Pedal LH/RH Thread - I don't understand

[Happiness Stan]

I was always told you had opposite threads to stop pedals unscrewing themselves but the pedalling action going forward unscrews the pedals if the pedal bearing was seized. Only by backpedalling a seized pedal can you can you hope to unwind it off the crank arm. Am I missing something or were the Wright Brothers on crack?

 

[gavintc]

LOL, I have just been replicating the action with fingers and arms and like you had just accepted the story without really thinking through the physics of it. But, you seem to be correct. Be interesting to see if someone can explain this.

 

[Happiness Stan]

Thank you, it's not me going mad(der) then.

 

[Ian H]

The pressure on the pedal, as you pedal round the stroke, tries to set up a counter-rotation in the spindle. But yes, it also acts as a safety feature should a pedal seize.

 

[Norm]

I was always told you had opposite threads to stop pedals unscrewing themselves but the pedalling action going forward unscrews the pedals if the pedal bearing was seized. Only by backpedalling a seized pedal can you can you hope to unwind it off the crank arm. Am I missing something or were the Wright Brothers on crack?

Hold a pencil point downwards on a desk then make a circle with your thumb and forefinger (yes, like a **ahem** rude gesture) around the pencil. Move your hand so the pencil runs around the inside of the thumb/forefinger circle and you'll notice that the pencil turns in the opposite direction to the circle you are making with your hand.

This shows that the crank does act to tighten the thread, even though (I think that) it appears counter-intuitive.

 

[andrew_s]

The pedal axle is slightly smaller than the hole in the crank arm, and when you pedal, you force the axle against the side of the hole. As the crank rotates, the point of contact moves round the hole as the direction of push is always downwards. The axle moves the point of contact by rolling round the inside of the hole, with the top of the axle moving towards the front of the bike. The pedal threads are arranged so that this rolling action screws the pedal inwards, hence the left hand thread on the left pedal.

 

[Ian H]

It isn't easy to explain, is it.

 

[andrew_s]

It would have been better to do some diagrams (worth 1000 words and all that), but ICNBA.
Maybe I'll get round to it and update the previous post some time.

 

[mistral]

It isn't easy to explain, is it.

It is if you know what you're talking about.....

For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.

Thanks to Jobst Brandt: http://sheldonbrown.com/brandt/left.html

 

[Deleted member 1258]

I can confirm that if a pedal bearing seizes the pedal unscrews, many years ago I had a pedal bearing seize, the end cap had vanished and I hadn't noticed, on the way home from work, I had to do the last two miles home pedalling with one leg.

 

[andrew_s]

It is if you know what you're talking about.....
Thanks to Jobst Brandt

That doesn't actually tell you anything about why the pedal axle turns relative to the crank.
It says that the direction of pedal force rotates relative to the pedal spindle, and that you can't eliminate the clearance between spindle and crank.

As an explanation, it's a FAIL (possibly due to selective quoting)
Anyone who understands a rotating load doesn't need telling why a pedal would unscrew if the thread is in the wrong direction

 

[mattsccm]

Just screw in a pedal half a turn.
Now "pedal" the bike on the stand and see what happens. Interesting.

 

[Scilly Suffolk]

The effect is known as "precession".

 

[Cyclist33]

It is if you know what you're talking about.....

For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.

Thanks to Jobst Brandt: http://sheldonbrown.com/brandt/left.html

I think your post shows that the more you know, the harder it is to explain, as this is just gobbledegook to me.