How do I know if a TT helmet fits my aero position?

The key test is whether the tail aligns with your upper back when you're in your aero position on the aerobars. Sit on your bike on a trainer and have someone photograph you from the side—the tail should smoothly transition to your back without a gap or overlap. Different helmets suit different torso angles and head positions, so try multiple models if possible. The helmet should also stay firmly in place when you're pulling hard without shifting on your head.