The first bicycles incorporating a chain and sprockets were fixed-gear (direct drive). People soon figured out that coasting might be nice, so various arrangements for freewheeling were created, and the braking operation was changed from using your feet to resist movement of the pedals to much safer braking systems.
Now, inventors were free to go crazy. The planetary gear systems mentioned in the first part of this book came along fairly quickly. Shortly thereafter, people figured out that one could combine a chain tensioner with a stack of sprockets (often called “cogs” or a “freewheel”), so that by moving the tensioner sideways, the chain could be aligned with one or another sprocket. These first versions of derailleurs, also spelled “derailer,” typically used only two three sprockets, closely spaced in gearing.
Then, for many years, until the late 1970s, ten-speed bikes were the norm, with five sprockets on the rear wheel, and two in front.
As you know, “ten-speeds” is a misnomer, since some of the speeds overlap, and one doesn’t shift from first through tenth, hitting every gear on the way. Just like a modern 24-speed bike doesn’t really mean you use all 24 speeds.
All modern rear derailleurs have two pulleys. The top one is called the guide pulley, and the bottom one is the tension pulley.
Some of the earliest designs had only one pulley, so shifting was a bit sloppy. At first, serious cyclists resisted two pulleys on a derailleur, figuring the extra pulley would eat up a lot of power with extra friction. It turns out that the second pulley uses only a microscopic amount of power.
During the late 1960s, some odd variations appeared, and became quite common. A French manufacturer, Simplex, made a derailleur in which the biggest parts were made from plastic. It worked quite well, but was prone to failure, as the points where the springs mounted would tear out of the plastic.
Another was by Huret. Their Allvit model was one of the very few that used ball bearings in the pulleys. This all-steel derailleur had a shell, sort of like a turtle, that protected the parallelogram mechanism which moved the pulleys in and out among the five sprockets. All the pivots were adjustable, with bolts and locknuts. Unfortunately, being adjustable, they frequently needed adjustment. Some of the Huret Allvits came with red pulleys, which became quite collectible for a while.
Index shifting did not become practical until the mid 1970s. Until then, one had to carefully adjust the shifter after selecting a gear to line up the derailleur properly for smooth and noiseless operation. The first index shifting systems generally put the detents (stops) in the derailleur, so it would precisely line up under each gear. Now, almost all systems have the detents in the shifter, depending on the cable to carry the message accurately to the derailleur. It seems like a system designed to fail, but index shifting seems to work well most of the time.
Most system have an adjusting barrel around where the cable casing meets the shifter. You can turn the barrel to adjust the system on the fly.
Simple but high-end, non-index shifters
One of the latest changes in derailleurs isn’t in mechanical design, but in the word itself. It is slowly evolving to be spelled “derailer.”