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First of all, let’s define what derailleurs are. Well, these are the mechanisms used for moving the chain from cog to cog or chainring to chain ring as well as "de-railing". This gives an opportunity to shift gears while riding. In reality, the derailleurs don't move the chain too much as they guide the latter from one side to another one. This is possible only in case of pedaling forward what the chain is moving. It’s important to mention that the front and rear derailleurs function with the usage of an internal spring that pulls the derailleur in the direction of the bike. It can also push the derailleur away from the bike. Besides, this takes place when the cable that’s attached to the bike's shifter obstructs the power of the spring. When the shifter plucks the cable, the latter vanquishes the spring and the chain is being moved toward or away from the bike. When the cable is released by the shifter, the derailleur gets back to the initial position. It can be a real difficulty if the cables and springs are pushing and pulling on the derailleurs if nothing prevents the derailleurs from turning the chain in any of the directions too far. But the benefit of the derailleurs consists in the fact that they have a pair of stop-limit screws that can prevent moving of the derailleurs in any direction. The screws have generally the mark of an H and an L (high and low). Thus, the high-limit screw is necessary for controlling ofon which the bike can be moved by the derailleur (or toward the higher gears), while the low-limit screw is responsible for controlling the distance of the derailleur’s movement toward the bike (or toward the lower gears). It’s important to say that limit screws prevent or decrease the movement when screwed deep into the derailleur. On the other hand, if the screws are turned clockwise they become unable to move to the great extent. As a result, the screws turned counterclockwise get loosened and, consequently, the range of movement is increased. Tightening the two limit screws as deep as possible will lead to the derailleur’s single position. It can turn out to be useful in case if a derailleur or chain gets broken. In this situation you’ll be able to locate the bike up to bring it home. By the way, the two derailleurs are extremely important in meeting the needs of a biker who needs to shift from higher to lower gears. If you move the chain toward the frame you’ll put your bike in a lower gear, in the same way, if you move the chain away from the bike you’ll put it in a higher gear. Here the situation is very similar to the one with an automobile: lower gears prove to be very useful if you need to lessen the speed (for instance, driving up the hill), while higher gears are necessary when it’s necessary to drive as fast as possible. The front derailleur has to move the chain sideways because it's under definite force. As a result, this area of the chain gives some power to the rear wheel. This is the reason why it’s too difficult for the front derailleur to shift when you're offering much power and driving slowly enough (driving up the hill). Rear derailleurs work hard to swing their arm or cage back and forth, guiding the chain from one cog to another one. This takes place under the freewheel. The cage of the rear derailleur has an S shape and two jockey wheels. The higher jockey wheel leads the chain to the cog, while the bottom jockey wheel is meant to retain tension on the chain. http://stanleylucasrevolution.com/sale_7588_457359847-SHIMANO-XTR-M971-FRONT-DERAILLEUR.htm
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