eveything i know about derailers and i hope to help you if u have any promblems with your derailer!!!

all about derailers!!!!!!!!How Derailers Work

Most modern bicycles use derailer gearing systems. Gear shifting is accomplished by leading the chain from one sprocket to another.
Derailers look complicated, but they are actually very simple, brute-force devices.

The front derailer simply consists of a cage made of sheet metal that can move back and forth from side to side. As it does so, it pushes the chain sideways until it can't run on the chainwheel it has been riding on, then the chain falls off and lands on the next chainwheel closest one to its new location.

The rear derailer looks more complicated, but the only additional complication is that the rear derailer also contains the arm with the spring-loaded pulleys, that takes up the slack as the chain goes to smaller sprockets.

Both derailers work by pushing the chain sideways, so that it runs at an angle onto the sprocket, instead of straight. When the angle becomes sharp enough, the chain can no longer mesh with the sprocket. It then falls off of the sprocket and lands on the next one in line.
When the chain is being shifted from a smaller sprocket to a larger one, it is slightly different. The chain is forced to rub against the side of the larger sprocket next to the one it is riding on. Eventually, it gets snagged on the sides of the teeth of the larger sprocket, and yanked up and away.

Newer sprockets usually have specially shaped teeth, and ramps on their sides to make this operation run more smoothly. This is the major reason that modern derailer gear systems work better than older ones. It has very little to do with the quality of the derailer.


Rear Derailer Adjustment
Before You Try To Adjust A Rear Derailer, Make Sure It Isn't Bent!
Before you try to adjust your rear derailer, you should really make sure it isn't bent. The rear derailer is the most fragile and exposed part of a bicycle, and they are always getting bonked.
When a rear derailer gets whacked, it bends inward, toward the spokes. In some cases, it will actually go into the spokes of the rear wheel while it is turning. This is very bad news when it happens. If you are lucky, the derailer breaks. If you are less lucky, it takes a few spokes with it, and can easily ruin your rear wheel. If you are truly unfortunate, the pull of the spokes on the derailer can tug it backwards so forcefully that the dropout (the frame part that the rear axle and derailer attach to) will be bent very sharply backward, or even broken. In some cases, this can render a frame a total loss.

When the derailer gets biffed, it is not usually the derailer itself that bends, but rather the hanger, the tab of the frame dropout that the derailer attaches to. This is because the part of the derailer that usually gets bumped is the strongest part, the parallelogram linkage that moves the cage back and forth. (Note; on many older or less expensive bicycles, the hanger is not part of the frame, but is a separate part, which comes with the derailer.
This "adaptor claw" is held on mainly by the rear axle nut, or the quick-release skewer.)
You can roughly check this by eye, if you look at the derailer from the rear. The two pulleys should be lined up exactly underneath one another, so that the chain runs straight up from the tension pulley to the jockey pulley, with no sideways slant.

Well-intentioned folks sometimes try to correct this by grabbing hold of the derailer and trying to pull it back outward to its proper position. Unfortunately, they usually wind up yanking on the derailer by its cage, which is the weakest part. As a result, they fail to straigten the bent hanger, and, instead, bend the cage as well.

When a derailer hanger is bent, it is generally necessary to remove the derailer to straighten it. Bike shops have a special tool, a bar with a sliding gauge on it, that screws into the hole in the hanger, in place of the derailer. This tool provides lots of leverage to straighten the hanger with, and has a gauge to judge when it is parallel to the rear wheel.

Note, wide-range Campagnolo derailers have cages that are not intended to be vertical; the lower pulley is farther outboard than the jockey pulley.

if you have any Question on anything please ask thanks Kevin!!