Q factor is a function of both the bottom bracket width (axle length) and the cranks. Bottom brackets axles vary in length from 102mm to 127mm. Mountain bike cranks are typically about 20mm wider than road cranks. A larger Q factor (wider tread) will mean less cornering clearance (while pedaling) for the same
bottom bracket height and
crank length. A smaller Q factor (narrower tread) is desirable on faired
recumbent bicycles because then the
fairing can also be narrower, hence smaller and lighter. 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.
Notable examples "Old Faithful" was a homebuilt bike that
Graeme Obree famously used in 1993 to 1995 at his attempts at the
hour record, which had a very narrow bottom bracket measuring only 40 mm wide, and famously also used "
washing machine" bearings. Walser model 5 designed by Andy Walzer from Switzerland was a narrow time trial bike with a 50 mm bottom bracket notably used by
Jan Ullrich in the
2003 Tour de France riding for
Team Bianchi, and from 2004 to 2006 for
T-Mobile Team. The bike also had a narrow rear hub spacing of 110 mm, which is 20 mm narrower than the 130 mm that was standard on road and time trial bikes at the time, resulting in the bike needing custom cranks and custom rear wheels. Narrow Walzer bikes are also said to have been owned by
Michael Rich and
Lance Armstrong, but for Armstrong the bicycle allegedly resulted in reduced power output "and the smaller Q-factor was rumoured to have induced tendonitis".
Pinarello Bolide F HR was a track bicycle used by
Filippo Ganna in 2022 to set the hour record, and featured a narrow profile with a 54 mm bottom bracket, 89 mm rear hub and 69 mm front hub. ==References==