STI Ultra 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 uses a 4 mm spec, making it 60 percent beefier than most heavy duty tubes. Specifically designed for the ultimate in severe applications and has already been proven in off-road competition. Priced considerably less than comparable rivals.