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We all know right pedals are screwed regularly in the cranks. Whereas left ones are screwed in the other direction.

Why is it like this and not the other way around?

Here is how I see things: under normal circumstances, the direction of the thread matters little if at all since the bearing allows pedals to spin freely regardless of how they are attached to the crank.

But when the bearing has troubles, I would think the thread direction is here to protect the pedal from unscrewing from the crank as one is pedaling.

But when you pedal forward, the right pedal is spinning counter-clockwise (since the crank is spinning clockwise) so you would unscrew the pedal from the crank.

Same works for the left: when you pedal, the pedal is spinning clockwise so in the direction to unscrew the inverted thread on the left.

I must be missing something,... but what?

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There are ball bearings on the axles, so the direction of rotation from the outer axle (which you mount into the crank) of the pedal to the inner axle that is attached to the body of the pedal. Think of it as an additional cog in between. – arne Sep 20 '13 at 14:00
The Physics Stack Exchange site has a question about this with good answers:… – amcnabb Sep 20 '13 at 15:57
Related question:… – amcnabb Sep 20 '13 at 15:59
up vote 18 down vote accepted

The reverse threading is to counter precession, not friction from the bearings. The wiki page explains it well (and I doubt I could):

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Yeah, loosely grip a pencil in your left hand (make a fist), and then use the right hand to move the end of the pencil around in a circle. The pencil will tend to rotate in the direction opposite to how you're moving the end. – Daniel R Hicks Sep 20 '13 at 14:58

superdesk's link to Wikipedia is excellent, but let me also add Sheldon Brown's explanation, which dismisses some other theories (like the ankle saving theory).

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