Q-Factor

            The previous post on crank-sets made reference to a dimension called the "Q-Factor" This raised a few questions along the lines of 'What the hell is a q factor???' So, here is the definition and why it is important.
            The Q-Factor is the distance from the point of pedal contact on one crank arm to the other, measured parallel to the bottom bracket. The little illustration below shows the measurement.

             Some people of a certain vintage refer to this dimension as the crank-set  tread, but nowadays it is universally called q-factor.
             In most cases we want the smallest q factor that is possible. The more this dimension increases the less clearance we have when cornering and for most body types the further out the pedal is the more the angle of attack under pedaling force. Think of this when you are running say, your legs prefer to function by being directly under your hips. If you widen your stance greatly you start to rock and loose some power. Also it puts pressure and strain on knees and hip joints.
             Why then don't we just make skinny crank-sets? Well we have clearance issues when building frames. We have to widen chain-stays on mountain bikes to get clearance for big tires, the chain rings have to be allowed for. All these things dictate the minimum q factor. With the compact double road bikes with minimum tire clearance we can get crank-sets with the narrowest tread.