MY 2006 yz450f has 3 -4 hours on it. I was a Wolfman's Saturday for the first time putting in a few slow laps and 10 minutes into it I was coming out of a corner and the engine stalled. I tried to bump it with no luck. When I stopped and looked down the sproket was wrapped around the swing arm . At the pits I looked closer and found that the bolts were still attached but the hub had broken in two. Does anybody know a Yamaha phone number to call? This thing could have hurt me if it did it over a jump. The rear wheel locked up.