What has become known as the Q factor, correct term is “tread”, is the distance between pedals at the inside of the crank arms. Tread or Q factor theory arises from anatomical studies measuring our stance distance when walking and running. Humans tend to place their foot towards the body line center for balance and most efficient force to propel us forward. Most efficient includes less stress to knee joint anatomy.
In order to fit triple chainring cranks without hitting the frame chain stays the BB must be wider setting the chainrings away from the frame; ergo, the “tread”/Q factor is farther apart i.e. 168 mm. Two ring cranks allow for narrower tread, one chainring cranks allow even narrower tread 156 mm among the smallest.
The other related crank number 150, 155, 160, 165, 170, 172.5, 175, 180 etcetera, refers to the milimeter length of the crank arm measured from center of BB hole to center of the pedal spindle hole. Again for avid senior riders there is an added benefit of less knee joint stress from shorter crank length, because smaller pedal revolution diameter reduces torque (stress) on the knee joint. Young riders with healthy and strong knees can support larger rotational stress for years.
Lots of anatomy, physiology, and sports studies are available touting crank length efficiency. My personal experience and that of older racing buddies support the shorter crank is definitely less painful for avid senior riders. Aside from the academic research, two weeks ago I listened to another common anecdote that supports efficiency of crank length theory. An avid female cyclist joined us on our Tuesday Off Road GBP excursion. She told us she could not climb steep grades until she had DaVinci make her some 155 mm cranks and viola, her climbing ability improved a hundred percent.
Tread a.k.a. Q factor from Wiki:
The Q Factor of a bicycle is the distance between the pedal attachment points on the crank arms, when measured parallel to the bottom bracket axle. It may also be referred to as the “tread” of the crankset. The term was coined by Grant Petersen during his time at Bridgestone Bicycles.
Q Factor is a function of both the bottom bracket width (axle length) and the crank arms. 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 arm 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. Sheldon Brown said that a narrower tread is ergonomically superior because it more closely matches the nearly-inline track of human footsteps.
Though it seems intuitive that a narrower tread is superior since a walking person must put their foot more to the centerline of the body to balance, this is not the case when pedaling a bicycle, where the “steps” are so very close together and balance a non-issue.
Scientific research has emerged from The University of Birmingham in the United Kingdom that shows 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.