Cervelo Handlebar - HB19 S5 MY26One Piece Handlebar+Stem The new one piece bar and stem is a big part of the new S5s increase in aerodynamic performance, but aerodynamic improvements are limited if the bike doesnt fit you perfectly. Nine different size combinations help you get the best fit possible.. Cervelo HB19 uses our Plus Four concept, meaning the dimension at the hoods is 4cm narrower than in the drops (e. g. 38cm 42cm). This approach helps riders maintain a more aero body