Bicycle drive systems are relatively simple: pressure applied to the bicycle pedal turns the crank arm, which in turn rotates the chain forward and turns the rear wheel. Wear in the chain or sprockets, however, can lead to this simple system malfunctioning. The result is a chain that slips out of gear easily. This slippage can disrupt your pedaling cadence or lead to the chain falling off the gear sprockets altogether.
Sprocket Wear
Most cyclists, whether road or mountain bikers, lubricate their bicycle chains with some type of oil. If this oil is not changed regularly, dirt and grime can build up in the chain, which wears down the gear sprockets with every crank of the pedals. Stanford-trained engineer and bicycle mechanic Jobst Brandt notes that excessive sprocket wear can cause a bicycle chain to sink deep into the grooves of the gear sprockets. When this occurs and the front chain ring turns, the bushings of the chain can become trapped in the grooves, which results in the chain bunching and slipping out of the gear. The only way to remedy this problem is by replacing the worn sprockets.
Stretched Chain
Though bicycle chains are made of metal and can withstand heavy forces from your pedaling, each time you make a revolution you stretch the chain a small amount more. When your chain becomes stretched too much, it can ride up the back of the sprocket and slip out of the teeth into a lower gear. According to Fred Milson in his book, "Complete Bike Maintenance: For Road, Mountain and Commuter Bicycles," you can check if your chain is too loose by placing one end of a 12-inch ruler on the center of a chain bushing and checking to see if the opposite end aligns with the center of another bushing. If the ruler is more than a 1/4 inch from the nearest bushing, the chain is over stretched and you need to remove a link.
Shifter Cable Tension
The cable that connects your shift controls to your rear derailleur may also be the cause for your gears slipping. Bicycle mechanic Alex Ramon states in his tutorial on how to adjust your rear derailleur that if your shift cable tension is too tight, the derailleur will be inclined to shift the chain into a lower gear easily. You can fix this problem simply by backing off the cable tension adjustment knob on the back of the derailleur. Turn the knob counterclockwise a quarter turn at a time until the derailleur shifts the chain only when you turn the shifting knob on the handles.
Derailleur Limit Screws
Apart from the shift cable tension, the limit screws on your derailleur can also cause your chain to shift into a lower gear accidentally. The low gear limit screw is located on top of the derailleur, typically closest to the gear cassette. If the screw is too loose, the chain will lean toward a lower gear and can occasionally slip off. Tighten the screw with a Phillips screwdriver until the chain sits centered on the gear sprocket to prevent this problem.
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