Just thought I'd post this even though it's been addressed by much more adept fellas than myself. This is a video of the belt rubbing on the pinion gear teeth. It's from a stock SC18 with no modifications. I thought they were RTR?
So, we moved the pinion away from the motor taking its teeth out of the path of the belt. On a second unit, we installed a 15T pinion to accomplish the same thing. In addition we were told it would afford us a little less tension on the belt. The rubbing is gone, but there wasn't much difference with the tension.
That said, the first unit we purchased is showing signs of the belts beginning to stretch a little. You can roll it about 3 feet where as out of the box it wouldn't roll more than 1 inch.
http://www.youtube.com/watch?v=1GJzzMNjTZY