You'll probably have to go to a wider wheel. I'd suggest the 120mm tire on a 21" wheel if you're wanting to stay with the 21. I've got an 18" x 3.5" wheel on order that will have a 140mm tire on it for my train. The radius will be approx .75" smaller. Hoping the wheel is in this week as it has already been over 8 WEEKS! Damn RC Components!