04, 11:39 AM
I get that fitting a Stronglight chainset to a Shimano bottom bracket. There may be some poor tolerance at the fit, with a square taper. You may be mixing a Euro and Japanes standard taper.

schwinnbikelove
03-15-04, 01:37 PM
I don't really think it's entirely possible to get a chainring completely circular, unless is alloy CNC'd. Sheldon Browne has an article about this somewhere.