Derailer or Derailleur?
May 29 '01
The Bottom Line Get to know your derailleur, learn how it works and treat it well.
Commonly spelled in the French style "Derailleur", this is
a mechanism for moving the chain from one sprocket to another to change gears on a multi-speed road, hybrid or mountain bicycle.
A typical derailleur consists of a parallelogram (a four sided geometric form, in which opposite sides are of equal length) which moves a cage. In the case of a rear derailleur, the cage will have two chain pulleys, that is, a jockey pulley and a tension pulley.
The jockey or upper pulley on a rear derailleur is the pulley which guides the chain from one sprocket to another. Shimano jockey pulleys are designed with a "Centeron®" mechanism that allows a small amount of sidewards motion to compensate for imprecise index adjustment.
The tension or lower pulley on a rear derailleur adjusts the tension (hence the name) on the chain as different-sized sprockets are selected.
The "capacity" of a particular derailleur model is the largest range of sprocket sizes it can handle:
For rear derailleur, the capacity relates to the amount of chain slack the derailleur can take up, and is equal to the front range (22 in the example above) plus the rear range. Thus, if you have a 52/42/30 crank set, and a 12-28 (16 tooth difference) cluster, the total capacity required would theoretically be 38 teeth (22 front difference + 16 rear difference).
Rear derailleurs are commonly designed for a particular maximum size rear sprocket. If you exceed this size the jockey pulley could rub against the sprocket when using the lowest gear.
Manufacturer specs are conservative. They do so, because they have to assume that some of their derailleurs will be sold to novice cyclists, who will abuse their drive trains by using the smallest chainwheel with the smaller rear sprockets.
Experienced riders can considerably exceed the official rated capacity, since they will not misuse the granny ring by running it with the smaller rear sprockets, so it doesn't matter if the chain hangs slack in those gears.
Quality rear derailleurs are manufactured by Shimano, Campagnolo and Sram. Most quality multi-speed bicycles will
feature derailleurs by one of these parts companies.
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