I thought you could not reverse a Brushless motor?

I have talked it through with my dad, its going to be his machine after all, and it seems that its speed 900 drives, with a speed 900 drum on the front, but what voltage can we gat away with?

The voltage range of a Speed 900 is 6-40V so what would happen if we ran all 3 speed 900's off a 10 Cell lipo, with victor 885's for the drive and a simple relay for the disc. or is using 10 cells a little mad.

The only thing is that until we know the voltage we cant get the speed of the motor and from that work out the gear ratio, I think we are sticking to the 40 shore Banebots 124mm wheels, 6 of them