21″ (front wheel) | Our X-GRIP tube 21″ HEAVY DUTY is made of butyl and is 4 mm thick. Butyl is known for its excellent air impermeability.
HD stands for HEAVY DUTY and the 4 mm material thickness. The tube is for a 21 inch front 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.