Very nice design!
There seem to be many variations of Vex holomic robots out there, probably because they are so fun to drive. Here's the beginnings of my first Vex bot, thanks to the RS sale:
Here's the underside:
My goal was to design a robot as clean and symmetrical as I could. It took one Starter kit and an additional hardware kit. The orange spacers in the motor mount bracket are the only non-vex parts, from an Erector set. They are conveniently in-between sizes of the vex plastic spacers.
To avoid having to support the axles on the outer sides of the wheels, I set back the motor with these spacers to allow a collar to hold in and support the axle on the motor side inside the bracket.
I've added sensors and have devised a scheme (still in the process of Implementation ) to have the robot detect walls and "bounce" off them before hitting with an angle of reflection equal to that of the angle of incidence. I'd like to put it inside a boxed-in area and have it bounce around like an billiard ball for robotics demos.