Cannondale Trail SL 2 2013 Mountain Bike. 1.5'' Head Tube: The oversized 1.5'' head tube increases front end stiffness for better steering and control. Smooth Welds: Trademark double-pass smooth welds showcase Cannondale's aluminium frame mastery eliminates the stress-risers and weak points found on traditionally welded frames, enabling us to build lighter, stronger frames everywhere in the line