What Our Gearhead(R) Experts Are Saying:"I wear the Fast Feet 4 Sock on race day or whenever I'm chasing every second. The aero fabric feels fast, and the footbed fits like a Rosso Corsa-tight, supportive, and super breathable. They stay in place all ride thanks to the grippy cuff."When every second counts, we slip into the Fast Feet 4 Sock. Part of Castelli's Fast Feet project, this sock is built to reduce drag where it matters most-on the leading edge of every pedal stroke. Engineered Lycra(R) up top features vertical grooves that help detach airflow, while the polyurethane grip at the cuff keeps the sock locked in place at high speed. Underfoot, the sock is built on the proven Rosso Corsa platform for in-shoe comfort and moisture control. It hugs the foot, supports the arch, and stays cool under pressure. Whether we're hunting PRs or racing full tilt, this is the aero advantage we reach for when marginal gains matter.