The derailleur consists of a clamp, a pivot, and a cage. A derailleur is usually attached to the bicycle frame's seat tube by a round clamp, or bolted onto a small, slotted metal piece which has been welded onto the seat tube. The derailleur has two metal plates which form a cage around the chain. The derailleur's clamp and cage are connected to each other by a pivot.
The front derailleur's function is to move the chain from one chain ring to another. The rider uses a lever connected to the pivot by a cable to move the cage inwards or outwards. When the cage moves inward, toward the bicycle frame, the outer portion contacts the chain and pushes it inward, off of the current chain ring and onto the next chain ring closer to the bicycles frame. When the cage moves outward, away from the frame, the inner portion contacts the chain and pushes it outward, off of the current chain ring and onto the next chain ring further away from the bicycle's frame.
Height-By adjusting the height of the clamp, it is possible to move the derailleur cage closer or further from the top of the chain rings.
Angle-By rotating the clamp to the left or right, it is possible to adjust the position of the derailleur cage relative to the chain and the chain rings, either parallel or out of parallel.
Travel-The front derailleur has two limit screws which limit how far the cage is able to travel either inward or outward. These function much like a door stop. If the cage is able to move too far inward, it can move the chain completely off the inner chain ring. If the cage is able to move too far outward, it can move the chain completely off the outer chain ring where it can get jammed between the ring and the inside of the crank arm. The screw which limits the inward travel is often labeled as "low" or "L". Turning this screw counterclockwise, or unscrewing it results in the cage moving inward as far as possible. Turning the screw clockwise results in moving the cage further away from the bicycle frame. The screw which limits how far the derailleur can move outward is often marked as "High" or "H". Since the outer chain ring is larger and therefore harder to pedal, associating "High" or "H" with "Hard" can make it easier to remember which is which. With the "High" adjusting screw turned completely counterclockwise, the derailleur cage will be able to travel to its outermost position, as far from the bicycle's frame as possible. Turning this screw clockwise will decrease this possible amount of travel.
Cable tension-With no cable attached, or with a cable attached in a slackened state, the derailleur is in its resting position, as far inwards toward the bicycle frame possible, depending on how the "Low" limit screw has been positioned. There is usually an adjusting knob attached to the front derailleur's handlebar-mounted shifting lever. Rotating this knob clockwise, or screwing it in, will lessen the cable tension. Rotating the knob counterclockwise, or unscrewing it, will increase the cable tension. Lessening cable tension results in the derailleur cage moving inward. Increasing cable tension results in the derailleur cage moving outward.
Braze-on mounted front derailleur-For frames with a welded on front derailleur "braze-on" mount, place the derailleur's convex mounting face so it mates with the rearward facing, concave mounting face of the braze-on. Insert the mounting bolt through the braze-on from front to rear, screwing it into the derailleur's mounting bolt hole. Take one end of the bicycle chain and route it over the top of the smallest front chain ring, inserting it through the derailleur cage, passing it on to the rear of the bicycle, wrapping around any of the rear wheel's gear cogs, down through the rear derailleur pulleys, and then back forward toward the underside of the front chain rings. over the cogs of the rear wheel and routed through the rear derailleur's pulley wheels and back toward the underside of the front chain rings. Final chain length should already be determined. If not, see "chain length" section of maintenance page.