Andrew Webster said:Quoted message said:I've always wondered.. why are the pedal and BB thread direction
such that forward pedalling direction would be loosening them?
Quoted message said:Quoted message said:If a pedal seized somehow (or a shoelace got wrapped around the
spindle - happened to me) then the pedal pops off. If something
goes wrong in the BB, and it seizes or something, then it unthreads
itself with pedalling.
Quoted message said:Quoted message said:I'd assume that these threads would be opposite, so the pedaling
direction would keep everything tight?
Quoted message said:Quoted message said:Like most bike traditions, I'm hoping there's an interesting story
involved in this?
Quoted message said:Quoted message said:Or maybe it's that way, so if something bad does happen in the
bearings, your pedalling won't tighten them so much that you can't
get them apart again?
Quoted message said:The pedalling action DOES tend to tighten the pedal, but not due to
bearing drag which, as you observe, applies a force in the wrong
direction. It is beceause the pedal spindle is supported at only
one end so, as you pedal the applied force tends to make the pedal
precess around its axis, applying a tightening force as it does do.
That the pedal is an overhung load is not the reason for precession.
That would occur even better if the load were not outboard.
Quoted message said:See http://www.sheldonbrown.com/gloss_p.html as search for pedal, or
search in rbt for "preccession pedal" as this topic comes up fairly
regularly.
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Subject: 8i.9 Left hand threads
From: Jobst Brandt <[email hidden]>
Date: Wed, 28 Apr 2004 16:14:11 PST
Left hand threads are used mainly in two places on bicycles, on the
left pedal and right bottom bracket (BB) bearing cup, and for the same
reason, that is not to prevent unscrewing if the bearing were to
seize, forcing it to unscrew. The more insidious force is that of
precession, in which a circular object rolling in a circular ring in
one direction turns in the opposite direction. This effect is what
causes such assemblies to unscrew under a rotating load.
The rotating load on a pedal arises form the downward force of the
foot on a spindle that rotates with the crank. The predominantly
downward force effectively rotates 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 on the faces of cranks where pedal spindles seat.
Precession of right side BB cups is less obvious because the rotating
load is only partial. The largest load being chain tension that
together with the moderately large downward force on the right crank
and the smaller upward force from pushing down on th left crank make
up 3/4 of a fully rotating load. That is why some right BB cups have
used right hand threads and why some with left hand threads loosen.
The left BB cup has no significant rotating load and does not unscrew.
Precession forces are large enough that no manner of thread locking
glues, short of welding, will arrest them. Mechanical fretting, the
micro-motion of tightly fitting parts moving against one another, is
the mechanism of this problem. Elastic motions in these joints cause
visible fretting rouge, red iron oxide, on the shoulder of the BB cup
on the face of the pedal spindle.
No left hand thread would be required on left pedals if the automotive
solution were used. Before the advent of conical lug nuts, left hand
threads were used on left side wheels. A conical face solved this
problem on wheels as it could on pedal spindles for bicycles.
However, unscrewing is not the main problem for pedals, but rather
crank failure caused by fretting erosion of the pedal eye. This wear
initiates cracks that can cause sudden and unsuspected pedal
separation when the eye of the crank breaks. This occurs equally with
right and left cranks and is the more important reason for using a
conical spindle face and crank socket. The method has been tested.
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Jobst Brandt
[email hidden]
