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Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Section - 8i.9 Left hand threads

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 4/5
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Date:  Wed, 28 Apr 2004 16:14:11 PST

On bicycles, left hand threads are used mainly in three places, on
left pedals, right bottom bracket (BB) bearing cups, and freewheel
cones, to prevent unscrewing under operating loads.  Unscrewing occurs
from precession, in which a round object rolling in a circular ring in
one direction will itself turn in the opposite direction.

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.

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 the left crank, make
3/4 of a fully rotating load.  For this reason some right BB cups have
used right hand threads and some with left hand threads have loosened.
The left BB cup with no significant rotating load has little tendency
to turn.

Freewheel cones are more obvious candidates for precession, their load
being mainly radial, and rotating continuously in the direction that
would unscrew a right hand thread.  There are other such but less
common threads on bicycles.

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 motion.  Motion in these joints causes visible
fretting rouge, red iron oxide, on the shoulder of the BB cup and on
the face of the pedal spindle.

Left hand threads would not be required on left pedals if a design
common on cars were used.  Before the advent of conical lug nuts, many
cars used left hand threads on left side wheels.  Today, stories of
wheels rolling away from cars no longer make news, the conical seat
having solved this problem on car wheels as it could on bicycle
pedals.

However, unscrewing is not the main problem for pedals, but rather
crank failure caused by fretting erosion of the pedal eye.  Fretting
initiates cracks that can cause sudden and unsuspected pedal
separation when the eye of a crank breaks.  Because this occurs
equally with right and left cranks it is the more important reason for
a conical spindle face and crank eye.  This has been tested.

---------------------

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