HD stands for HEAVY DUTY and the 4mm material thickness
The tube is for a 19 inch rear wheel. All enduro and motocross bikes are nowadays supplied with inner tubes in the tyres. These tubes are often very thin (1.4 mm) and you easily get a flat tire. Our X-GRIP tube 19″ HEAVY DUTY is made of butyl and is 4 mm thick. Butyl is known for its excellent air impermeability.