The index system

The index system.

Until the mid-eighties was the derailleur system constructed so that the rear and front derailleur with the switches had to be precisely positioned in the right place.
Today, the derailleur system is indexed. That is to say that the switch for the rear derailleur has fixed positions T, 8 or 9 (depending on the number of sprockets and the rear derailleur, and is thus displaced over a fixed distance and comes always end up in the proper position.
The switch for the front derailleur has a number of fixed positions also, so that the front derailleur always comes right above a chainring.
For the proper functioning of the indexed shifting, two things are important:

  • The rear derailleur should be just right under the chain wheel in any position. For this purpose, the switch is provided with a fixed number of positions corresponding to the distance between the center lines of the chain wheels. See Fig 1. Through the inner cable, this displacement is transmitted to the rear derailleur. Switches, cables and derailleur should be matching, they work together to ensure that the derailleur in any position is directly below the chain.
  • In order to ensure that the chain slides smoothly from one chain wheel to the other, the chain wheels are cut in a special manner. This makes it possible for the derailleur to exchange the chain to any chainring, even though the chain is loaded. The cyclist can simply switch while cunning.

Because the parts are tailored for a perfect functioning of the index system, they can not be easily exchanged. The various components manufacturers provide complete components groups, whose individual components are aligned.

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