The best tetra grabber that i've seen would have to be team 188.
It's a little biased, but team 357's drive train.
So far it hasn't been explained too well, so here ya go...
The drive train is a mecanum wheel hybrid completely manufactured by our team. The rollers are configured on 45 degree angles on our custom hubs. In order for the wheels to achieve their designed motion, each roller's bearings are actually molded into their urethane rubber material. The angles make an "X" pattern.
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Forward motion is achieved by driving all four wheels forward, the same concept applies for backward. Though there is a specific way to do it with these wheels, spinning can also be done in the traditional tank steering manner.
The translational movements are achieved on a screw principle. Diagonal wheels move in the same direction while the other diagonals move in the opposite. For example, the front right and back left move forward, while the others move backward. The roller configuration causes the robot to "slide" left and right.
These wheels can also achieve omni-directional movement: for example, spinning about its own axis while driving in an arc.
The mecanum wheel's rollers prevent the robot from being pushed freely like traditional onmi-wheels.
Videos and other info can be found here:
http://www.chiefdelphi.com/forums/sh...ad.php?t=36079