help with sons bike
 

I cant figure out what wrong with my sons bike. The chain doesn't stay on he'll be able to ride for about 5 min then it pops off I've tightened the chain a couple times still don't work. both sprockets look straight and the chain does too when its on. Any other ideas?
Thanks for the help

single speed? probably the rear wheel nut is not sufficiently tightened

Yes its only a 16" single speed.

I would say it's likely to be that the axle nuts aren't tight enough, allowing the rear wheel to slip forwards and so loosen the chain. You need to pull the wheel back as far as possible, making sure the wheel is straight. Then really crank those axle nuts down to hold it in place.

1. Pull the rear wheel back and tighten the rignt side axle nut just snug.
2. Now make sure the rear wheel is straight and tighten the left side nice and tightly.
3. Lastly do a final tightening on the right side.

If you don't follow that procedure, there's a tendency for the hub to "walk forward" in the dropout as you tighten the right side axle nut.

yeah I was doing that. I started watching it closer as I pedaled it the chain seamed like it was trying to jump off the rear sprocket before it finally does. So it might not be as straight as I thought. Is there a way to test this and a way straighten it besides a hammer and solid place to lay it?

T_T, Is it jumping off the bottom or the top of the rear sprocket? Is there a wobble at the rear sprocket? A wobble at the chainring (front sprocket)? When my son had a 16" tired bike ANYTHING could be wrong! ;)

Brad

PS Grab the two crankarms and see if there's any play when they're horizontal and vertical.

Get low down behind the bike, and sight down the chain. It should be nice and straight from the rear cog to the crankset, without any left or right deviation. If it's not straight, then you have a chainline problem, which you might be able to fix. Pictures would help. Is this a new problem with the bike ?

Just to make sure the obvious isn't being overlooked; tight or damaged links in the chain?

When you adjust the chain tension on a one speed bike by pulling the wheel back in the dropouts you must loosen the brake arm first. After pulling the chain taut and tightening the axle nuts you must retighten the brake arm. For a complete description of the procedure go the the Park tool website and locate the article "Chain tension on one speed bikes."