Your bmx bikes chain keeps skipping?

Answer 1

If it's a bike with external gears the most usual reason is that the derailers haven't got their limit screws set right, so when the rider attempts to change gears the derailers push the chain right off. For Single-speed bikes the most common reason is that the chain is too slack. With too much slack the chain is free to move sideways enough to come off the sprockets if the bike is jostled around hard enough..

Answer 2

Bike chains "stretch" as they wear, which cause them to ride higher on the teeth of the cogs. This wears down the flank of the teeth, eventually to the point of the chain skipping. If it's a bike with external gears it can also be the derailer being slightly out of alignment.

Replacing the chain MIGHT fix the problem, but only if the sprocket hasn't gotten too badly worn too. For a certain fix - replace both.

Answer 3

The chain tension can be too low, but most probably either the chain or the freewheel/sprocket at the back has become too worn. If you've been riding with worn parts too long you might also have worn out the chainring.

I'd start with replacing the chain and the freewheel/sprocket.