The Q-factor on a bicycle is the horizontal distance between the outer surfaces of the two cranks. It is also referred to as the pedal distance, crank width, or bottom bracket distance and therefore indicates how far apart the pedals are on the left and right. The term Q-factor is short for quack factor, with quack referring to the English word quacking, meaning the waddling motion of a duck. However, it is not only the crank width that determines the distance between the feet, but also the width of the pedals. The width between the feet is known as the stance width. Some pedals specify a so-called Q-factor, which by definition does not correspond to the Q-factor, but can make a difference to the rider and should be taken into account. Ultimately, the stance width is determined by the combination of the pedal width and length of the pedal axle, as well as the cleats selected and how they are adjusted.
Why is the Q-factor important?
The Q-factor influences the economy of movement. A smaller Q-factor usually allows for a more natural, straighter leg movement and can be more efficient. The individually “correct” Q-factor is important for riding comfort and healthy ergonomics. A distance that is too large or too small can cause knee or hip problems. A Q-factor that is too large or too small can cause knee or hip problems. It is important to find the Q-factor that maximizes overall mechanical efficiency and minimizes knee variability. However, since the strain on the muscles also changes with the incline of the terrain, it is additionally difficult to determine the optimal geometric settings under laboratory conditions. MTBs often have a larger Q-factor. There are various reasons for this. Wider tires and chainstays sometimes necessitate a larger Q-factor. Different cranks, pedals, and bottom brackets influence the Q-factor in terms of their compatibility. However, riding stability also increases with a larger Q-factor. A narrow Q-factor reduces the frontal area and results in supposedly better aerodynamics. Supposedly, above all, because the Q-factor can influence the seating position and a narrow Q-factor can sometimes lead to an unfavorable seating position.
Typical values for different types of bikes are (deviations possible):
Road bike: approx. 135–150 mm
Track bike: < 135 mm
Gravel bike: 140–160 mm
MTB: (depending on type) 155–190 mm
Fat bikes: > 200 mm
However, hardly anyone chooses their bike based on the Q-factor. There are various ways to influence the standing width, which is important in practice, with the existing Q-factor. As already mentioned, pedals with different geometries can be used. Shoes and cleats offer further adjustment options.
The more athletic and forward-leaning the pelvis, the narrower the Q-factor/stance width should be. The more upright and relaxed the pelvis, the wider the Q-factor/stance width can or should be.
Attention: gender differences!
Much of what we know about optimal cycling biomechanics comes from studies in which predominantly men were tested. However, women differ significantly in terms of hip anatomy, pelvic position, mobility, and muscular control. Women have different pelvic mechanics when sitting and pedaling. Unfortunately, most bike fittings do not take this into account and are based on findings obtained from men. This may also be appropriate for women, but there is a high risk that women will be forced into positions in which they cannot optimally develop their strength. Women can tilt their pelvis forward (anterior tilt) more easily. The weight and pressure can be distributed more widely across the saddle and shift further forward from the center of gravity. This naturally results in a narrower leg axis. Women have more internal and external hip rotation. This allows the knee to move straighter upwards, even if the feet are positioned slightly narrower. Women have wider pelvises, but NOT wider knee paths. A wider pelvis does not automatically mean a wider pedaling motion. In fact, many women’s knees run closer together than men’s. Women more easily form a “natural hollow back” when leaning forward. This stabilizes the hips and allows for narrower crank distances without pushing the knees outward. This means that women often biomechanically tolerate a NARROWER Q-factor than men of the same size. While handlebar width and crank length can now be adjusted to suit smaller/narrower riders, there is only one unisex (male standard) option for crank width and pedal axle length.
Another highly controversial topic is the optimal crank length. Crank lengths generally range between 165 and 175 millimeters. Occasionally, 160 millimeters can also be found on small bikes. Crank length is primarily based on body height. The topic recently made headlines when world champion Tadej Pogacar, who is 1.76 m tall, switched from a 172.5 mm crank to a 165 mm crank. Tadej Pogacar was not the first to make such a radical change. The crank length can also influence the optimal stance width, as the knee line may (but does not necessarily) change.
In general, your sitting position and hip alignment also have a major influence due to your musculature. It is therefore important to know your individual physique. It may help to test this with flat pedals without a click system: Where is your foot when the pedaling motion feels best? Your knee should be as vertical as possible above your foot.
My case study: I personally have a slightly inward-rotated right hip (muscular cause). This leads to more pressure on the saddle on the left side, more pull on the left side, and means that I am positioned differently on the left side of the leg axis than on the right. My left knee falls slightly inward and actually needs a greater distance between the crank and my left foot. This is clearly shown by the recordings from my power pedals. My left foot has its center of gravity on the outer edge of the foot. In fact, I feel more comfortable on my bikes with a wider stance. Since I – like most people – am not symmetrical, it is a challenge to determine the optimal stance width. I feel that I can achieve more with a wider stance, and in theory that should be true. This is difficult to measure, as almost all wattage systems and wattage pedals are unable to display the power along the pedal axle. For me, this means moving the cleats more inward on my road bike and choosing pedals with a longer pedal axle for my gravel bikes with SPD systems.
In short: The Q-factor and stance width are two important factors that are still given far too little attention. Biomechanically, women can often tolerate a narrower Q-factor than men of the same height. In general, however, the optimal Q-factor or stance width is a very individual metric that should definitely be taken into account when fitting a bike.
Literature:
Disley BX, Li FX. The effect of Q factor on gross mechanical efficiency and muscular activation in cycling. Scand J Med Sci Sports. 2014 Feb;24(1):117-21. doi: 10.1111/j.1600-0838.2012.01479.x. Epub 2012 May 21. PMID: 22612455.
Bieuzen F, Lepers R, Vercruyssen F, Hausswirth C, Brisswalter J. Muscle activation during cycling at different cadences: effect of maximal strength capacity. J Electromyogr Kinesiol. 2007 Dec;17(6):731-8. doi: 10.1016/j.jelekin.2006.07.007. Epub 2006 Sep 22. PMID: 16996277.
Disley BX, Li FX. Metabolic and kinematic effects of self-selected Q Factor during bike fit. Res Sports Med. 2014;22(1):12-22. doi: 10.1080/15438627.2013.852093. PMID: 24392768.
Thorsen T, Hummer E, Reinbolt J, Weinhandl JT, Zhang S. Increased Q-factor increases medial compartment knee joint contact force during cycling. J Biomech. 2021 Mar 30;118:110271. doi: 10.1016/j.jbiomech.2021.110271. Epub 2021 Jan 27. PMID: 33567380.
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