The correct chain length
The correct chain length ensures the optimum chain tension on the rear derailleur. If the chain is too long, then it will no longer be properly tensioned when running on the smallest of rear sprockets and the chain will start to flap uncontrollably when cycling over uneven ground. If the chain is too short, then the ratio of riding on the largest chainring and the largest rear sprocket will no longer be possible. Should the shifter accidentally be pushed into this position, then it is highly possible that the rear derailleur and the derailleur hanger itself could get severely bent and damaged.
There are two ways to correctly determine the length of the chain:
For the general practitioner:This is the way that the majority of chains are measured. To start, take the chain by the end with the pertruding pin, add the number of teeth on the largest sprocket and the largest chain ring together, divide this number by two and finally add two to this number.
For example: Largest chainring has 44 teeth, largest sprocket has 28 teeth
44 + 28 = 72 : 2 = 36
36 + 2 = 38
For the mathematition:
The following formula can be used to calculate the precise length the bicycle chain needs to be:
LK = 0,157a + 1/2 Z1 + 1/2 Z2 + 2
LK = chain length in links (number of pins)
a = chainstay length in mm (quick release skewer to bottom bracket center)
Z1 = number of teeth on the largest chainring
Z2 = number of teeth on the largest sprocket
Chainstay length a=420mm, Z1=44 teeth, Z2=28 teeth
LK=0,157 x 420 + 44/2 + 28/2 + 2
LK = 103,94 which equates to 104 links
The result is always rounded up or down to the next even number. This is due to the fact that one end of the chain has to have the opposite type of link to the that of the first link in order to allow the chain to be reconnected.