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I understand the reason that bicycle pedals are oppositely threaded on either side. What I don't understand is why it works because I'm missing something.

Take the right pedal for example. It's threads are right-handed, so it would be tightened by turning it clockwise (if the pedal were in between you and the bike). Now imagining this from the same perspective, a person pedaling makes the right pedal constantly turn counter-clockwise, relative to the crank. If there's any friction between the pedal and the bolt attaching it to the crank, I would imagine that it would create a force that would try to turn the bolt counter-clockwise along with the pedal.

This is the opposite of what happens, and I've seen the bolts unthread after pedaling backwards a lot myself. What am I missing?

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I don't totally understand the question, but friction points in the direction opposite the motion. If something is spinning clockwise, friction accelerates it counter-clockwise. –  Mark Eichenlaub Mar 7 '11 at 19:34
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@JustcallmeDrago: this might be a good question for bicyles.SE too. –  freiheit Mar 7 '11 at 19:37
    
@freiheit: Oh man! I didn't see that in the SE list. If a mod feels that this could be moved, move it! –  JustcallmeDrago Mar 7 '11 at 19:41
    
@Mark: Right... but that friction is on the pedal. There is an equal and opposite friction on the bolt itself (or maybe not equal, as I don't know the mechanics of bearings) –  JustcallmeDrago Mar 7 '11 at 19:43
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@Georg: the "help" post on bicycles SE is clearly totally off-topic as-is because it's not a question. Actually asking a question with a link here is the right way to do it. –  freiheit Mar 7 '11 at 21:40

5 Answers 5

up vote 10 down vote accepted

Take the right pedal as an example. It uses a right-hand thread, so turning the pedal spindle clockwise (CW) relative to the crank will screw the spindle in, counter-clockwise (CCW) will unscrew it.

Say you put the bike in a repair stand, grab the right pedal and gently simulate the motion of someone riding the bike (always keeping the pedal platform horizontal as if your foot were there). The body of the pedal will turn CCW relative to the crank.

Intuitively you would think this might help unscrew the pedal! But in reality the clamping torque of the threads will be far, far greater than any friction in the bearings could generate. Even if the bearings were to seize, it would be very difficult to unscrew the pedal (unless it was never tight to begin with).

Now consider someone riding the bike, putting weight on the right pedal. This applies a force perpendicular to the ground, no matter where the foot is in the pedal stroke. Relative to the crank arm, this radial force rotates CCW, which - via the process of mechanical precession - creates a CW torque on the pedal spindle (thus tightening it). The rotations on the left side are all reversed, so it must use the opposite threading to prevent the pedal from coming loose.

Also see "Precession" on Wikipedia, from where the below illustration was taken (CC-SA).

Illustration of precession on rotating bolt

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Thanks a lot. I didn't realize that there was a different force at work and that the bearing friction was negligible. –  JustcallmeDrago Mar 9 '11 at 23:45
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Another thing to do is to take a pen or pencil in one hand and hold it near the end. Then loosely grip the body of the pen with the other hand and move the end (held by the first hand) in a circle. Then pen will tend to rotate within the loose grip of the second hand in a direction opposite the direction the first hand is moving. –  Hot Licks Aug 17 '12 at 20:11
    
In the illustration on the Wikipedia page, I'm interpreting the green arrow as the direction of the force from the foot (assuming you're clipped in and can pull as well as push). Is that right? –  Trevor Alexander Jan 17 at 6:56

I don't know if it's right or not, but Wikipedia has this to say:

The right-side (usually the drive-side) pedal spindle is right-hand threaded, and the left-side (usually the non-drive-side) pedal spindle is left-hand (reverse) threaded to help prevent it from becoming loose by an effect called precession.

Although the left pedal turns clockwise on its bearing relative to the crank (and so would seem to tighten a right-hand thread), the force from the rider's foot presses the spindle against the crank thread at a point which rolls around clockwise with respect to the crank, thus slowly pulling the outside of the pedal spindle anticlockwise (counterclockwise) because of friction and thus would loosen a right-hand thread.

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Wikipedia here is correct. The assumption that the bearing friction across the entire surface of the spindle is equal is incorrect. The friction is highest at the top of the spindle, where the rider's force is pushing down on the pedal. If you look at this point of highest friction, it moves opposite around the spindle to the direction the pedal is moving - as the pedal rotates clockwise around the spindle, this point moves counterclockwise. Regardless of which way the pedals are threaded, they won't loosen if they are being spun either direction without this downward force. –  lantius Mar 8 '11 at 0:15
    
Also the bearing cups for the crank can have right and left threads. –  Chris K Jan 17 at 11:06

I think pedal shaft threads are right handed on the right side and left handed on the left side is so that the pedals, which are asymmetric, will fit only on the proper sides. The bearings of the pedals are so low in friction that during pedaling there is completely insufficient torque to overcome the static friction of the threads which hold the pedals to the crank arms.

However, if there were any significant friction the net action of forward pedaling would be to unscrew the pedals from the crank arms on both sides.

This can be seen as follows. Consider the right side seen from the right looking from the outside end of the right pedal toward the crank arm. Imagine a spot of paint placed on the end of the crank arm on the outside face and imagine the crank turning CW. The spot of paint will be seen to rotate CW as the crank rotates CW. But we know that a right handed nut must be turned CCW as seen from the head of a bolt to tighten the bolt because this would be CW as seen from the nut end for right handed threads.

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In my opinion the easiest way to see this is the following:

take two identical screws, plug them a little into the pedals and then start to cycle. If you look at the bike from behind both the screws will move to the same side, one tightening and the other loosing.

If you want them both to tight, then they have to move to opposite directions, so their helicities must be opposite.

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I once built a custom bicycle, and used a crank from another bike. I found that the crank's design differed on the donor bike, and thus, would only fit my custom bicycle when installed in the reverse direction. I saw no other problems with this solution, until one fine Spring morning, while riding my newly fabricated custom bicycle downhill at great speed, the right-hand pedal un-threaded and dropped onto the roadway. Unfortunately, my right foot followed; as did my right leg. In what seemed to be nearly a full minute of slow-motion events, my entire body followed for a meet-and-greet with the road surface on that fine Spring morning. I lay crumpled and severely injured, against a metal road sign for an unknown period of time, until a passing motorist stopped and called 911. I was transported to Emergency Central, where surgeons labored for hours to stop the bleeding and to remove my custom bicycle from its painful resting place in my rectum. Fearing the worst, a priest was summoned by the hospital staff, and my family was notified of my condition. Fortunately, I pulled through, and am able to operate my wheelchair without assistance, thanks to many months of rehabilitation and therapy. So, to answer your question; it matters little, why the pedals possess a tendency to loosen when a left-hand thread is installed on the right side. However, for the love of God; it would be in one's best interest to remember to be absolutely certain to place the pedal with right-hand threads on the right-hand side of the bicycle.

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protected by Qmechanic Jan 17 at 8:48

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