The front derailleur will only operate properly when the chain rings are engaged in the clockwise rotation since it is the top side of the chain (drive side) that must be forced onto the larger or smaller chain ring in order to change the gear ratio. Also, the chain used for a multi-speed bicycle is thinner pitch than the type used on a single speed cycle, and it is also more flexible from side-to-side, allowing it to be forced over to the next chain ring. Adjustment and setup of a front derailleur is a simple task once you understand the basic operating principals.
Figure 1 - A standard front derailleur
The typical front derailleur shown in Figure 1 represents the most commonly available steel plate style that most modern cycles are equipped with. The front derailleur sits just over the largest front chain ring, held in place by the clamp that is secure around the seat tube. When the shifting lever is moved, the chain cage is pushed or pulled, forcing the chain to jump off the current chain ring and onto the next one. Newer shifting hardware is “indexed”, meaning that the shifting hardware is matched to the derailleur so that one “click” translates into one shift. Older shifters are simple levers that allow you to move the derailleur position in either direction until you have shifted gears. The older systems are much easier to setup and maintain, but do make an annoying ratcheting sound as you move between gears.
Figure 2 - The rear derailleur takes up chain slack
To make it possible for the chain to ride on multiple diameter chain rings, a chain that is long enough to wrap the two largest rings must be used. This requires that the “slack” or return chain be picked up and pulled somewhat tight by the spring loaded rear derailleur. Figure 2 shows the rear derailleur pulling back on the lower chain, keeping the tension tight. The lower part of a bicycle chain is called the “return chain” because it never has any real tension, nor does it drive the transmission in any way.
Figure 3 - The cage derails the chain
Figure 3 shows the movement that the front derailleur makes as it changes gears. The steel cage pushes against the side of the chain, causing it to jump over to the next chain ring where it is picked up by the sprocket teeth and wrapped around that chain ring in the clockwise (pedaling) direction. Back pedaling while you shift will jam up the chain since the front derailleur cannot force the chain off of the chain ring in the counter clockwise rotation.
More of this free and other DIY plans, www.atomiczombie.com .