As bicycle designers settled on having a chain run to the rear wheel, this opened up the opportunity for the invention of a derailleur. The funky spelling due to the invention taking place in France in the early 20th century. Derailer has no place in any English dictionary.
Just like the bicycle itself, the derailleur had to go through a number of inventors and phases to get it just right. The first attempt used rods to move the chain from one gear to another. In early models this was between two gears, because, of course, it had to start somewhere. It became quickly apparent that more gear options were needed, as anyone who has ridden a single-speed up a hill could attest to.
As more people began using bikes in differing landscapes, the ability to change the stress placed on the rider’s legs was essential, as doing so improved the longevity of riders through the safety to their knees. Using too high or low a cadence while riding puts stress on the knee that can lead to injury and decreases riding longevity. This was especially common in early bicycle racing. Only in the 1930’s were more than two gears allowed in racing. Previously, there were two gears allowed, with a spoke on either side of the back wheel. In order to change gears riders had to stop, take of the rear wheel and flip in around before remounting it.
Rods were eventually replaced with paddles. Though only slightly more effective than the preceding design, they kept the process moving forward. The game changer was the invention of the slant-parallelogram rear derailleur by the company Suntour. It was a design so revolutionary that derailleurs used today are only slightly different. The invention dropped cycle racing times dramatically, and was quickly incorporated into even recreational bicycles. Designed in 1964 it used a series of pulleys that kept gears equidistant from each other. The design was so sound that when the patents expired, it was immediately copied by all other companies in operation. Though feeling bad for whoever held the patents to begin with is probably a mistake, as I’m sure they made a great deal of money from their invention.
Because of this freedom to the slant-parallelogram design, until 1985, there were a wide range of companies producing derailleurs. But that was before the company Shimano introduced index shifting that needed all parts made by that one company. This reduced the field of manufacturers down to only a handful, and today it is three companies that handle the large majority of derailleur production. Kind of like Apple ruining everything for everybody. It can happen in any market.
Nonetheless, derailleurs continue to evolve and new additions are continually designed to make cycling more bearable for those of us that have no other means of transportation— and a huge hill at the end of our commute. Sometimes I wish I lived in flat Iowa instead of Colorado.