Built for speed on the summer's toastiest days, the Castelli Climber's 4. 0 Jersey provides us with the airflow and rapid cooling we need to ride at our best through the summer heat. This jersey combines the CFD-based seam construction we know from the Aero Race 7 Jersey with aerodynamic 3D mesh fabrics to keep us cool without sacrificing speed. Though form-fitting, the Flusso 3D and StradaPro 3D fabrics are stretchy and soft, allowing the jersey to move with us as we move in and out of the saddle.