Q-Factor - briefly explained

Q factor?

Anyone who is a little interested in bicycles has probably already read or heard the term “Q factor”. But what does that mean?
In general, the term refers to the distance between the outer sides of the left and right cranks.

Good to know, but why actually?

For all of you performance-oriented people, according to research it is supposed to influence driving performance: narrower = better watt output. For anyone interested, Jan Heine has devoted himself to this aspect in great detail.
So simply mount a crank with a lower Q factor and increase pedaling efficiency? Rather not.

Not every crank fits every frame. Racing bikes and track bikes have narrow tires and therefore a "narrower" frame - this refers to the rear triangle width, i.e. the distance between the dropouts on the rear tire.
Mountain bikes have wider tires; they require more space in the frame. Due to the design, the rear triangle becomes wider, but the bottom bracket area also has to be wider. The chain and seat stays also move outwards. As a result, the pedal crank must also be “wider” so that the crank arms do not hit the frame. This means that racing bikes have a lower Q factor, mountain bikes/trekking bikes/gravel bikes have a larger one.
E-bikes usually also require a larger Q factor due to the motor. Some manufacturers' solution: asymmetrical installation of the motors and cranks with different offsets.

To make things a little more complicated: pedals also have a Q factor. This can be found in the specifications of the respective manufacturer.
The Q factor of the pedals in particular can be of interest to certain people. This has a significant influence on the possible foot position. People with V-shaped feet or very large feet may benefit from pedals with a longer axis (larger Q-factor). These enable a more natural foot position and prevent the ankles or heels from rubbing on the crank or frame. Some knee pain has already been “treated” in this way. If you have problems in the knee area or other health problems, a visit to the doctor and a bike fitting are recommended!
Crankbrothers, for example, offer sets for converting to a longer pedal axle. SQlab also has pedals with different axle lengths in its range.
Small note: I am very sensitive about heel clearance. Monk alert! For all non-Monks: This is the distance between the heel and the chainstay when pedaling. If this is very low, the heels can rub on the frame. That's why I pay particular attention to the frame construction and Q-factor of the crank and pedals.

Oh, the Q factor of the crank is often related to the chainline.
You can read about why and what that is again here Kettenlinie - pretty weird .

Influencing “factors”:

  • Frame or rear frame width (indirect influence)
  • Tire width (indirect influence)
  • crank
  • Pedal axle or pedals

Ergonomics and effect:

  • Q factor around 150 mm: supposedly for better performance. Such a value is probably not possible on a mountain bike
  • Larger Q factor: more secure stability on terrain
  • More distance from pedal to crank and/or chainstays: non-parallel foot positions are also possible
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