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Subject: Cracking/Breaking Cranks
From: Jobst Brandt

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 (link goes to wayback archive)

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.

Jobst Brandt

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