Mountain bikes often contain anywhere between 21 and 27 different speeds. These speeds are adjusted through various combinations of the rear sprockets and front chain rings, sometimes referred to as gears. If the chain of your bike will not stay on the gears properly, it may slip onto another gear, have little traction or fall off the bike entirely. This can make for a difficult ride or may leave you stranded if you do not take care of it.
Worn Sprockets
The leading cause of a gear slipping on your mountain bike is that the sprockets are worn down. This occurs naturally as the chain creates friction against the gears and grinds them down over time. Stanford-educated engineer Jobst Brandt states that as the teeth on the sprockets wear down, particularly on the front chain rings, a new chain will sink deep into the sprocket grooves and stick there. This causes the chain to bunch up and may result in it slipping off the gears.
Prevention
The teeth of your mountain bike's gears become worn down when dirt and dust from the trails collects in the chain grease and grinds on the metal. One way to slow the effects of this dirt is to soak your chain monthly in a wash of solvent, such as kerosene, then oil the chain again before putting it back on the bike. No matter how clean you keep your bike, however, you will need to occasionally replace your gear cassette and front chain rings. This process requires special bicycle mechanic tools such as a chain whip, lock ring nut and possibly a notched freewheel wrench.
Chain Stretching
Every time you turn your pedals, force is transferred to the front chain ring and the chain becomes stretched between the front and back gears. If you are climbing hills regularly, this force is greater in order to pull you up the climb. Eventually, this tension stretches out the links in the chain. When this occurs, the chain will ride up on the teeth of the gear sprockets and slip off while riding.
Repairing the Chain
As author Fred Milson notes in his book, "Complete Bike Maintenance: For Road, Mountain and Commuter Bicycles," the easiest way to check for chain stretching is with a ruler. Place the beginning end of the ruler over the center of a rivet in a chain link near the derailleur. If the 12-inch mark of the ruler is more than 1/4-inch from the center of the nearest chain link rivet, the chain is stretched too far. Use a chain link removal tool to take out as many links as necessary to better fit the chain. You may need to adjust the derailleurs to accommodate the shortened chain.
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