Lifted from Sheldon Brown (RIP)
Tread (“Q Factor”)
The tread, or “Q factor” of a crank set is the horizontal width of the cranks, measured from where the pedals screw in. The wider the tread, the farther apart your feet will be. It is generally considered a good idea to keep the tread fairly narrow. There are three main reasons for this:
The hip joint is optimized for walking, and in normal walking the footsteps are pretty much in line, with little or no “tread.”
For standing pedaling, the farther out the pedals are from the centerline, the harder you have to pull on the handlbar to counterbalance the tendency of the pedaling force to tip the bike sideways.
The wider the tread, the higher the bottom bracket needs to be to prevent clipping a pedal while pedaling through a turn.
Older bikes were generally designed to keep tread to a minimum, but starting in the late 1970s there has been a trend to wider tread, for a variety of reasons:
The popularity of triple-chainwheel cranksets has moved the right side outward.
Front derailers designed for triple-chainwheels have a more 3-dimensional shape to the derailer cage, which requires more clearance between the large chainring and the right crank.
Mountain bikes have wider-spaced chainstays for tire clearance, which requires moving the chainwheels outward so they won’t hit the chainstays.
Newer bikes with more sprockets in back move the chainline outboard.
