Training wheels for 16" of 20" kids bicycles. The size is the wheel size of the child's bicycle. Bolt on to existing axle. Please note that training wheels require an axle to extend 1/2" past the axle nut. Make sure there is some axle sticking out past the axle nut (rear wheels), the training wheels bolt on between the frame and the axle nut. For riders up to 100 lbs. Steel wheels.