STI Heavy Duty Tube (80/100-21) STI Tire & Wheel, acclaimed for groundbreaking ATV / UTV products such as Black Diamond tires and HD Alloy Series wheels, has brought its innovative thinking further into the off-road motorcycle market with the expansion of its recently-introduced motorcy How could a fundamental item such as a dirt bike inner tube possibly be made different or better? Glad you asked.Each is 50 percent thicker than most standard tubes.HD tubes are made from high-quality butyl rubber with a 2 mm thickness.Priced considerably The HD "Heavy Duty" model is 50 percent thicker than most standard tubes on the market today, and STI Motorcycle Tubes are economical. Top quality and performance at an affordable price make STI motorcycle tubes stand out from the crowd.Tube Specifications:Size: 80/100-21.Valve: TR-4 (straight metal stem (8 mm diameter)).