Gotcha,
If you are using a s4, how are you zero'n it?
http://puu.sh/jxgeU/4eb20e2530.png
Hopefully this shows how we did it, we used the tube axle by VEXpro and used r6-5zz bearings. The forks also had a recess for the tube axle to sit into and it was held in with a 1/4" bolt. We liked this layout but we will be updating the use of the R6-5zz bearings to a metric flanged bearing to retain the wheel better. Atm the wheel can slide off the bevel gear, while it never happened during season it did happen to one of our pbot wheels.
Unsure how thick your forks are as well, we did .5" thick and had room for an 11mm OD bearing. Also used a thrust washer this year, will most likely be switching to a thrust roller bearing.
-RC