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Sugino XD Crank Failure on "Vanity Ridge"

Sugino XD crank failed in the middle, away from features, where the arm section is somewhat D-shaped; failure intiated in the high point of the "D".

Crank from approximately 2007; used on a single-speed for commuting. Regular use, but mainl "sit and spin" riding style.

Rider was thrown to the ground without warning, with minor injuries.

        

Note dark marks starting at the outer face, suggesting the crack started at the dark spot and grew from there until the crank failed. Note also "beach marks" mid-way across the crank. The significant blackening at the edge, plus depth of the beach marks suggests the crank failed over significant time.

Pedal load near the bottom of the stroke tends to put D-shaped arm under highest load on the curved outer face. In such bending, the highest load is at the "skin". The narrower the face (the rounder the D), the higher the skin stress for a given pedal load.

Further, the outer face tends to be scuffed by shoes, etc., in normal use. Scuffing can give rise to surface defects that accelerate failures.

Compare to other D-shaped arm failures HERE, HERE, HERE, HERE, HERE, and note scuff marks on this not-failed hollow D-shaped arm: HERE.

A squared-section arm is probably more durable against these loads, but many square-ish cranks have a milled relief, e.g., HERE. The milled relief means that the loads are concentrated in the remaining ridges. As the milled groove saves little weight but causes failures, it is sometimes called a "vanity groove". By association, arms with a ridge (such as the Sugino XD) are sometimes described as having a "vanity ridge".

See also FAIL-201.html.