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Traditional mountain bike tires, commonly called "knobbies," have the familiar large knobs protruding from all angles. They cover the tire in pyramids, blocks, circles and other geometric patterns, but they're definitely not designed for looks. They do one thing: provide traction. This type of tire performs well on mountain trails, and in soft dirt or sand. They can run at lower pressure, allowing them to "float" on loose material. They don't move fast, and when covering hard-packed ground or pavement, buzz loudly enough to vibrate your arms.
Tamed-down versions of knobbies, semi-slicks traditionally combine small treads along the outside perimeter of the tire with smooth rubber down the middle. Others use a slightly sticky but pliable rubber compound for grip designed to keep the tire from washing out in corners. Semi-slicks provide speed on hard-packed dirt or trails with large flat rocks. They do not work well in sand, but they can run more pressure than knobbies on pavement and do not vibrate as much.
City slickers resemble car tires and are used mostly on pavement. The tread pattern, a weaving line or series of lines, contributes more to the tire's aesthetics than function, with little or no effect on the way the tire performs. Fashioned from a hard rubber compound that provides a stiff ride, they are the fastest of all the mountain bike tires. They run at high pressure to make your mountain bike feel almost like a road bike. They can be used on hard-packed dirt trails, but do not work well in sand or loose material. They don't provide much traction off-road but stick to pavement sufficiently to cut corner without problems.
Approximately 99 percent of all mountain bike tires fall into the category of clinchers, but one other type of tire -- now on the endangered species list -- can still be found on expensive bikes. Known as a "glue-on," this type of tire gets glued to a special rim. Some old-school enthusiasts insist the tire provides better traction and a more comfortable ride. This type of tire is hard to repair and change. If you do get a flat, you have to replace the entire tire, and you always have to carry one with you for this purpose.
Clinchers set the standard for mountain bike tires. This means the edge of the tire hooks over the edge of the rim. They contain a separate inner tube and air pressure holds everything in place. When air gets added, the tire clinches onto the rim. Traditional clinchers used a wire bead around the perimeter of the tire to hold it on, but many of today's contemporary clinchers use a Kevlar bead. This type of clincher costs a bit more, but is much easier to remove and install on the rim.