The RAM-B-231 consists of a 1" rubber ball designed to sit at right angles to the mounting rail. The rust proofmetal u-bolt accommodates rails in diameter from 0.5" to 1" in diameter. Included are black cover caps to be placed over the exposed ends of the u-bolt and a composite adapter that can narrow the rail to a minimum width of 0.5". 
U-Bolt Dimensions:Accommodates rails from 0.5" to 1" in diameter.