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Homework Help: Forces on a Bike Pedal Crank

  1. Apr 17, 2009 #1
    I am trying to determine the forces exerted on a bicycle pedal crank by the rider so I can analyze the fatigue. I believe that I understand how to conduct the fatigue analysis but I think that the forces on the bike crank are more complicated then I have assumed them to be so far.

    I know that the forces from the rider will be treated as alternating forces, but am also trying to determine if non-alternating forces exist. I was thinking that the weight of the crank could be considered a non-alternating force.

    So far I have broken up the force cycle into four parts to simplify the problem. I am considering the crank forces when the bike pedal is at the top (0 deg), when the pedal crank is parallel to the ground with the pedal near the front wheel (90 deg), when the pedal is at the bottom of the cycle nearest the ground (180 deg), and when the pedal crank is parallel to the ground again with the pedal near the rear wheel (270 deg). At 0 deg, I am assuming that there is a tensile force on the crank normal to the crank ends. At 90 deg and 270 deg, I am assuming that the tensile force is angled. And that the tensile force is normal to the crank ends at 180 deg. Any suggests on whether or not these assumptions are correct would be very helpful.

  2. jcsd
  3. Jul 15, 2009 #2
    At 90 and 270 degrees, the force of that the rider is exerting onto the pedals is *tangent* to the crank.
  4. Jul 15, 2009 #3


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    tmccraig: The self weight of the crank is also an alternating force, because the gravitational force is directional. However, the crank self weight is probably negligible compared to the forces from the rider.
  5. Jul 15, 2009 #4
    The only time during that 4-stroke pedal cycle that 100%(disregarding mech. loss) of the rider's effort(force applied to the pedals) is being directly converted into torque is at 90 degrees and 270 degrees. this is because it is only at these two points that the pedal force is directly tangent to the cranks. at any other point in the pedal stroke, only a *component* of the rider's pedal force is converted into torque.
  6. Jul 15, 2009 #5
    There is an online "Cyclists Pedal thrust calculator" that I think you will find helpful at

  7. Jun 12, 2011 #6
    Can I offer an improvement to the sine wave cycle of forces when pedalling? If the downward foot movement is connected to a vertical chain which runs on two small cogs (top and bottom) and then onto a large cog behind , then all the applied force is "tangential " and is much easier and more efficient. The basic shape to keep in your mind is a large cog running forward to the small top cog, then vertically down to a second small bottom cog and then back to the large cog.All that can be geared down for a bicycle or a generator. That would represent a square wave rather than a sine wave.
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