why is the left bike pedal left-hand threaded?

April 18, 2011 at 10:06 am | | hardware, science@home

Any cyclist knows that the left bike pedal is left-hand (i.e. reverse) threaded. This is so the pedal doesn’t unscrew itself while you’re pedaling. But go grab a bike and spin the pedal and crank around and you might be a little confused. Last time I did this, I thought, Wait why isn’t the right pedal reverse threaded? When you spin the pedal and crank forward, as if you’re actually powering the bike, the effective spinning of the pedal around its axle (AKA the spindle) should actually unscrew both pedals: lefty-loosey on the right pedal and righty-loosey on the left. Did every bike manufacturer get this wrong?!?

Of course not, and the real answer blew my mind. (Probably because I’m not a mechanical engineer.)

It is not, not mind you, because of the effective unscrewing force from the non-zero friction of the ball bearings. Instead, it is an effect that works in the opposite direction (in this case): 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.”

The source of the screwing/unscrewing force is thus radial on the spindle—the downward force you put on the pedal—instead of the twisting force from the ball bearing friction. This radial force translates into a screwing/unscrewing force because there is a small amount of clearance between the spindle and the threaded hole in the crank. I picture it like a pencil in a toilet-paper tube: crank the end of the pencil around, and there is a force that wants it to rotate on its long axis (from friction with the wall of the tube).

The screwing force from precession (on a reverse-thread on the right pedal) is much stronger than the unscrewing force from friction of ball bearings, so bike manufacturers ignore the latter.

I wish I could find an animated gif of mechanical precession, but I haven’t found one. Anyone have a book on “advanced thread theory” and want to make an animation?

UPDATE: Here’s a nice animated figure from Wikipedia:

Animation_of_mechanical_precession

Animation of mechanical precession” by Chris ShannonOwn work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

20 Comments »

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  1. …and, for that matter, WTF do they do with hipster bike pedals? i can’t get my head quite around how fixie pedals work now!

    Comment by psi*psi — April 18, 2011 #

  2. you should invent pedals that don’t spin and convince hipsters that that’s what the real die-hard fixies use.

    Comment by sam — April 18, 2011 #

  3. Hi Sam,

    Another place where gyroscopic precession pops up is in aviation. The precession of spinning gyro-based instruments used in flight in the clouds requires the pilot to occasionally reset them to other indicators. The directional gyro for instance (which is basically a steady copy of a compass) will precess up to 10 degrees or more per hour. So occasionally as you are flying along you have to reset the direction to the compass heading. Anyway it’s interesting how these small things play a large part in our lives, without us even realizing it!

    – Austin

    Comment by Austin Blanco — April 28, 2011 #

  4. That’s clever but the very short path through the crank arm v. the longer lever of the pedal creates an increased pressure or contact area on the inside top and outside bottom of the arm at the upper position transitioning throughout the rotation which should effectively cancel out the effects of precession (which I do not think is correct in this example) The remaining constant force upon unscrewing the pedal would still be the rotation and friction from the ball bearings. The elliptical path generated by the rather insignificant space between the mated parts would only serve to allow the effects of cumulative rotation to eventually separate the parts, hence the need for left and right threaded pedals. Just my 2 cents worth…

    Comment by Dave — January 22, 2012 #

  5. There’s a nice animation on the Wikipedia page about precession:
    http://en.wikipedia.org/wiki/Precession_%28mechanical%29

    Comment by Geert — May 30, 2012 #

  6. My sister’s new Huffy mountain bike left her walking home about 2 miles when the pedal fell off, luckily not on a hill! both pedals screw the same direction on that bike, meaning the left pedal unscrewed as she was pedaling.. i looked up reviews on the bike and others had the same problem, one lady even fell off the bike.. I wrote to Huffy and they told me this is the industry standard in pedal design.. well wtf, if a car maker gets a hint that their visor can cause someone to chip a nail they recall millions of cars so they can be fixed before harm is done.. but bike makers tell me ‘it’s how we do it’.. so we have to have in our minds the entire time that the pedal may be about to come off again… great. How freaking hard is it to turn the thread around on the left pedal? because it would be impossible to come off if it were the other way as you’re pedaling.. you put force on the forward downstroke, not the reverse.. people have crashed and they still won’t fix design.

    Comment by avemaria — April 28, 2013 #

  7. […] à billes, mais qui ne m'a pas convaincu (en fait elle est fausse). Et puis j'ai trouvé une réponse circonstanciée sur Everyday Scientist que je vous traduis ci-dessous: La bonne réponse m'a stupéfait, probablement parce que je ne […]

    Pingback by Pourquoi les pédales de vélo se vissent à l’envers | Pourquoi Comment Combien — August 12, 2013 #

  8. Fuck that stupid ass design….i bought a sweet ass fixie and the fucking left pedal keeps sticking and unscrewing. I am not about to go buy a new fucking crank set either cause i already sunk 600$ into it and im broke so now i gotta sell it and lose my ass. There is no god. Por quoi??????????

