Castellli Fast Feet 4 Sock Watts on Your Feet Most aero socks are tested statically, but a foot is never static on the bike. Castelli built a jig that replicates the actual movement of the lower leg, then verified the results on real riders. The Fast Feet 4 can save 2 watts at 50 km/h - a number that holds up under dynamic conditions, not just in a lab. The latest version adds an updated cuff fit for more comfort and aerodynamic gains. If you're racing on the road and can't run a shoecover, this sock is the next best thing. Features Wind-tunnel and on-rider tested aerodynamics verified 2-watt savings at 50 km/h under dynamic conditions Grooved Lycra(R) cuff engineered surface trips airflow at the leg for maximum aerodynamic performance High-density elastic midfoot band locks the sock in place and prevents bunching inside the shoe Flat-seam toe construction eliminates pressure points where they matter most | Castelli | Fast Feet 4 Sock Men's | Size Large/extra Large In Black