dwood

OK . . . I've had alot of sugar today so I might not be thinking clearly. But it seems to me that the L pedal with a left-hand thread . . . and the R pedal with the right-hand thread is exactly backwards.

As I see it the only force [torque in this case] that would try to unscrew a pedal from the crank would be friction in the bearings in the pedal itself.

To help visualize . . . assume the bearings in the left pedal are frozen solid [will not rotate]. As your foot pushes down on the pedal on the power stroke, the pedal is moving in a clockwise direction relative to the crank. This would unscrew the pedal.

Ditto the right-hand pedal, just reverse everything. What's wrong with this picture?

DON

cooker

The bearings roll, and that reverses the direction of the pedal rotation effect.

Cadfael

If it were the other way about you may not be able ever to unscrew them, they would gradually screw further in, and eventually bind. Basically it does rely on torque to to keep the pedal spindle tight, not on the movement of the pedals. IF the worst case should happen and a bearing suddenly locks, you will probably make the spindle over tight by pedalling, even for half a revolution. It could then sheer, and that is another problem.

cooker

https://www.sheldonbrown.com/gloss_p.html

"Pedal Threading
Direction
The right pedal has a normal thread, but the left pedal has a left (reverse) thread.
The reason for this is not obvious: The force from bearing friction would, in fact, tend to unscrew pedals threaded in this manner. The fact is, however, that it is not the bearing friction that makes pedals unscrew themselves, but a phenomenon called "precession".

You can demonstrate this to yourself by performing a simple experiment. Hold a pencil loosely in one fist, and move the end of it in a circle. You will see that the pencil, as it rubs against the inside of your fist, rotates in the opposite direction.

Ignorant people outside the bike industry sometimes make the astonishing discovery that the way it has been done for 100 years is "wrong." "Look at these fools, they go to the trouble of using a left thread on one pedal, then the bozos go and put the left thread on the wrong side! Shows that bicycle designers have no idea what they are doing..."

Another popular theory of armchair engineers is that the threads are done this way so that, if the pedal bearing locks up, the pedal will unscrew itself instead of breaking the rider's ankle.

The left threaded left pedal was not the result of armchair theorizing, it was a solution to a real problem: people's left pedals kept unscrewing! I have read that this was invented by the Wright brothers, but I am not sure of this.

Note! The precession effect doesn't substitute for screwing your pedals in good and tight. It is very important to do so. The threads (like virtually all threads on a bicycle) should be lubricated with grease, or at least with oil."

dwood

Sorry . . . I'm not buying the precession answer [with all due respect to SB]. You can't have it both ways: either the friction in the pedal bearings *OR* the effects of precession exert a greater rotational force on the pedal threads.

If precession was the dominant force . . . it WOULD NOT be necessary to tighten the pedals firmly.

My grandkids both received Razor 'big wheel' trikes for Christmas this year. Don't know who assembled them . . . but after only riding them at our house for a few minutes each had a pedal come loose. One actually fell out. The pedal bearings on both trikes were on the stiff side. I tightened them properly and that was the end of the problem.

Just because something has been done one way for 100 years doesn't necessarily make it valid. One of these days I might swap sides with the cranks on one of my bikes and see what happens. Viva aposteriori!

DON

MudPie

+1.

Same deal with the bottom bracket cups. The drive side cup is reverse threaded (tightens in a counter-clockwise direction, loosens in a clockwise direction), so it appears that as you rotate the bb spindle looking at the drive side, the action would want to loosen the cup.

An engineering "free body diagram" shows how the bearings actually reverse the rotation effect.

rodri9o

Wordbiker

Just buy a tandem crank. They're threaded the opposite way due to the chainring on the left side...but maybe they're wrong too.

urbanknight

An even better explanation. They are threaded that way so that if the bearing does freeze, the pedal unscrews itself instead of yanking your foot upside down, throwing you to the ground.

HillRider

You really don't have to "buy" it, it's correct anyway.

Bicycle pedals (and bottom brackets) use rolling elements, either ball or needle bearings, and these rolling bearing elements roll in the reverse direction of the spindle. The minor friction between the rolling elements and the spindle does tend to tighten the spindle if the threading is the way it's done now.

And, inspite of what was written above, pedals only have to be snug, not really tight. Many mechanics overtighten them and there is no reason to do so.

Your grandchildren's trikes have very cheap pedals with bushings, not ball bearings. There is no rolling element to reverse the direction of force on the spindles so the situation isn't the same as real bike pedals.

dwood

Grab a roller bearing . . . one that the ID fits snugly on one of your fingers. Rotate the OD of the bearing in whichever direction pleases you.

Now tell me that the drag you feel of the inner race on your finger is turning the opposite direction to the OD.

Paleeeease!

