I bought a 36" metro and the belts at first were great. They never slipped, not even when hauling me up hill. But after being in the rain, dust and other such elements, the wheel belts, not the transmission belt, have begun to slip. I adjusted the springs to make more tension, but they still slipped and made it much more difficult for me to use. Is there somthing I can do to the belts to get them in the shape they were when i bought it a month ago?