Product Details + Product Description Helmet Friendly Compatible with bike helmet, can be worn under the helmet when cycling in winter. Skull cap ROCKBROS helmet liner perfect for riding, skiing, snowboarding and other winter outdoor activities. Lightweight and soft of polyester mesh fabric to keep head breathable and cool when cycling Cycling cap extended visor is flippable, sun-proof and dust-proof Compatible as bike helmet liner absorb sweat and provide shade in the hot weather Multi-Functional:Fits for cycling,running,fishing,hiking and other outdoor sports Adjustable elastic design,one size fits for head circumference from 53 to 61cm ( 21-24inch)