DON

Wordbiker

I believe you dwood. The whole thing was just a marketing gimmick by the Wright Brothers. Their "cones" for hub bearings will never catch on either.

cooker

The reason the pedals fell out of the grandkids bikes is because the pedal bearings were stiff and failed to reverse the torque of the pedal. Or maybe Razor provides some kind of cheap bearingless pedals, I don't know. I suggest you do an experiment. Lubricate the pedals as best you can, until they spin freely. Then mount the bike on a stand, or upside down, loosen the pedal thread in the crank, and spin the cranksl by hand while holding the pedal so it doesn't rotate with the crank. The pedal will either get tighter or looser. It's -12C here, plus I'm leaving town for a few days, or I would do it myself and film it for interest.

EDIT Never mind. If HillRider is correct that these pedals have no bearing balls, then yes, the thread would work better the other way, and the best solution is to firmly tighten the thread.

Calli46

HillRider and many others are correct. Here is a rough drawing I made for the visually inclined.

nafun

The picture is correct. The predominant explanation thus far is incomplete.

As SB said, it is due to precession. The bearing ball must slip against either the inside or the outside race and cannot role smoothly on both. This is simply because the diameter of the outer race is larger than the diameter of the inside race. This means that the ball must roll farther against the outside race in one revolution than it does on the inside race. Obviously, for a rigid ball, this is not possible without slipping against one race.

Now consider the ball rolling without slipping against the outside race. The ball is turning to fast for it's rotation around the inner race, so the slippage, and hence the friction, is opposite the direction of travel.

As for the bearing on a finger test, it is not a valid test as the bearing is not radially loaded. With no load, the balls can slip easily on both races freely, and the force the axle feels is just from the viscosity of the lubricant.

dwood

What you've drawn makes it look like a standard ball bearing is like a planetary gear system with the planets held stationary. It is not. The balls are not held stationary . . . the cage containing the balls rotates in the direction of the outer race [in the pedal example]. And because the ball/cage assembly is allowed to rotate . . . there is no scuffing of the balls on the races.

As an example of this take a planetary gear system. The gears in it do not 'scuff' [they can't], yet the outer and inner rings turn freely with respect to each other. The ball bearing is no different . . . imagine it as a planetary gear system with infinitely small gear teeth.

As for precession that SB talks about . . . I believe it has nothing to do with the bearings in the pedal. It is the precession effect of the rigid spindle and the way it is threaded into the crank. The spindle is cyclicly loaded as you pedal. Any looseness in the thread allows the pedal to attempt to 'walk out' of its thread. See SB's fist/pencil example.

The solution: tighten the pedal properly.

DON

EDIT: I've altered some of the wording in the above since I originally posted to make my points more clearly [also corrected some typos].

At any rate . . . tradition is a wonderful thing. Pedals have been threaded this way for 100 years or so. I don't know what the problems were with the pedals 100 years ago . . . possibly they selected too coarse a thread, poor equipment produced poor threads, possibly inferior materials. At any rate . . . there is no reason for a modern pedal to unscrew itself for any reason IF PROPERLY TIGHTENED regardless of being left or right hand.

nafun

Yes. You've convinced me. My other post was BS. What was i thinking?

Wordbiker

It's too bad the Wright brothers couldn't listen to guys on the internet explaining exactly why their plane couldn't fly.

Seriously, do you really think this one slipped by everyone for 120+ years, and that you just had an epiphany that will free us all from the Matrix? Just as you said that because it's been that way for 100 years doesn't make it right, the fact that you don't understand it doesn't make it wrong.

dwood

My point is that "it doesn't make it necessary!" [to thread them a certain way]

DON

HillRider

Your further explanation is an excellent refinement of what several of us have said in more simplistic terms.

However the finger test is illustrative since the viscosity of most bearing lubricants is sufficient to transmit a bit of torque and all we need for this demonstration is a small amount.

dwood

HillRider,

Did your read nafun's retraction a couple of posts above yours?

DON

HillRider

Yes I did but I just reread the post he cited. The planitary gear set analogy is a good one but the thrust on the inner race is still finite even if there is no real "slippage" as the bearing rotates.

dwood

HillRider,

I'm assuming you are still talking about thrust in the direction opposite the pedal rotation. This is impossible. In order for there to be any force in the opposite direction the original force has to "act against something".

In a planetary gear system you get that by holding the planets. This causes the outer and inner rings to rotate in opposite directions. There is nothing in a bearing that 'holds' the balls, they freely revolve [as a group] around between the races.

You can't pull yourself up by your own bootstraps [as my dad would say].

DON

HillRider

The revolve freely, i.e. there is no slippage, but there is a friction force, otherwise the balls wouldn't rotate at all.

dwood

Correct! But the friction causes the races to revolve in the same direction.

DON