A puzzle to ponder - The paradox of the pedal

[ColinOldnCranky]

On a bike we know that the act of pedaling introduces a tightening torque to the pedals. the left and right hand cranks and pedals have opposite threads to accomplish this. Indeed if you were to pedal backwards far enough then you would wind up unscrewing the pedal.

So, in principle you should be able to work out which thread is for the left pedal and which for the right.

So, as you ride forward, the left pedal is rotating clockwise with respect to the left crank.

But you will notice that the left pedal has a LEFT HAND THREAD! Should that not cause the crank to unscrew?

Like all good puzzles, the answer is obvious - ONCE YOU KNOW IT!

(PS On a unicycle you do not have the chain wheel to tell you which crank goes on which side. And so once, in haste, I put the wheel of my unicycle the wrong way around. The pedal unscrewed itself within a km of starting sending me onto the road in an ungainly fashion. And in a very public location.

[P!N20]

Mechanical procession

[ColinOldnCranky]

Mechanical procession

You've got it (other than the small matter of vowel ) that was quick.

But best to not explain it in plain english or detail the components so that others can ponder a little longer.

[P!N20]

^ Oops, got all religious on you.

Anyone with Italian bottom bracket threading knows all about it.

[John Lewis]

Similar reasoning for bottom bracket threads.
I never thought of that problem with unicycle.
In guess I never rode far enough to see the result.

[uart]

Mechanical procession

You've got it (other than the small matter of vowel ) that was quick.

For those of us who don't know what mechanical procession is, is that the counter rotation of the ball bearings (in relation to the spindle)? Because that is what I was thinking.

[Duck!]

Mechanical procession

You've got it (other than the small matter of vowel ) that was quick.

For those of us who don't know what mechanical procession is, is that the counter rotation of the ball bearings (in relation to the spindle)? Because that is what I was thinking.

That would be precession (hence the comment about the vowel).

What is not taken into account though is that the pedals are subject to a considerable non-rotational load from being stood on, and this loading puts a lot of shear force across the threads and can strip them if they're not sufficiently tightened. So you should always properly tighten the pedals and not rely on hoping that the act of pedalling will do it for you, because you'll likely end up with no thread in the crank first.

[twowheels]

Thx all above for the explanations and terms. I had known this, then a short time ago had a memory lapse when I was trying to explain "why is it so" to myself.

[DrShifty]

You can do a simple example of precession with a pencil.

Close your left hand so your little finger is touching your palm and your index finger touching the end of your thumb. You now have a cone shape. Put the pencil into the cone so the point touches your little fingertip. Now rotate the pencil anticlockwise around the inner surface of the cone. You will see that the pencil itself is rotating clockwise.

This is what happens to the thread in your pedals. The very slight clearance in the thread is sufficient to replicate the action of the pencil in the cone.

[BJL]

Great thread!

I had the Google what mechanical precession was and there's a nice little animation on the wiki. I hope it's accurate because it took some time for me to get my head around it. I like learning about how things work.

[Duck!]

Another good way to see precession in action is to get hold of a bearing, either an industrial cartridge or a cup & cone system, and pick the covers off so you can see the balls. As you rotate the inner race or cone, you'll notice that the balls also move around around the outer race in the same direction, but at a slwer speed, but they spin in the opposite drection[/].