Q-Factor

Summary

Q-factor is the lateral distance between the outside faces of a bicycle’s crank arms at the pedal mounting points. It describes the width of the crankset, but not the rider’s complete stance width. Pedal spindle length, cleat position, pedal spacers, and footwear also determine where the feet sit. Q-factor is influenced mainly by crank design, chainline, frame clearance, rear-hub spacing, and—in e-bikes—motor width. It can affect how a bike fits, but narrower or wider is not universally better.

Quick Facts

• Category: Drivetrain and fit measurement
• Also called: Crank tread
• Units: Millimeters
• Measured between: Outer crank-arm faces at the pedal threads
• Related to: Stance width, chainline, bottom-bracket design
• Not determined by: Frame size or rider height alone
• Adjusted primarily through: Crank or pedal selection

Overview

Q-factor describes the total width of a crankset at the pedal mounting faces. If a crank has a Q-factor of 150 mm and is symmetrical, each pedal mounting face sits approximately 75 mm from the bicycle’s centerline.

The term is commonly credited to bicycle designer Grant Petersen, who used “Q” as shorthand for “quack,” referring to the wide stance of a duck. The measurement itself is more important than the name: it allows crank widths to be compared across road, gravel, mountain, fat-bike, and e-bike systems.

Wider Q-factors are usually driven by packaging requirements rather than a deliberate attempt to change rider biomechanics. Large tires, wide chainstays, suspension pivots, motors, chainring position, and wider rear hubs can all require additional crank clearance.

Q-Factor Versus Stance Width

Q-factor and stance width are related but not interchangeable.

Changing pedal spindle length or adding an approved pedal spacer changes stance width but does not change the crankset’s Q-factor.

This distinction matters during bike fitting. A rider can use two bikes with identical crank Q-factors but experience different foot positions because the pedals or cleat settings differ.

How It Is Determined

Several design factors influence Q-factor:

Crank Shape and Spindle Length

Crank arms may run nearly straight or curve outward to clear the frame. The integrated spindle or bottom-bracket axle must also be long enough to position both arms correctly.

On square-taper and other three-piece cranksets, spindle length can significantly affect crank position. On modern two-piece systems, Q-factor is usually fixed by the crank and spindle assembly.

Frame and Tire Clearance

Mountain, fat, and cargo bikes require room for wider tires and chainstays. Suspension pivots can also occupy space near the crank arms. The cranks must clear these structures throughout the pedal stroke.

Chainline and Rear-Hub Spacing

The chainring must align with the intended cassette position. Cranks designed for wider rear-hub standards may use a different spindle, arm shape, chainring offset, or Q-factor.

Bottom-bracket shell width contributes to the design, but it does not determine Q-factor by itself. Some cranksets fit more than one shell width while retaining the same Q-factor through the use of specified spacers.

E-Bike Motors

Mid-drive motors occupy more space than a conventional bottom bracket. Their housings, drive spindles, and chainlines commonly require wider crank positioning.

Representative Measurements

Q-factor varies by model rather than following one universal value for each discipline. Current Shimano examples illustrate the progression:

These figures are examples, not category limits. Fat-bike, cargo, adaptive, and some e-bike cranks can be considerably wider.

Rider Experience

Most riders can adapt to a reasonable range of stance widths, especially when the difference is small. A substantial change—such as moving between a narrow road crank and a wide mountain or e-bike crank—may be more noticeable.

Possible observations include:

• Feet feeling unusually close together or far apart
• Knees tracking differently during the pedal stroke
• Heels contacting the crank arms or chainstays
• Discomfort appearing after changing bikes, cranks, pedals, or shoes
• One side feeling different because of asymmetric cleat or crank setup

These observations do not prove that Q-factor is the cause. Saddle height, saddle setback, cleat rotation, foot support, leg-length differences, previous injuries, and training load can produce similar symptoms.

Biomechanics and Efficiency

There is no single ideal Q-factor for every rider. Anatomy, mobility, preferred foot position, and cycling discipline all influence the result.

A controlled study of trained cyclists found slightly higher gross mechanical efficiency at the narrower tested widths, but the experiment does not establish that every rider should use the narrowest possible crank. Disley and Li, Scandinavian Journal of Medicine & Science in Sports

Other research has found that increasing stance width changes hip and knee movement and can alter knee-joint loading. These findings support treating stance width as an individual fit variable rather than assuming wider or narrower is always superior. Fife et al., 2020 and Wilbert et al., 2024

Mechanic’s Perspective

When a rider reports a stance-width problem, measure the complete setup rather than reading the crank specification alone. Check:

• Crank model and published Q-factor
• Pedal spindle length
• Cleat lateral position and rotation
• Bottom-bracket and crank spacers
• Whether both crank arms are fully seated
• Chainring chainline
• Crank-arm and heel clearance
• Left-to-right symmetry

Incorrect bottom-bracket spacers can create poor bearing preload, incorrect chainline, or unequal crank positioning. On square-taper systems, fitting a different spindle length can alter both Q-factor and chainline, sometimes preventing proper front shifting.

Some crank families are offered in standard and wide versions that look nearly identical. Ordering by brand and group name alone can result in the wrong spindle, chainline, or Q-factor.

Adjustment Options

Small stance-width changes may be possible through:

• Pedals offered with different spindle lengths
• Lateral cleat adjustment
• Manufacturer-approved pedal washers
• Purpose-built pedal extenders
• A crankset with a different Q-factor

Do not stack ordinary washers or use extenders without verifying thread engagement, crank clearance, and manufacturer approval. Moving the pedal outward increases leverage on the pedal spindle and crank threads.

Changing cranks solely for Q-factor may also affect chainline, bottom-bracket compatibility, frame clearance, and front shifting.

Buying Considerations

Q-factor deserves attention when:

• Moving between road, MTB, fat-bike, or e-bike platforms
• Reproducing a successful fit on another bicycle
• Addressing a known stance-width requirement
• Choosing between standard and wide crank variants
• Using pedals with unusually short or long spindles

For most riders without fit problems, Q-factor should not outweigh correct crank compatibility, chainline, gearing, and frame clearance. If persistent pain is involved, component changes should be based on a structured fit assessment rather than Q-factor alone.

Common Misconceptions

Q-factor and stance width are the same.
Q-factor is only the crank portion of the final foot spacing.

Narrower is always more efficient.
Laboratory results cannot establish one ideal width for every rider.

Mountain bikes use wide cranks for added leverage.
Their width is primarily driven by tire, frame, chainline, and suspension clearance.

Pedal spacers change Q-factor.
They change pedal stance width, not the crank’s measured Q-factor.

Related Topics

• Crankset
• Bottom Bracket
• Chainline
• Pedal Spindle Length
• Cleat Position
• Bike Fit
• Rear-Hub Spacing

References

• Shimano crankset specifications
• Disley and Li: The Effect of Q Factor on Gross Mechanical Efficiency and Muscular Activation in Cycling
• Fife et al.: Effect of Q-Factor Manipulation via Pedal Spacers on Lower-Limb Kinematics
• Wilbert et al.: Effects of Q-Factor Changes on Knee Biomechanics During Cycling
• Grant Petersen: Just Ride