dianne_1234 writes:

The University of Limerick Mountain Bike Club has collected some
interesting failures.

http://www.skynet.ie/~mtbclub/Photogallery/0001_Broken/

I see mostly pictures of a failed tight crank that unfortunately is
focused on the pedal rather than the fracture. Just the same, it
also shows the failure mode to be one of bending from standing on
the bottom of the stroke as most previous pictures of crank
failures have.

That one interested me because in several photos I think I can see
the darker side of the crack on the /inner/ side of the arm, as if
the crack had originated *opposite* to the "standing on the pedal at
6:00" argument would suggest. Photos 8-10 are the ones.

You will probably agree that the crank broke crosswise, no in torsion
(spiral) or to the outside (for which there are no forces).
Therefore, without going further into fracture analysis, it is safe to
say that it broke inward from the standing load on the pedal.

It also appears to me there might be a sudden change in cross
section near the break, but I'm not familiar with the crank model.

That is not ruled out, but as I said, the pictures are all focused on
the pedal leaving the fracture blurred. We need a better photographer
and one with a flash or other illumination.

Jobst Brandt