[MrForbes]

3% stretch is working ok for us. We are using 1-1/2" ABS (about 2" outside diameter) rollers that are about a foot long. We're using ball bearings in end caps to support the rollers. We don't have any grooves in them, instead we have a simply plywood "comb" above the lower roller to keep the cords in place.

Pictures of your robot would help us help you figure out what's wrong...