Castelli Nano Flex 3G Knee Warmers On those days when there's a chill in the air but you don't want full coverage over your legs, the Castelli Nano Flex 3G Knee Warmers are ideal. Your knee joints and surrounding muscles will stay warm and the fabric is soft against the skin, and breathable, while additionally offering protection against the rain. The material is stretchy and the seams minimal, so you'll retain full freedom of movement, while elastic grippers at the top hold the warmers securely in place throughout your ride.