STI Extreme 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 uses a 3 mm spec, making it 30 percent beefier than most heavy duty tubes.Specifically designed for the most severe applications and has STI's XD "Extreme Duty" butyl rubber uses a 3 mm thickness, making it 30 percent beefier than most heavy duty tubes now available on the market, and STI Motorcycle Tubes are economical. Top quality and extreme duty performance at an affordable price make STI motorcycle tubes stand out from the crowd.Tube Specifications:Size: 80/100-21 (3.00/3.25-21).Valve type: TR-6 (straight metal stem (8 mm diameter)).