I just changed the sprocket for a friend of mine that had 5 missing teeth in a row, then one good tooth, then 4 missing teeth in a row..

It is a chain drive with a 530 chain.

What causes this??????

 

Don't think I've ever seen one that bad. Rocks? Crappy Chinese steel?

 

Tooth fairy?
Probably not very helpful...

 

Is there any chance the chain stretched then rode up on the teeth and broke them off?

 

Couldn't be!!!
The tooth fairy leaves money and my friend is broke as hell!!!!

 

front or rear?

 

Did it look kinda like YOU?

 

Cheap nasty sprockets would be my guess. They don't usually break off until they are totally hooked and worn. If they snapped off and the remaining teeth have a reasonable profile I would make sure he doesn't fit that brand of hardware again....and tell us the make so we can avoid it.

 

Definately sounds like way over due replacement. I wouldn't do anything less than replacing both sprockets and chain (along with a talk with your friend about pre-ride inspections and preventative maintenance.

 

Is it a steel or alloy sprocket? Some bright sparks like the weight savings of cheap alloy sprockets.