what causes broken sproket teeth?

[woodyracing]

a while back on a group ride a guy showed up on a GSX-R750 with NO teeth on the rear sprocket and he still did the entire ride (couple hours, nothing huge).  I can't imagine how he managed that, everytime he'd start off the chain would spin around the rear sprocket a little bit before the wheel finally moved. 
Anyway, not really relevant to your situation but it was hilarious none the less

[Desmo Demon]

I check it once a week since it's my primary. I just don't know what I'm looking for. So far I only know what it looks like once it's broken or OK. Still can't tell what breaking or worn looks like.

Aaaahhhhh...I see.......first off, broken teeth is a huge no-no.   

You cannot go by the number of miles a chain has on it to determine when it needs to be replaced. There are several different things that you can check for. Here are some that I can think of off the top of my head. Some people use some of these and others use different ones, but here are a few suggestions...

1) If you can pull the chain more than half a tooth length at the 3:00 position on the rear sprocket, you may need to replace your chain....
2) If you remove your rear wheel, grab a section of chain, and bow it from left to right with more than 1" deflection fro a 12" section of chain, you may need to replace your chain...
3) If you have any rollers or o-rings missing, you may need to replace your chain....
4) If your manual stated to check the overall length of a certain number of links in your chain and the length is longer than the recommended maximum length, you may need to replace your chain....
5) If you have a kink in your chain resulting from some stiiff links that you have difficulty pivoting with your hands or it won't pivot on the chain's roller pin, you may need to replace your chain...
6) If you have rust on anything than your outer plates on the chian, you may need to replace your chain...
7) If you adjust your chain tension and then rotate the wheel and the chain gets a LOT tighter or very sloppy loose compared to how you just adjusted it, sections of your chain are worn more than other sections and you may need to replace your chain....
If your chain breaks.... 
9) If you have any broken or "hooked" teeth on your sprockets, you may need new sprockets....
10) If halfway around the rear sprocket youc an tell that the rollers are not touching the teeth, your chain is probably stretched too much and needs replacing or your sprocketis excessively worn...
11) If there is a groove on the outside of your sprocket where the inner plate of the chain has been rubbing, not only might you need a new chain and sprockets, but your sprockets are not properly aligned....


With my experience, I get around 10k-15k miles for a 520 chain, 16k-22k for a 525 chain, and 18k-25k to a 530 chain. The quality of the chain and the maintenance and preventative maintenance that you do to the chain can have a huge affect on these components. I have been known to change out my chain a little sooner than most people, but I also like to try to run two chains to one set of sprockets. Generally the second chain will not last as long as the first, but I view it as being cheaper than running the risk of breaking a chain and crashing or taking out an engine case....and is cheaper than replacing the sprockets. Case in point with my ST2.

The first chain lasted around 18k miles, but I really could have run longer. I figure another 5k miles and I would have finished taking out the sprockets and be running a much higher risk of breaking the chain. I opted to buy an EK x-ring chain that I found for $75 and running the stock sprockets that still looked pretty good. I ran that arrangement for over another 16k miles. Had I bought new sprockets, I figure I would have gained another 2k-6k miles, but the sprockets would have cost me another $100+. So, I ran two chains on the original sprockets for over 34k miles, and the rear sprocket still looked pretty good, to tell you the truth (but the front sprocket was worn fairly well).

[bigiain]

I wouldn't know anything about worn out front sprockets...

http://flickr.com/photos/bigiain/2535181995

big