Q-factor is the distance between the two pedal attachment points on a crankset, measured parallel to the bottom bracket axle. In practical terms, it determines how far apart your feet sit while you pedal. A typical road bike has a Q-factor around 145 to 150 mm, while mountain bikes run wider at roughly 170 to 180 mm due to frame clearance needs.
How Q-Factor Is Measured
The measurement is taken from the outer face of one crank arm (where the pedal threads in) to the outer face of the opposite crank arm. It is sometimes called the “tread” of the crankset. Q-factor is not the same as stance width, which measures from the center of one pedal to the center of the other. Stance width includes the length of the pedal spindle, so it’s always wider than Q-factor. Two riders on the same crankset can have different stance widths depending on their pedals and cleat positioning, but the Q-factor stays fixed.
Why It Matters for Your Knees
A wider Q-factor pushes your legs farther from the centerline of the bike. During the power phase of each pedal stroke, your foot pushes outward more, which changes the angle of force passing through your knee. A study published in the Journal of Sport and Health Science found that increasing Q-factor raised peak knee abduction moment (the inward-collapsing load on the knee joint) by 47% to 56% across different power outputs. That loading concentrates on the medial (inner) compartment of the knee, meaning a Q-factor that’s too wide for your body can contribute to inner knee pain or accelerate wear in cyclists who already have early osteoarthritis.
At the same time, frontal-plane knee range of motion decreased as Q-factor increased. The knee essentially locks into a less natural tracking path. If you’ve been experiencing persistent medial knee pain and can’t trace it to saddle height or cleat angle, your Q-factor is worth examining.
Narrower Is Generally More Efficient
The late bike mechanic and writer Sheldon Brown argued that a narrower tread is ergonomically superior because it more closely matches the nearly inline track of normal human walking. Research at the University of Birmingham led by Dr. Xavier Disley supported this, finding that pedaling was more efficient at narrower Q-factors than at wider ones. The likely reason is mechanical: when your feet are closer to the bike’s centerline, more of your force goes straight down into the pedal rather than being wasted pushing outward.
There’s also a small aerodynamic benefit. Track bikes carry the narrowest Q-factors of any discipline, often under 140 mm, partly to keep the rider’s legs closer together and reduce frontal area. For road riders the aero gain is marginal, but the efficiency improvement can be meaningful over long distances.
Typical Q-Factors by Bike Type
- Track bikes: 130 to 140 mm. The narrowest option, optimized for aerodynamics and direct power transfer on a fixed-gear setup.
- Road bikes: 145 to 150 mm. Shimano road cranksets tend to sit near 146 mm, while SRAM road cranks are similar.
- Gravel bikes: 150 to 156 mm. Slightly wider to accommodate wider chainstays and tire clearance. Shimano’s GRX gravel cranksets run a few millimeters wider than their road equivalents.
- Mountain bikes: 168 to 183 mm. Frame design demands more clearance for wide tires, suspension linkages, and boost hub spacing. SRAM’s Eagle AXS mountain cranks share the same Q-factor as their DUB mountain cranksets.
These numbers mean that switching from a road bike to a mountain bike moves each foot roughly 10 to 15 mm outward. That’s enough to feel noticeably different, and it’s a common source of discomfort for riders who split time between disciplines.
How to Adjust Your Effective Stance Width
Q-factor itself is fixed by the crankset you choose, but your effective stance width (the true distance between your feet) can be fine-tuned in a few ways.
Pedal spindle length. Some pedal manufacturers offer longer or shorter spindles. Adding a longer spindle moves your foot outward without changing the crankset. This is the most common adjustment for riders who feel cramped on a narrow Q-factor.
Pedal washers and spacers. Thin washers (typically 1 to 2 mm each) thread between the crank arm and the pedal. They’re a simple, inexpensive way to push one or both feet slightly outward. Bike fitters often use these to correct asymmetry if one hip is wider than the other.
Cleat position. On clipless pedals, sliding the cleat inward on the shoe moves your foot outward relative to the pedal spindle. The range is limited, usually only a few millimeters, but it’s enough to fine-tune stance width without buying new hardware.
Crankset swap. If you need a significantly different Q-factor, the most direct solution is choosing a crankset designed for a different width. Some manufacturers make “narrow Q-factor” mountain cranks that bring the measurement closer to 165 mm, and road cranks exist in slightly wider versions for riders with broader hips.
Indoor Bikes and Q-Factor Mismatch
One of the most overlooked Q-factor issues shows up on indoor smart bikes and spin bikes. Many stationary bikes have Q-factors between 170 and 200 mm because of their wider drive systems, which is significantly wider than a typical road setup. If you ride a road bike outdoors with a 146 mm Q-factor and then hop on an indoor bike at 185 mm, your knees are tracking a different path for every pedal stroke. Over weeks of consistent indoor training, that mismatch can produce knee or hip discomfort that seems to come from nowhere.
If you train indoors regularly, it’s worth checking the Q-factor spec of your smart bike and comparing it to your outdoor setup. A direct-drive trainer that uses your own bike avoids this problem entirely, since you’re pedaling on your own crankset.
Finding the Right Q-Factor for You
There’s no universal ideal. Riders with wider hips or those who naturally walk with a wider stance often feel better on a slightly wider Q-factor, while narrow-hipped riders tend to prefer tighter setups. The goal is a stance width that lets your knees track straight over your feet without any lateral wobble or inward collapse at the top or bottom of the stroke.
A professional bike fit is the most reliable way to dial this in. Fitters use motion capture or visual assessment to watch your knee tracking under load, then adjust stance width with spacers, spindle swaps, or crankset changes. If a full fit isn’t in the budget, pay attention to where you feel strain: persistent inner knee pain often points to a Q-factor that’s too wide, while outer hip tightness or a feeling of being “squeezed” can mean it’s too narrow.