Measuring a new chain is a vital part of setting up a smooth drivetrain. Too short and you won’t be able to shift to higher gears or worse, you can damage your expensive rear derailleur. Too long and the chain will slap around unnecessarily and increase the risk of dropping a chain.
Most people, however, do not take chain growth into account when fitting a fresh chain onto their full suspension frames.
Ignoring chain growth can be really bad news. If the chain is too short, if a rider bottoms out their rear suspension in a tall gear there will not be enough chain to accommodate the chain growth. This will lead to the derailleur being ripped off of the hanger and potentially into the wheel.
This is a dangerous and costly failure to replace, with at the least a trashed derailleur and hanger, and almost certainly a wheel rebuild! Seems unlikely, but it can happen more easily than you think!
In this video, we learn how to measure a chain to take the suspension system’s chain growth into account.