I have a fixed gear bicycle with a chain that gets periodically tight and loose as I complete a rotation of the pedals. How do I center my chainring so that the chain has even tension throughout?
First off, apologies for the first comment. OP is a friend of mine and I intended to post that as a joke. For some retribution here is some real advice:
- Check to see the drivetrain is real gritty or dirty, or the chain is worn out or unevenly stretched
- Check to see if there is a bend in the chain ring:
-take the ring off and lay it on a really flat surface
-If it wobbles like a chair with one short leg than you need to replace it.
It might be one of these two things.
I want to provide an initial answer to this question with some resources that I have discovered and attempted.
- Loosen the chainring bolts until they are finger-tight
- Rotate the chain until you reach the tightest section of the chain
- Strike the top of the chain lightly with a rubber mallet until you have loosened this tight section.
- Continue to rotate and strike any tighter sections that you encounter, gradually softening your blows on subsequent strikes.
- Once your chainring is centered and no more tight spots are observed you can tighten the chainring bolts in a star pattern.
- If after several attempts you can't loosen a tight section there are two things you can check:
- Check for a bent chainring / crank, or off-center bottom bracket (suggestions on how to do this?)
- Check for an unevenly worn or stretched chain
- Use a chain wear tool and measure the chain wear along the entire length of chain. Lay the chain down on a piece of paper and write down the wear measurements for each area, then crossmatch those measurements with the tight spot.
There are some cases where this problem occurs when there is an incompatibility between the crank and the bottom bracket, because the ISO square taper to the JIS taper differ on their angles.
If for example you got a Sugino 75 crank with a JIS taper you will get that effect, whereas with an ISO taper it centers perfectly.