A mtb 26" rim is 559 mm. A 650C (road 26") is 571 mm. Depending on the frame and brake you might be able to make it work.father_k said:I am looking to build a road bike from a mountain bike frame. I thought of using 650c rims but I dont' know if they are compatable with V brakes. I don't have any to measure without ordering them in. What is the exact diameter of 650c rims?