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Shimano SLX Crank Arm Fail

From http://www.pinkbike.com/u/wtt-tmr/album/broken-crank/ as of 2013/08/24:

              

Posted 2011/11; "SLX" model cranks, history otherwise unknown.

"Snapped off when going to work", no injury reported.

Here is a photo of a not-failed carnk, note the area near the pedal eye is flat, with no vanity groove or ridge, but that it develops a vanity ridge approaching the spindle, which suggests the ridge is at fault:

  

At the same time, note the blackening (oxidation) of the failure is on the back side of the arm near the spider, and there is a shadow in the phot suggesting a sudden change in section:

  

Further, the arm has a "bump" on the inside of the back of the arm next to the fracture:

  

This suggests the crank failed on the back side due to cyclic load and design decisions about section.

See also FAIL-187.html.