    Comment by Jesse — November 7, 2013 #

  9. I love how some of the ones commenting clearly didn’t understand this post. I like this example to explain its meaning:

    take a pen or pencil in one hand loosely holding it near the end. Then rotate the other end with the 2nd hand & keep the 1st hand stationary. Then pen will tend to spin within the loose grip of the 1st hand in a direction opposite the direction the 2nd hand is rotating.

    Comment by Asks — November 14, 2013 #

  10. Sam, could it be that what you wanted to say is “The SCREWING force from precession is much stronger than the unscrewing force from friction of ball bearings”? That would make the whole subject clearer a bit. It took me a while to realize that precession takes place between already partially unscrewed outer thread on the pedal spindle and the inner thread in the hole in the crank.

    Comment by V. — February 5, 2014 #

  11. yup. i’ll edit this…

    Comment by sam — February 6, 2014 #

  12. Thanks — I’ve been wondering about that!

    http://en.wikipedia.org/wiki/Precession_(mechanical)

    has an animated diagram

    Comment by Dennis Hevener — August 21, 2014 #

  13. Thanks, Dennis! I added that animation above.

    Comment by sam — August 25, 2014 #

  14. if the bicycle is going to the left, and that is the left pedal, it should have a right hand thread on it to tighten it, if it has a left hand thread, it would tend to unscrew, since he is showing the left pedal, we assume the pedal is going to the left, therefore it should have a right hand thread, that is my comment by experience.

    Comment by pedal — March 19, 2015 #

  15. pedal, your comment is the most concise and clear explanation of a scientific phenomenon that i have ever heard. and all in one sentence, too!

    Comment by sam — March 23, 2015 #

  16. OK, I get it. If you were to label the diagram from Wikipedia, it would become very clear the phenomenon that is occurring here. First, this is the view of the pedal on the right side of the bike (right hand thread). Second, the red circle is the crank shaft arm female threaded hole. Third, the blue circle with square inside is the pedal shaft male threaded (right hand). And finally the green arrow represents the force of pedaling forward to the right. As you pedal forward the shaft tends to turn counter-clockwise (left). But due to precession effect the pedal shaft (blue) is actually turning clockwise inside the crank shaft arm (red).

    If you take a pedal wrench to the right pedal and free spin the crank counter-clockwise the pedal is tightened in to the crank shaft. Therefore the opposite is true if you spin the crank in the forward direction clockwise. The pedal becomes loosened. The same is true of the left pedal, as it is left hand threaded.

    I just bought a bike second hand refurbished and the right pedal kept unscrewing itself as I was pedaling. The bearings in the pedal were very sluggish. I ended up destroying the threads inside the crank shaft arm and had to replace the crank set, and of course the pedal too.

    So I submit the following for consideration:

    Which of these two forces is greater the precession effect or even brand new bearings in the pedal? And for that matter there is a precession effect that is not taken into account and that is of the bearings themselves. In any case I feel that the latter bearing resistance is always going to be greater than the precession effect. I also feel that I would much prefer that the pedal not unscrew itself during pedaling the bike say uphill. Work those bearings loose once the pedal has fully tightened itself into the crank shaft arm threaded hole. I feel that for these reasons the industry should re-evaluate this and reverse this standard to left hand thread on the right and right hand thread on the left.

    Comment by Gary J. Howell — June 3, 2015 #

  17. Gary, it is quite obvious that the force from (good) bearings is not sufficient to unscrew the threads. You can test this by turning the pedal with your hand and see if it unscrews. :)

    I suspect the “industry” started left-handed threading the right pedal after *experience* of precession unscrewing the threads.

    Comment by sam — June 4, 2015 #

  18. Since nobody brought it up, the idea to reverse the thread on the left pedal is attributed to the Wright Brothers. Who says America isn’t great?!

    Comment by Mark — April 15, 2016 #

  19. So was the first bike pedal ever designed with a reason in this way, or is more likely it was stuck on with normal threads, the left one kept falling off, providing laughter for old cycle guy’s best friend (until his fell off too), and so the next version was made with reverse threads. Old cycle guys: smiles, shrugs, hi-five, let’s go ride.

    Comment by Dave — May 12, 2016 #

  20. I feel that the pedal manufacturers are missing the whole point, I get the reverse thread for fixing the pedals what is annoying to me is that they don’t do it for the bearing cones and one always goes loose and one always goes tight at the bearings.

    simple – reverse the thread for the cones also one the left side so it rotates it’s setting better to stay without the need for constant tweaking or stripped threads trying to get the pedal to hold tolerance.

    Comment by Paul — August 7, 2017 #

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