Bontrager Crank Failure

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Broken left Bontrager crank.

Rider knee injuries led to 19 stitches, no muscle or tendon damage. The rider says:

It happened while climbing a 7% grade in standing position at a relatively slow speed. That is the second crankarm to fail on me on this bike.

The Break occurred at 2 1/4" up from the end of the crank. [...]

[...] When your pedal breaks off the opposite pedal immediatly bottoms out and turns the other into a spear. [...]

Rider weight and crank use uknown. The crank looks to be a model under 10 years old.

Note dark area and "beach front" lines; the crank was already cracked rougly half-way through before final failure.

One of the large forces on cranks is the rider's weight at the bottom of the pedal stroke. That causes a tension in the outermost surface. A rounded shape such as this has much less material to support that tension than a flat arm of the same section.

A seocnd poster says:

[It looks like] a forging fault [...] where it has broken. It looks like the molten metal had some debris in it but the [sic] poured it anyway and then painted it black. I think I would only be using polished, plain aluminum cranks from now. They couldn't hide such a fault with paint and it would have to be rejected.

A third poster says

An unpainted part would have made it much easier to see the crack, if it was gone over that is. Posts like these should make people with bikes new and old go out and clean there [sic] bike to check critical areas like the cranks bars and stems. I've got a box with Cinelli stems that are cracked all the way through by the pinch bolt where there is a huge stress riser.

Just curious but did the crack start at a point where the shoe contacted the crank-arm? check the pieces for scuffs in paint. Aluminum is much more notch sensitive than steel, or carbon! Older cranks back in the days of straps and toe clips would often fail where the straps would rub the crank. Another area is the threaded area for the pedal spindle.

Take off the pedal on the drive side arm and clean it thoroughly, check for cracks in bright light!

See also FAIL-108.html