Bicycle Derailleur

On a bike in a stand, note the sprocket tooth counts — say 11 to 32.

Chain wrapped around a sprocket covers roughly half its circumference. The difference in chain needed between smallest and largest is about (32 − 11) / 2 ≈ 10 teeth of length.

Write that number down. The derailleur cage has to absorb it, and cage length is chosen from exactly this arithmetic — which is why a long-cage derailleur is required for a wide-range cassette.

Find the upper (guide/jockey) pulley and the lower (tension) pulley.

Turn the cranks and shift while watching only the upper pulley: it moves sideways, and the chain follows it onto the next sprocket.

Now watch only the lower pulley: it swings fore and aft as the cage rotates, taking up slack.

Two motions, two jobs. Confusing them is why derailleurs look more complicated than they are.

Look at the four-bar linkage in the body as you shift across the full range.

Expect the guide pulley to move sideways while staying at a roughly constant distance from the sprocket teeth.

That is Campagnolo's contribution. A simple pivoting arm would swing the pulley away from the large sprockets and into the small ones, so shift quality would vary across the block. The parallelogram holds the geometry constant.

Find the two screws marked H and L. Shift to the smallest sprocket and turn H until the guide pulley sits directly under it. Repeat with L on the largest.

These are hard mechanical stops, not adjustments to shift quality.

Get L wrong and the derailleur pushes the chain into the spokes. Get H wrong and it drops the chain off the outside, into the frame. Both are the classic ways a bicycle destroys its own transmission, and both are one screw.

With modern indexed shifters, one click must equal exactly one sprocket. Adjust the barrel adjuster until it does across the whole range.

Test in both directions. Expect it to need slightly different tension going up than coming down — the chain is being pushed one way and released the other.

Indexing is why a derailleur that worked yesterday rattles today: cable stretch of a millimetre is enough to put the pulley between sprockets.

Campagnolo's actual contribution. The Gran Sport of 1949 established the parallelogram rear derailleur, which is the layout still used. He did not invent gear-changing by derailing the chain — French manufacturers were selling derailleurs in the 1920s and 30s, and the concept goes back further still.

The story about the freezing pass. Campagnolo is said to have conceived his quick-release skewer, and by extension his interest in gearing, after losing a race in 1927 on the Croce d'Aune pass because his hands were too cold to undo the wing nuts and flip his wheel. It is a good story, it is told by the company, and it should be read as company history rather than documented fact.

The Tour de France banned derailleurs until 1937. Henri Desgrange considered them unmanly and required riders to flip their wheels to change gear. That is not a footnote — it held back the technology's adoption at the top of the sport for years, which is a reminder that governing bodies shape engineering as much as engineers do.

Why it survived against better ideas. A derailleur is mechanically crude: it changes gear by deliberately misaligning a chain under load, it is exposed to everything the road throws at it, and it cannot shift at rest. Hub gears are none of those things. The derailleur won because it is light, cheap, efficient and offers a huge ratio range — around 95–97% efficient against a hub gear's 90–95%, and a fraction of the weight. It is a case where the crude solution is genuinely the better engineering answer.