Derailleurs are responsible for the smooth movement of the chain across a bike’s chainrings, and consequently see a lot of wear and tear, and abuse.
The first derailleurs were introduced to bicycle racing in 1938 by French company Simplex. In 1949, Campagnolo debuted a refinement of SImplex’s design, the Gran Sport, giving us the cable operated parallelogram derailleur that we know today. In 1964, Japanese company Suntour invented the slant parallelogram derailleur, which allowed the derailleur to maintain a constant distance from the sprockets, irrespective of sprocket size. Today, this has become the de-facto rear derailleur design from all the major manufacturers, with the only changes since then being the usage of different materials such as magnesium, titanium and carbon fiber, and the introduction of servo assisted electronic shifting.
Rear derailleurs consist of a cage holding the jockey/guide pulley at the top, and the tension pulley at the bottom, a parallelogram that is spring loaded and moves the cage in and out, and an arm that attaches to the frame and holds the parallelogram. The parallelogram allows the cage to move from side to side, as well as pivot up and down to clear the sprockets.
Front derailleurs are a lot simpler in operation, having only to move the chain from side to side. While front derailleurs have fewer moving parts than a rear derailleur, they will still require periodic maintenance.
Rear Derailleur Adjustment
Shift the rear derailleur all the way down into the smallest sprocket.
Turn the barrel adjuster all the way in so that there is room to adjust it later.
Adjust the low limit by rotating the screw on your derailleur marked ‘L’ so that the pulley aligns with that smallest sprocket.
Tighten the tension in the cable by unscrewing the cable anchor, pulling the cable tight and re-tightening the anchor. Make sure the cage doesn’t move when taking up the tension.
Shift the rear derailleur into the 4th or 5th sprocket.
To adjust the index, turn the barrel adjuster so that the derailleur pulley lines up underneath the correct cog. With a Shimano rear derailleur, line up the pulley slightly inside of the cog. Shift the derailleur several times to make sure it is adjusted properly.
Shift the rear derailleur into the largest sprocket. Adjust the high limit by rotating the screw on the derailleur marked ‘H’ so that the pulley lines up with that largest cog.
Adjust the B-tension screw so that the pulley on the rear derailleur is as close to the largest cog on the cassette as possible without dragging.
Front Derailleur Adjustment
Adjust the position of the derailleur so it is 1-3 mm above the large chainring. Make sure that the derailleur cage is parallel with the chain ring before tightening everything up.
Turn the front derailleur barrel adjuster all the way clockwise to release any tension that may be in the cable.
To set the inner limit, first shift the front derailleur into the small chainring and the rear derailleur into the largest sprocket. Turn the inner limit screw until the inner part of the derailleur cage is as close as possible to the chain without rubbing.
Tighten the cable tension by loosening the cable anchor, pulling the cable tight and then retightening the cable anchor.
To set the outer limit, first shift the front derailleur into the large chainring in front and your rear derailleur to the smallest sprocket. Turn the outer limit screw until the outer part of the derailleur cage is as close as possible to the chain without rubbing.
Check to see that the front derailleur is shifting properly into the small and large chain rings.
Additional Maintenance Steps
- Lubricate the pivot points on the derailleurs with a light lube.
- Bearings in the derailleur cage should be inspected and replaced if necessary.
- Ensure that cables are tightly fastened and not fraying at the cable ends.