Troy Lee Designs D4 Polyacrylite Helmet – Linear Blue X-SmallTroy Lee Designs D4 Polyacrylite Helmet Linear Blue X Small. The reigning king of full face helmets the D4 range expands with a Polyacrylite model packing the same safety and comfort features as the premium carbon and composite models into a price conscious shell. Details make the D4 the most coveted full face in mountain biking including Mips C2 brain protection an air intake system for ventilation and an unmatched anatomical fit. Mips C2 system and