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Old 2019-04-06, 03:12 AM   #1
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Pedals threading. What am I missing?

The more I think about it, the less the physics makes sense. Yet I can't be right while billions of people over a hundred years have all been wrong. Right?

This is about the threading orientation on both (obviously) the pedal and the crank being reversed between the sides.
If the pedal bearings were perfect - with absolutely zero friction - it wouldn't make any difference. The whole idea is that if there is some friction, then the rotation (when pedaling forward) tends to tighten the pedals, not loosen them.
This is what puzzles me. The threading on the right side has the regular orientation: clockwise to screw, counterclockwise to unscrew (the other way around on the left side). But, when pedaling forward, the right pedal in regard to its crank rotates counterclockwise. Same reasoning for the left side, only in reverse. In other words, the pedals' rotations when pedaling forward tend to make the pedals come loose!

So, what part of the problem right under my nose am I not seing?
Or is it one hell of a long lasting conspiracy by the Loctite people?

Last edited by Garp; 2019-04-06 at 03:27 AM.
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Old 2019-04-06, 03:30 AM   #2
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You are correct, it is a little counter intuitive. As it turns out it is not the rotation of the pedal that caused the problem but the rotational bending or stress of the pedal shaft. With out any rotation of the pedal and just flexing or stressing the pedal shaft in a circular motion would tend to loosen the pedal thread if the wrong thread was used on the wrong side. I suppose you could test this process by theading a long rod into a fixed nut and flexing the rod in a circular motion.

Mechanical precession:
“Precession is the process of a round part in a round hole rotating with respect to that hole because of clearance between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force.”

More info at:
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Old 2019-04-06, 04:03 AM   #3
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Thank you for the explanation, JimT. This had been bugging me for a while.
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Old 2019-04-06, 11:44 AM   #4
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An easy, and classic, demonstration is to wrap your hand around one end of a pencil and move the other end around in a circle. Circle it forward, and watch it rotate backward at the same time - voila!
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