Q Factor is a measurement of how far apart the pedals of a bike are. When the crankset is installed on the bike it's a horizontal measurement from the outside of one crank arm to the outside of the other where the pedals are installed. Variations in crank spindle length or bottom bracket width will directly affect the Q Factor, however; a crankset is generally designed with a specific Q Factor in mind so it's best not to play with it too much.
Q Factor affects how far apart the riders feet are so it's directly related to proper riding posture. A person who is 6ft tall will probably have a wider stance than somebody who is 5ft tall so the pedals should be positioned wider. Shims do exsist which are designed to space each pedal further from the crankset resulting in a wider Q Factor. These shims come in a number of different widths, they have a male and female thread interface so they can fit between the pedal & crank arm.
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