Matt Mannelly, the president and chief executive of Cannondale Bicycle, is stepping down in September. Mannelly, who oversaw Cannondale’s acquisition of Sugoi Performance Apparel and subsequently the company’s sale to Dorel Industries in February, has been CEO since 2005, two years after being named president. He will be replaced by Jeff Frehner, president and CEO of Dorel’s Cannondale Sports Group.