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:
"For a pedal, a rotating load arises from 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."
Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is
several orders of magnitude greater than that caused by a jammed pedal bearing.
It's clear as day now.
Originally Posted by
fietsbob
Pedal washers for the Win !
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:
http://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...