Crank failure, seatpost failure
See other failures under
Crank broke on first downstroke after standing up crossing a
bike bridge. Rider's foot hit pavement but kept control while "sitting"
on the top tube and holding the handlebars.
Minor injury from saddle nose.
Rider continued riding one-legged.
Soon, the rider leaned back on seat to clip remaining foot in,
whereupon a bolt on the seatpost broke, releasing the saddle
and dumping the rider.
- Parts age unknown. Derailleur on same bike marked Patent 1973.
- First owner big/strong rider who raced for University of Michigan.
- Second owner purchased in 1980.
Fit rider, typical weight 77kgf (170#) to 82kgf (180#)
- Used for commuting and occasional touring and racing until 2000
- Used rarely 2001-2005.
- Used for light commuting (70 mi/week) 16 months 2004-2006
until crank and seatpost/seat bolt broke.
- Crank: Campagnolo, Marked "Strada 172.5" and pedal eye marked "9/16 X 20 F."
- Seatpost: Zeus. Standard 1970's two-bolt design with bolt heads
above saddle rails reaching down to threaded barrels inside the
seatpost. Marked "27".
- Left crank shows classic "beach" lines indicating failure started in the
root of the outside "vanity groove" and progressing until the crank
was cracked nearly half-way across. Thus weakened, the crank
presumably failed due to overload in the remaining section.
- Root of "vanity groove" shows machining marks rather than a smooth
polish. Marks may have acted as stress risers
and contributed to failure.
Note the part of the "vanity groove" nearest the pedal may have
highest stress for loads applied near the bottom of the pedal
- Following failure, the right crank was examined. It shows significant
cracking of both leading and trailing "webs" supporting the chainring
It appears relatively common that when one arm fails
the other is near failure.
- Forward seatpost bolt failed. Bolt shows smooth/oxidized evidence of
some on the leading edge and more on the trailing edge, overall
reading area by about 50%.
- These failures are interesting in part because of apparent "cascading".
Both parts were near failure, but the first failure caused increased
load to the second. Although the first part failure did not
damage the second part, it nonetheless served as a trigger to increase
the load on the second part.
Arm/spider failures occur because the load is transferred along a thin
tapering knife-like edge that concentrates the greatest force along the
An alternative is to use a flattened transition,
as in the following non-failed Sugino crank.
See also FAIL-039.html
(FAIL-038.html for more broken seatposts)