[Jon Stratis]

One more thing to add... if you're using the cubic function, make sure all your variable types are longs - 127^3 will overflow an int.

As for which wheels to look into changing, you'll want that to be the ones that hold the most weight. Also, do you have your wheels offset so the traction wheels in the middle are slightly lower than the ones on either end? You need to do that (and introduce a slight rocking in your robot) to ensure your traction wheels are always on the ground.

Finally, no one's asked yet if you're test driving on carpet or not... We use the same wheel setup as you, and it does slide a bit on hard flooring, but on carpet it works just fine!