Training wheel set will fit 12" and 16" bicycles. The size is the wheel size of the child's bicycle. Bolt on to existing axle. Please note that training wheels require an axel to extend 1/2" past the axle nut. Make sure there is some axel sticking out past the axle nut (rear wheels), the training wheels bolt on between the frame and the axel nut. For ridiers up to 75lbs. Steel wheels.