Derailleurs are the small mechanisms mounted to the frame that shift the chain between the chainrings (front derailleur) and cogs (rear derailleur) when you operate the shift levers. Unless you crash, drop or abuse your bicycle, derailleurs last a long time. In fact, they can last as long as your bike with only minor maintenance, such as occasional lubrication (any time they look dry or after riding in the rain).
Rear derailleurs are more frequently damaged than fronts because they protrude from the bicycle. This means that if someone knocks over your bike, the derailleur or the part of the frame it's attached to, can get bent. When this happens, the derailleur will not shift properly. It will hesitate shifting onto the smallest cog and may overshift throwing the chain off the largest cog. Worse, if it's bent badly enough, on some bikes it might even get hooked on a spoke, pulled into the wheel and end up mangled.
To ensure this never happens, always carefully inspect your bike anytime it falls over or anytime the rear derailleur isn't shifting correctly. To tell if it's been bent, stand behind the bike and see if an imaginary straight line bisecting the cogs will also bisect both derailleur pulleys. It should. If the derailleur looks canted inward, bring the bike in for us to have a look and realign the derailleur and/or frame with our special tools.
Reasons you might want to install a new derailleur include upgrading to save a little weight or to try the latest technology. In order to provide a derailleur that will work with your drivetrain, we'd like to see your original derailleur so we can match its capacity. Or, if you no longer have the old derailleur, we need to know:
Front derailleurs are less often damaged than rears, though it is possible to bend them by getting baggy pants or shoelaces caught in them. Still, like rear derailleurs, you may wish to upgrade to save weight or improve shifting.
We'll need to know several things, which we can determine by seeing the original derailleur. Or, check it and provide the following details:
If you have any questions at all, just ask and we'll be happy to help!