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Knobby tires are used mainly on off-road vehicles, such as motocross and enduro motorcycles, ATVs, and mountain bikes. They may be fitted as an aftermarket option on four wheel drive vehicles. On dirt surfaces, rubber knobs on the surface portion of the tire come into contact with the ground, and provide substantially more traction than is possible with street tires. Rather like boot studs, the vehicle's weight causes the knobs to dig into the surface. Knobby tires provide maximum grip on loose dirt surfaces, but on paved surfaces, such tires have much less grip than street tires. Tires with less aggressive knobs (smaller knobs and the shape of the tread cross-section closer to that of street tires) can provide a compromise, giving less grip off-road, but a grip closer to that of street tires on paved surfaces. Such tires are useful for enduro and dual-sport motorcycles and other vehicles designed to be used both off-road and on pavement.
When the vehicle is used on loose surfaced, low tire pressure will be used to improve grip. In this case, the tire now has a chance of coming off the wheel, or the tire may slip around the wheel rim. To prevent this from happening, a beadlock or rim lock can be used to clamp the tire to the wheel.
Depending on the model of tire, a knobby tire may or may not be approved by the DOT. A knobby tire approved for street use is referred to as a "DOT knobby" or a "D.O.T. knobby." The markings on the sidewall of a DOT knobby include a DOT code (U.S.) with the last four digits representing the week and year the tire was built, which identifies the tire as street legal. If the knobby tire has markings like "Motocross Use Only" or "Not for Highway Use", then the tire is not street legal. Even if street legal, knobby tires are generally not suitable for long distances on roads or paved surfaces, because the pavement wears down the knobs and ruins the tires for off-road use.
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