This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Since the advent of the crank cottage industry, crank failures have
become more common than previously because most of these "home made"
cranks, usually produced in prototype quantities on numerically
controlled (NC) milling machines, have not been designed with sound
engineering practices, nor have they been tested destructively in
repeatable tests on testing machines. My comments are mostly aimed at
major brand production cranks.
Cranks break primarily because they are aluminum and because they have
high stress principally at two places. Aluminum has no distinct
fatigue threshold in contrast to steel, so that with increasing use
and load cranks are destined to break at the two most failure prone
places, the pedal eye, and the junction of the spider fingers and the
The pedal eye is weak because the joint is incorrectly designed, but
being standard, it may not be changed since it appears to work. This
joint always moves under load and through its fretting causes an
undercut in the face of the crank. Removal of a pedal, that has been
ridden any considerable amount, will reveal a recess in the face of
the crank with cracks around its circumference caused by fretting.
The cracks often propagate into the crank and cause failure. A
solution to this problem would be a 45 degree taper in place of the
flat shoulder at the end of the pedal thread.
The thin web between the crank and the adjacent legs of a five legged
spider is also a place where most cranks crack. The Campagnolo
C-Record as well as Ritchey cranks address this problem by using the
crank itself as the fifth leg of the spider, and transmitting pedal
torque directly from the crank to the chainwheels. By this
arrangement the spider merely supports the chainwheels radially and
laterally and the driving torque is delivered by a solid anchor.