Often times people refer to chainline when discussing drivetrain choices. Whether geared or single, chainline is important. But what is the common usage of chainline?

Basically, “good” chainline is straight, whereas “bad” is not. Huh? Think of a single gear setup, with only one chainring up front and one cog out back. The drivetrain runs most efficiently when these two rings are in plane with each other, with the chain making a perfectly straight line between the two. Out of plane, the the chain will make a noticible bend in its path when viewed from behind the bicycle.

On a geared bike, chainline is also important. For optimal drivetrain performance and longevity you want your most used gears to be just about in plane. Such as, the middle ring should line up with the middle of the cassette. The allows low gears (small chainring, large cogs) middle gears (middle chainring, middle cogs) and large gears (large chainring, small cogs) to have reasonably straight and thus efficient chainlines.

Soon I’ll get into the formal definition of chainline involving caliper measurements and manufacturing specs.