The catenary derailleur.
On bikes with derailleur gears, the chain may not always be in a perfect line.
Especially when using the small chainring and on the smallest sprocket wheels behind and the largest chain wheel in front and the largest rear sprockets, the chain is running awry. See fig. 33. </ Strong>
The chainrings and sprockets have to be mounted so that the center of the chainrings pure lies in line with the center of the chain wheels.
At 3 x 7, the middle chainring must lie in line with the fourth chain wheel.
At 3 x 8 must be the middle chainring between the fourth and fifth sprocket.
At 2 x 7 the middle sprocket must be between the two chainrings.
At 2 x 8 the line between the chainrings must be between the fourth and fifth sprocket.
When the chain line is not correct, it can be corrected by installing another crank or – when the bottom bracket unit is equipped with two separate adjusting rings – to shift the crankshaft.
However, any deviation from the ideal chain line is not such a drawback, because the derailleur chain is smooth and the chain with most combinations are not perfectly in line.
In case of an oblique position of the chain will be the chain, the chain wheels and the rear derailleur wear out much faster.
The most extreme combinations may not be used.
The outer chainring should not be used with the two inner chain wheels.
The inner chain wheel should not be used with the two outer chain wheels.
With triple chainrings the middle chainring may be used with all sprockets.
This rule applies to both 7-, 8- and 9-speed systems.
In a bicycle with 2 x 7 accelerations can, in practice, only 10 of the 14 gears are switched.