Not sure if adding weight to the back will help much. The motors are just standard drills so they'd probably not be able to pull much against another robot trying to drive in the other direction. But I'll have a play about with things and see if I can improve it.

No spinners in the qualifiers, and I'm not expecting it to make it through to the latter stages. The sides are pretty well armoured though, if it does.
I didn't expect to get it done so quickly either, but glad I did as it leaves more time for other bits and pieces.