The rear derailleur.
Not every rear derailleur can be used with any sprocket combination.
The use of the rear derailleur possibility is limited by four factors.
First, the distance between the chain guide pulley and chain wheels are minimized.
When using a cassette having a smallest chain wheel with 11 teeth and a large chain wheel with 28 teeth, the distance from the roller and to the chain wheels are different than in the case of a smallest chain wheel with 12 teeth and a large chain wheel with 21 teeth.
The largest chain wheel which can be connected yet with the rear derailleur is the rear capacity.
Derailleurs for race groups usually have a rear capacity of 26 or 28 teeth.
Derailleur for mountain bikes usually have a capacity of 32 teeth behind.
Secondly, at all sprocket combinations the derailleur must be able to tension the chain.
If the difference between the sprockets is large, the lower derailleur pulley must be moved to much to tension the chain.
The longer the cage of the derailleur, the more the chain tensioner can move .
A derailleur with a long cage derailleur is called a long stroke, a derailleur with a short cage a short stroke derailleur and derailleur with a stroke in between a medium size derailleur.
The difference in teeth between the chainrings and the chain wheels whereby the derailleur still can tension the chain, is the total capacity.
The total capacity is the difference between the largest and smallest sprocket plus the difference between the largest and smallest chainring.
The total capacity of a short stroke derailleur is usually 28 teeth and a long stroke derailleur 38 teeth.
The third limiting factor is the number of teeth which the large chain wheel may be different with the smallest chainring.
When derailleurs for use in combination with triple cranks this is usually 22 teeth.
When derailleurs for use with double cranksets, this is usually 14 teeth.
The final limiting factor is the type of system that is used.
An SRAM ESP system has a different gear ratio of the shift cable.
The ESP system is therefore not compatible with the systems of Shimano and Sachs.
Manufacturers give every derailleur a capacity number.
When mounting new sprocket wheels you must ensure that the chain wheel combination is within the capacity of the derailleur.
In some derailleurs, it is possible to change the cage.
This allows the derailleur to be converted from short to long stroke.