Drive train suggestions

[ksafin]

Rookie Team 3992 here,

We currently made a drive train which is as so:
4 Wheels;
The two front wheels are 10 inch pneumatic wheels from harbor freight.

The back two "wheels" are 3.5" diameter swivel casters.

We thought this would allow for good maneuverability, but instead it's harder to control with the casters on the back.

However, we have been driving it:
1) On painted concrete floor (more slippery than the carpet)
2) With no weight on the drive train (pneumatics get more traction with weight, and casters may act differently)

So I ask you,
Are our two testing factors giving us the unsatisfactory result, or is our drivetrain design not very efficient and we should change it?

Thanks!