How is pedal motion measured?
Cycling pedaling motion is cyclical and described as cadence, the number of cycles per minute. It is important because it defines instantaneous foot speed which when multiplied by average pedal force gives the instantaneous pedaling power.
How fast are your feet moving based on your cadence?
As your feet pedal, they cover some distance which is easily determined from your cadence which is the number of cycles you are turning per minute or per second.
This is an easy mathematical computation. A standard crankarm is 7 inches which means in one cycle the pedal covers a distance of 2πr or 3.67 feet. Divide this by the cycle time and you get Pedal or foot speed in ft/sec.
For a cadence of 60 rpm, ω is 1 cycle per second which means the pedal covers 3.67 ft/sec or 2.5 mph. The following chart shows Pedal or Foot speed as a function of cadence:
What should be surprising is that in the 60 to 80 pedal rpm range, your feet are only moving approximately 2.5 to 3.5 mph. Even at an Elite 100 rpm, they are only moving a bit over 4 mph.
What does pedal speed tell us about the importance of the drivetrain?
Recreational riders are nominally assumed to have cadences between 60 – 80 rpms. This projects to foot-speeds between 2.5 – 3.3 mph. Elite riders are capable of riding upwards of 100 rpms which projects to 4.2 mph foot-speed.
All of this is to simply point out pedaling motion is a fraction of cycle-speed. How is it my feet moving at less than 5 mph, can produce the necessary energy to propel the cycle at speeds well over 20 mphs?
Anyone familiar with cars or motors knows the answer to this. It is what drivetrains do “for a living.” Drivetrains take one form of motion, usually rotational, and transform it into another, usually forward, with the intent of moving vehicles at higher speeds. Once we have explained the role of gearing, we will examine how the cycle drivetrain accomplishes this.
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