Nice shots! Just wondering if this is your first time shooting motocross? I'm guessing not as these are pretty good.
I like 5,7,8,9 the most. Some flash would have really helped out on #9. 5, 7 and 8 show great expressions of concentration, also #6 but the crop is too tight for my taste.
For me, #1 looks a bit awkward and I prefer to see the ground as well when they're in the air.