realskiers

Hmmm, longer cranks?
Are you guys sure?

I've heard longer cranks, more torque on knees.
Not  knowledgeable enough argue about it though.

Lynn

Thought I would respond to Lynn's message, though I'm not knowledgeable enough to say much other than this.

Torque is twisting force, so crank length won't affect or createu any "twisting force" on the knee (unless your knees really flail around while you pedal).

 To go a given speed, it takes less force on the pedal as the crank gets longer.
For a given amount of pedal force, a longer crank will create more twisting force (torque) on the chain rings because torque is the result of force and lever arm.  (1 foot-pound of torgue is a 1 pound weight/force on a 1 foot lever arm.  If you keep the weight/force the same and increase the lever arm to 18", then you have a 50% increase to1.5 foot-pounds of torque).

However, increasing the crank arm also increases the diameter of the circle created by the pedal, with longer downstroke (for longer legs) but also a higher upstroke.  Slightly more forward pedal position but also slightly  more rearward on the back side of the pedal stroke.  A shorter legged person with a long crank will create more torque, but will also increase their range of motion requirements in the knees and hips (and that may be bad for the average person, not sure).

The pedal being forward is good for the knee angles of larger riders, helps get their BOF (Ball of Foot) more forward and under the COM (center of mass of the knee). 

If Lynn puts 75 pounds of force on her 170mm cranks, then she is creating 502 pound-inches of torque.
If Svend puts 75 pounds of force on his pedal with 175 mm cranks, then he creates 517 pound-inches of torque, a 3% increase in torque for a minimal 2.5 mm crank length change.
With a 200mm crank the torque goes up to a whopping 590.5 pound-inches with only the same 75 lbs on the pedal.

The pressure on the knee is the same, but the angle of the pressure on the knee might be different depending on leg length and stuff and that might be a big downside.  Note also, of course, that Lynn doesn't need to create as much torque because she isn't doing as much work since she only has to move 120 lbs compared to me.

FWIW, my Mtn Bike has 175 cranks, my road bike's are 172.5, and the authority that BushWacka referenced uses 203mm cranks (and did so when he raced professionally).   



PS.  But there's no "free lunch" in physics, so somewhere the person with the longer crank is still having to do 3% more work, probably in the longer circle that they have to pedal.