You might find this white paper helpful.
http://www.chiefdelphi.com/media/papers/1917
Understanding why some robots turn and others don't is a big part of making a successful drive train, and I've blogged a bit about it for my students here
http://botshop.wordpress.com/page/2/
We're using an 8 wheel drive system, with the axles evenly spaced about 10.5" apart. We are using 6" IFI (VEXPro) Traction wheels with 1.5" wide roughtop tread on the middle two axles, and 6" AM "kitbot" wheels from 2007 and 2008 on the ends. The end axles are raised 1/4", so the end wheels don't contact the ground when we are on the flat.
The wheels are driven directly off the output shaft (we have the extended output shaft) of a 14.88:1 toughbox.
The performance is astounding. It outperforms EVERY drivetrain we've built, inlcuding a mecanum drive. I figure we can go from full speed to stop, turn 90 degrees and get back up to full speed in about the same time it took the mecanum to come to a stop and begin to strafe sideways.
Admittedly, the mecanum had a higher top speed, (12.75:1 driving 8" wheels) but it rarely reached it, and we do have a bit more weight to add to this machine, so the accelleration may drop off a wee bit.
This is our seventh year of building robots and we keep getting better at it, it seems, and I'm pretty sure after looking at what some other teams have built this year we can do better still. So good luck figuring out your drive and using that knowledge to form the starting point for your next 'bot.
Jason