Derailleurs perform an important job on a bicycle. Sometimes called "derailers," they facilitate the movement of the chain through each sprocket of the chain wheel.
Derailleurs can be located on the front or back wheels of a bike. In the front, a derailleur is basically a metal cage that moves and pushes the chain along the chain wheel. When the chain has moved as far as it can at that angle, it falls off and onto the next closest chain wheel.
On the back wheel, derailleurs work in the same fashion, but with an added component. In the back, derailleurs contact a pulley, so that the slack in the bike chain is collected and wound around smaller sprockets.
Newer bicycles usually have sprockets with specially shaped teeth. This helps the derailleur system work more smoothly than in older models.
Derailleurs are the most exposed component of a bicycle, and are therefore likely to suffer some damage during a ride. Unfortunately, they''re also one of the more fragile parts of a bike. Damage to the derailleurs creates a domino effect of problems with other components, including the wheel, spokes and chain wheel.
When a derailleur gets hit by anything, the component itself may become bent. Or, more likely, the hanger may get bent instead. The hanger is the area of the frame to which the derailleur attaches.
To see if the hanger on your bike is bent, look at the derailleur from behind. The two pulleys on the rear derailleur should be lined up exactly one on top of the other. The chain should run straight up, without slanting.
If the hanger is bent, you''ll probably have to remove the derailleur in order to fix it. Bike repair shops have a special tool that slides onto the hanger in place of the derailleur. The tool straightens out the hanger, and it even has a gauge to estimate when the position of the hanger has been corrected.
Another potential problem is the loosening of the limit screws. These screws tell the derailleur where to stop in its movement back and forth. If these screws need frequent adjustment, the derailleur is probably bent. Adjusting the screws during a ride is fine, but it isn''t a long-term solution for a bent derailleur. Experts recommend taking your bike into a repair shop if the derailleur is bent.
If you need to make a minor adjustment in the field, look for labels on the stop limit screws. Newer models feature an "H" for "high" and an "L" for "low." You can give each screw a half-turn at a time. Note that if the low screw is set too loose, the derailleur can move into the spokes of the bicycle. But a little fussing with the screws is generally harmless and can help you get home during a ride.
To maintain the derailleur, simply lubricate it every time you oil your bike chain. Be sure that you oil the barrel areas, or ask a bike shop pro to show you where to lubricate the derailleur. Use a light oil such as WD-40. Motor oil just clogs the mechanism.
Traditional derailleur models have cages that are positioned vertically. Popular brands include Shimano, Ultegra, and Dura-Ace. Another is the Campagnolo, which has cages that are next to one another, rather than stacked vertically.
Interestingly, most bikes have a front and a rear derailleur. Single speed bicycles have no derailleur, of course, so you don''t have to worry about your child denting the derailleur on a starter bike. A single-speed bike teaches the rider how to pedal correctly.
Some cutting-edge bike models are doing away with the derailleur to streamline gear shifting and reduce overall bicycle weight. These models can cost upwards of $4000.
Most people who ride bicycles may never know what a derailleur is, but those who ride often would do well to become familiar with this delicate, important mechanism. The ability to diagnose problems related to a derailleur and the capability to repair those problems can be vital in certain situations.
Bicycle Tutor (n.d.). How to Adjust Your Rear Derailer. Retrieved April 17, 2008, from the Bicycle Tutor Web site.