I hear about guys breaking (Spinning) stationary gears. How is this so? Is the material so weak that under nice power the teeth break?

 

I dunno, but on my rear rotor there is apparently a couple of teeth missing on my "hardened" gears

 

The gear profile on the Mazda unit dates back to the 60's and when you actually look at the gear engagement model the teeth are only just meshing!

Here in OZ we are breaking a lot of them in our high rpm circuit racers. Did you see the new stationary gears at Sevenstock made by Rohan Ambrose (from Australia) , he finally has them done and not for too bad a price either.

 

The stock gears will break at anything over 9000rpm and even as low as 8500 if under prolonged revs. That's why racers use hardened gears that allow shifts at 10k+ rpms.

Some of these hardened gears may be better than others, but since I don't race I'm not in a position to comment further on them.

I've also heard horror stories from the racing community about rotor internal gears that can "walk", ie: shift from their mounted positions so that they are no longer in sync, with the same devastating results as broken stationary gears.