The zipp 40439s with its firecrest rim technology offers superb aero performance straight line sprinting stiffness great stability in crosswinds and are ideal for climbing also being lightweight they deliver precise handling on tricky descents and crisp accelerations when the pressure is on firecrest moves the center of pressure of the wheel backwards giving the 45mm deep rim the crosswind stability of a box section rim at the same time the 264mm wide brake track at center allows