[Wayne TenBrink]

A few items to consider for a good 6 or 8 wheel tank drive (or any other chassis, for that matter):
1) For better turning, keep the track width (side/side spacing) large relative to the wheelbase (front/back spacing). For a long aspect ratio chassis, this requires either lowering the center wheels or using omni wheels at one (6WD) or both (8WD) ends.
2) For better stability, keep the wheelbase long relative to the track width.
3) Ideally you want a good balance between turning ability and stability. A wide aspect with wheels at the corners, or a 6WD with dropped center wheels is about right. If you can fit more traction wheels between the other rigid (non-omni) wheels, that is good and can be done without lowering them.
4) Keep the wheels as close to the frame perimeter as possible.
5) You will spend a lot more time turning, lining up on a game piece, accelerating, and pushing than you will going fast. A "slow/strong" single-speed is usually better suited than a "fast" single-speed, and simpler (to build and drive) than a two-speed.
6) Develop and test chassis designs during the fall/off-season. Try to develop a "system" (chassis structural modules, gearbox mounting, chain tensioners, axle mounts, maintenance & repair accomodation, etc.) that can be adapted to a wide variey of final configurations (wide/narrow, # drive wheels, wheel
size & type, ground clearance, etc.). After kickoff, it is relatively quick to finalize the design and get started on building the "bottom end" while you are still figuring out the "top end" (manipulator). We build our own chassis's and are usually driving by the end of week 2.