The Castelli INEOS Climber's 3. 1 is a race-cut, lightweight, breathable jersey that epitomizes the "all-in" mantra of performance climbing. Heat-loss and aerodynamics are two combative forces -- and typically heat loss wins out -- with aerodynamics suffering as you unzip the jersey to cool-down. Castelli finds a welcomed middle ground with the 3. 1. Adopting the fit from their Aero Race 6. 0 jersey, Castelli optimizes the wind flow at higher speeds -- think at the base and lower slopes of the climb at 16+ miles per hour. When you hit the slopes and ramp up the intensity, the breathable Flusso 3D front-facing fabric enhances wicking and heat loss, while helping you keep the jersey zipped to maintain aerodynamic efficiency.Castelli continues the pro-level features of the 3. 1 and adds a transparent race-number sleeve to the rear, which smoothes airflow even further and reduces the damage caused by pinning. And at a lightweight 116g (about the same a four energy gels), the 3. 1 is a natural climber with pro pedigree.