Minibot pulling away from pole

[Sb28000]

Early on, we settled on a three-wheeled design, set at 120 degree angles to each other. However, we discovered (a bit late) that the third wheel, built on a pivot, would not maintain enough tension on the pole. Trying to come up with a new design, we thought of using basically trapping the pole between a length of Ttrix C-channel and the two powered wheels (that were parrallel to this bar). However, the minibot tended to pull away from the pole in the direction of the motors, and even with magnets we could not correct for this. We have since built something similar to Team 166's design, http://www.youtube.com/watch?v=ERVuEe9yytM
using four wheels, with two wheels driven from each motor. But we are still strugggling witht the minibot pulling away from the pole, backwards. If anyone could tell us if they encountered this problem or how they overcame it, we would appreciate it.

Team 2656