The castelli viva donna skully is simple but effective the cap is warm and stretchy perfect for wearing underneath your helmet through the winter months castelli have constructed the skully with clever raw cut edges that provide you with a comfortable fit that wonrsquot bunch under your helmet castelli viva donna skully features thermoflex is warm soft and stretchy the raw cut edge lies flat and doesn39t bunch under helmet fabric 85 polyester 15 elastolefin olefin