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Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Section - 8f.10 Cracking/Breaking Cranks

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Date:    Fri, 18 Jun 2004 12:47:58 -0700

Aluminum alloy cranks develop cracks principally at two places
although other failures occur as can be seen in samples at:

http://pardo.net/pardo/bike/pic/fail/FAIL-001.html

The two most common failures are the pedal eye and the junction of the
trailing spider leg and the right crank.  The trailing spider leg
adjacent to the crank generally has a thin web that connects it to the
more rigid shaft of the crank, while the three preceding legs are more
flexible, stress is concentrated at this web.  These cracks are
relatively benign because they are easily seen and rarely progress to
failure.

In contrast, the most common and most dangerous failure, one at the
pedal eye has a different cause not directly related to a stress
concentration, but one that might be apparent to a critical observer.
That the left pedal has a left hand thread is taken for granted and
seems not to be questioned because it has "always" been that way.
What is less well remembered is that automobiles also used left hand
threads to secure wheels on the left side of the vehicle before the
advent of the conical lug nut commonly used today.

The pedal attachment, as wheel nuts on cars of old, has a flat face
that bears against the crank, a surface that cannot transmit any load
except by friction because it is parallel to the applied force.
Therefore, this joint always moves under load, a microscopic type of
motion known as fretting.  Fretting causes erosion of the interface
and develops an undercut in the face of the crank that is visible when
the pedal is removed.  Besides, a left pedal without a left hand
thread unscrews, regardless of how tightly the pedal is installed,
proving that there is motion.

Removing a pedal, ridden for a longer time, reveals erosion in the
crank face having tiny cracks radiating from its circumference.  In
time, some of these cracks propagate into the crank and cause the end
of the pedal eye to break off, releasing the pedal, usually at the
worst possible moment, that of high stress of a rider pedaling in the
standing position.  Such failures generally cause the standing rider
to fall to the side of failure because that foot is suddenly standing
on the road at speed.

A solution to this problem is to use a tapered face (~90 degree
countersink) similar to the face of an automotive wheel nut in place
of the flat face at the end of the pedal thread.  This design has been
tested in prototype with a rider who previously had more than two
dozen such crank failures and has subsequently not had any for five
years on the same cranks.  Not only does it suppress fretting motion
that causes failures, but it makes the left hand thread unnecessary, a
bonus for manufacturing while secondarily giving one to tandem riders
who generally have difficulty finding cranks with threads opposite to
convention.

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Previous Document: 8f.9 Crank noises
Next Document: 8f.11 Installing Cranks

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