The pedals would unscrew when pedaling forwards if not for a phenomenon
called "precession". Here's what Sheldon Brown has to say about it in
his 'glossary pages under 'p' for pedal'
(http://www.sheldonbrown.com/gloss_p.html)
Quoted message said:
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.
And another page at Sheldon Brown's site about 'left-handed threads'
(http://www.sheldonbrown.com/brandt/left.html)
A search of the forum for "precession" will find other threads about
it.
Make sure that you don't have pedals with 1/2" threads. There are two
primary pedal sizes. 1/2" for kids bikes and bikes with one piece
cranks and the more common 9/16" size that most unicycles now use.
1/2" is just slightly smaller than 9/16". A 1/2" pedal in a 9/16" crank
hole will undo itself very quicly.
It is also possible that your pedals are just slightly out of spec and
the threads are just a little bit too small. It has happened before.
If that is the case you may be able to return them to the place you
bought them for a replacement.
--
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