DougG

Everyone knows (or discovers to their detriment!) that the right-side pedal has right-hand threads and the left pedal left-hand threads. Many times, even in repair manuals, it gives the explanation that it has to do with the direction you pedal and the keeping the pedals tight.

But this is demonstrably not so. If your pedal is loose and your pedal bearing is stiff, pedaling in the forward direction will unscrew the pedal, not tighten it. In fact, I use this as a lazy way to unscrew the pedals with the bike on a stand: break them loose, hold the inner shoulder by the flats, and pedal the crank forwards to back them out.

So is there a known valid reason for the opposite threads, or is it something that's just become a standard over the years?

(BTW, anyone remember when some cars had left-hand threads on the left-side wheel lugs? I almost broke off a stud trying to change a tire on my '55 Chrysler before thinking to try the reverse direction.)

Bianchigirll 

It is only done to keep you up at night thinking about it.

cny-bikeman

In short it's about the actual forces on the pedal when operating normally, as opposed to what you note when it's frozen. Rather explaining the physics here I'll refer you to Google. Reverse pedal thread, left hand pedal thread, why bike pedal thread reversed, etc - any of those should work.

...and btw rotating the cranks rather than the wrench to both remove and install pedals is not lazy, but rather efficient. That's the way most shop mechanics do it.

ThermionicScott 

And then car lugs went to a tapered interface which makes a much better connection, and we're stuck with the "good-enough" technology from 1900 or whatever.

Wil Davis

Something about the Wright Bros. (yes, those Wright Bros.) deciding to make it a standard when they built bikes back in the early part of the last Century. (Sheldon Brown has something about it on his site, and I'll leave it as an exercise for the reader to find it!) Good luck!

- Wil

Looigi

If a the left pedal was loose, precession would cause the left pedal to unscrew if it had right handed threads. Precession is the reason for left hand threads on the left pedal.

Kimmo

I'm highly dubious about any precession forces managing to loosen a pedal, but meh...

What I'd like to see is a tapered seat, to prevent the spindles eating into the crank and resulting crank failures.

fietsbob

Pedal washers for the Win !

Airburst

I've seen it. We had a BMX bike come into the co-op once that the owner had converted to left-hand drive by simply swapping the cranks over (obviously he had a left-hand drive rear wheel too). One pedal (I forget which) had worked itself loose. The shoulder on the pedal spindle was about a turn away from the crank face, and he hadn't ridden it very far. We tightened whichever one it was up, but while I wasn't there he apparently brought the bike back again with the same problem.

HillRider

I've heard two explanations. The first is precession, which does occur and is also why English and Swiss bottom brackets have left hand threads on the drive side cup. The second sounds a bit more like an urban legand and is if the pedal bearings seize, the pedal will unscrew instead of breaking your ankle.

well biked

I've read that the left pedal often loosened up when in use, and that the Wright Bros., being the clever, innovative fellows that they were, came up with the idea of reverse threads on the left pedal. Simple as that......I wasn't there, so I can't swear this is the way it went down, but it sounds plausible to me.

Bezalel

The precession forces are real. that's why I can't use the same hex key to revove padals that I installed them with. If you hang out with people that try to put together their own tandem drivetrains you'll see precession forces loosening pedals

caloso

Anyone who's ever used an Italian BB knows that precession is a real thing.

Bill Kapaun

You think everyone here is a youngster?

onespeedbiker

But the reverse threads on a pedal is on the other side as with a bottom bracket. Precession defined as a reversing of direction on a rotational axis, explains the the reverse thread on the drive side of a bottom bracket, but how could is be the same with pedals where the reverse thread is on the other side? There must be something the mitigates the procession with the pedal bearings..

cny-bikeman

The pedal axle is turning opposite to the BB spindle in relation to the balls, so of course the precession would occur on the opposite side.

onespeedbiker

Wow! I actually understand! I figured it had to have something with the orientation of the axle and bearings. Thanks!

Dan Burkhart 

I'll give you another example of precession at work. The cone ring on a freewheel is left threaded to prevent it from unthreading while freewheeling. In normal use, the hub rotates clockwise within a stationary cog set while coasting in a forward direction. But what happens when a freewheel is re-purposed as a mid drive?



In this case, the hub is mounted stationary, and the outer body rotates clockwise about it. As I found out with this example, no amount of tightening would keep the ring from unscrewing. In the end, only loctite would keep the bearings from getting lost on the road.

RubeRad

This question turns up a lot on BF. (I can say that with certainty, because that's what everyone told me when I asked it a while ago!) The answer is indeed precession. What made me understand was realizing that threads and grooves are not machined perfectly, they only touch in some places, there's tiny gaps in a lot of places, and depending on which direction sideways pressure is applied, the metal flexes a tiny bit and the contact points and gaps shift around. Voila -- precession! This is what drives your pedals into your cranks and leaves an imprint, and what tightens your drive-side bottom-bracket cup so that herculean efforts (giant cheater bars, the "Sheldon Brown Tool", boiling water, etc) are needed to loosen it. If the threading was opposite, then the riding your bike would cause these parts to loosen, not tighten.

Looigi

The term precession describes a motion, not a force. Gyroscopic precession, for example, is the resultant motion of the axis of a rotating body subject to a torque perpendicular to that axis.

Kimmo

Ah, I get it now - every time I looked it up in the past, I found myself looking at the general article on Wiki, which was mystifying. This time, I spotted Precession (mechanical) under See Also.

I'd always assumed it was some obscure force coming off the bearing balls, but it's simply the load moving around the axis. The article quotes our own Mr Brandt:
It's clear as day now.
Pedal washers might prevent wear at the crank/pedal interface, but IMO they're just likely to spread half the damage over a slightly larger area. Speaking of Jobst Brandt, he came up with a much better solution: https://www.pardo.net/bike/pic/fail-001/FAIL-016.html

Seems to me, this evolution is a no-brainer and quite feasible to introduce; mobs making pedals should start doing them with a taper and providing inverse cone-shaped washers for normal cranks, and making a tool to create the chamfer in the crank available to shops. The collets could be made available in order to fit square-shouldered pedals to modified or new cranks. A relatively painless thing, as far as transitions in standards go IMO.

I just scored some sweet hollow Dura-Ace cranks, and the last thing I want to see is them die due to a stupid old standard like this...

DougG

Great explanation -- I totally understand now. The "tight pedal bearing" explanation also didn't right true since it would take a completely frozen bearing and a lot of force to break loose a pedal that was halfway properly tightened. The mechanical precession due to the axial load on the threads explains all (and also why pedals are so hard to remove even when you swore you didn't install them that tightly).

osco53

Lots to know, so much on this subject.

I've never had a pedal come loose,
All I do Is tighten them down by hand and an open end wrench to a good snug feel and
an unknown torque,,,

That should be all the average riders needs to know right ?

ollin

Oh wow; I was about to start a cult for the unknown supernatural force that tightens pedal to infinity. Shouldn't have quit the dayjob! ._.

davidad

When in doubt ask The Man. https://draco.nac.uci.edu/rbfaq/FAQ/8i.9.html