Chain tightens....

[ChrisK]

My son has a BMX bike with freewheel. When you pedal the chain will get tight then loosen up. i checked the crank and it appears that it is not bent. i do not see anything on the back sprocket that is out of whack. Any suggestions on what to look for?

Thank you.

[trevgbb]

usual cause is the wheel out of line with the frame, check that the gap either side of the tyre is the same both sides at the chain stays and that it doesn't alter as you tighten it, can be caused by damaged drop outs.
If it persists, you may want to use a chain tensioner like these;
also check for damaged teeth and the chain ring out of round.

[ChrisK]

(03-22-2011, 12:36 PM)trevgbb Wrote:  usual cause is the wheel out of line with the frame, check that the gap either side of the tyre is the same both sides at the chain stays and that it doesn't alter as you tighten it, can be caused by damaged drop outs.
If it persists, you may want to use a chain tensioner like these;
also check for damaged teeth and the chain ring out of round.

Thanks Trevgbb.

the wheel is aligned with the frame okay. What are drop outs? i never thought to check to make sure the chain ring is flattened on one side i will do that.

[DaveM]

I've found that this usually comes from the front chainring being out of round or not centered on the crank as trevgbb mentions. Depending on how the chainring mounts to the crank, you may be able to adjust this.