Q factor (bicycles)

[6] A larger Q factor (wider tread) will mean less cornering clearance (while pedaling) for the same bottom bracket height and crank length.

[6] Sheldon Brown said that a narrower tread is ergonomically superior because it more closely matches the nearly-inline track of human footsteps.

[7] Research from the University of Birmingham in the United Kingdom suggests narrower Q factors are more efficient, likely due to improved application of force during the pedal stroke, as well the potential for reduced knee variability and risk of injury.

[10] Walser model 5 designed by Andy Walzer from Switzerland was a narrow time trial bike with a 50 mm bottom bracket[11] notably used by Jan Ullrich in the 2003 Tour de France riding for Team Bianchi, and from 2004 to 2006 for T-Mobile Team.

Narrow Walzer bikes are also said to have been owned by Michael Rich and Lance Armstrong,[13] but for Armstrong the bicycle allegedly resulted in reduced power output "and the smaller Q-factor was rumoured to have induced tendonitis".

The Q factor is an important factor affecting how close the pedals are laterally, and therefore the affects the spread of the riders legs
The quack factor is a reference to the wide-stance gait of ducks - the larger the stance, the larger the quack factor