Tuesday, October 23, 2007

Fatigue failure of crank arm

This is an interesting problem. Stole it from Road Bicycles Tech forums. Not that it can't happen, but its interesting that the non-engineer thinks his massive lower body "PROWESS" broke the crank-arm. Read on :

Hey All,

I was working my regular pedicab driver shift the other day,
accelerating from a stop light with 3 large guys seated in my trailer
type pedicab. I sort of jumped forward in an unexpected way and heard
a scattering, clank-clank. I then noticed the my left pedal was
missing. Then I noticed my left crank arm, and the end of the bottom
bracket spindle that would normally keep the afore mentioned parts
attached were gone too. Upon inspection, the missing bottom bracket
stub was still pressed into the crank arm that I was holding in my
hands. In spite of my displeasure of not being able to earn more
money that night dragging drunken louts to their next whisky bar, I
was amused by the massive power I exhibited by ripping the crank
nipple right off.

So here is my question:
Was it really my massive lower body prowess that killed this BB
spindle? Or is it that Shimano is making crappy spindles that shread
after 5 months? Or is the pedicabbing I do on this bike just that
hard on the torquey bits?

Perhaps it was a combination of the three. I bring this to the R.B.T
group as none of my pedicabber buddies have had this particular
mechanical failure before. Chalo on the other hand has done it a few
times before resulting in his oh-so-fashionable grillz. (him 300+ lbs)

-Ken Cameron
humble neighborhood pedicabber

Answer :

Although such spindles are usually hardened so they can take the high
contact stress of their bearing balls, this is most surely a fatigue
failure that was the summation of many loaded torque repetitions.
You'll note that only the torque of the left foot goes through the
spindle, the right foot going directly into the chain. The fracture
should be a slightly helical crack, part of which is at 45° to the

Low quality material and/or heat treatment.

Jobst Brandt

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