Rear Derailleur
Bike Derailleurs Basic
Derailleurs are the mechanisms for “de-railing” or moving the chain from cog to cog and chainring to chain ring, allowing you to shift and change gears when you ride. The derailleurs don’t so much move the chain as they guide it from one side to the other, which is only possible if you’re pedaling forward and the chain is moving.
Both the front and rear derailleurs work through the use of an internal spring, which pulls the derailleur toward the bike or pushes the derailleur away from the bike, while the cable attached to the bike’s shifter opposes the force of the spring. When the shifter pulls the cable, the cable overcomes the spring and moves the chain toward or away from the bike. When the shifter releases the cable, the derailleur moves back to its normal position.
The fact that springs and cables are pushing and pulling on the derailleurs would be a problem if there were nothing stopping the derailleurs from moving the chain too far in one direction or the other. Fortunately, derailleurs are designed with a pair of stop-limit screws, which limit the movement of the derailleurs in both directions. These screws are usually marked with an H and an L, for high and low. The high-limit screw controls the distance that the derailleur can move away from the bike (toward the higher gears), and the low-limit screw controls the distance the derailleur can move toward the bike (toward the lower gears).
Limit screws physically limit movement as they’re screwed deeper into the derailleur. Turning the screws clockwise drives them in deeper and restricts the range of movement; turning the screws counterclockwise loosens them and increases the range of movement. If you were to tighten both limit screws to their maximum depth, you would limit the derailleur to a single position. This is good to know if you break a derailleur or chain and need to set the bike up for just one gear to get home.
Both derailleurs have difficult jobs when it comes to keeping up with the demands of a biker who is shifting to higher and lower gears. Moving the chain toward the frame puts the bike in a lower gear, whereas moving the chain away from the bike puts the bike in a higher gear. Just as with a car, lower gears are useful when you’re going slow (for example, climbing a steep hill), whereas higher gears serve you on the descent, when you’re riding at a faster speed.
The front derailleur has the unenviable task of moving the chain sideways as it’s under force. This part of the chain transmits power to the rear wheel, which is why when you’re applying a lot of power and moving slowly (such as climbing a hill) it’s hard for the front derailleur to shift. Rear derailleurs have the job of swinging their arm or cage back and forth under the freewheel, guiding the chain from one cog to the next. The rear derailleur cage has the familiar S shape, with two jockey wheels. The top jockey wheel guides the chain onto the cog and the bottom jockey wheel is designed to keep tension on the chain and take up slack.
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How to install and adjust rear derailleur-FULL guide
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