We have the 71T belt rings. They are a little too big on a stock Co-Motion frame that they were basically designed for - with the eccentric in the loosest position, the belt only JUST slides on, with a little help of turning the cranks that goes against Gates' instructions. The belt is then already at the recommended tightness, and so the eccentric stays where it is. After less than one year, Gates realized that this was a problem and so switched to the slightly smaller 69T rings.