Re: Thomson Elite Seatpost Failure

Ron wrote:

Followup to the Thomson failure article. Analysis from Thomson here :
http://cozybeehive.blogspot.com/2009/04/broken-thomson-seatposts-overtightening.html

Interesting read.

As an aside, the Thomson rep mentions anodizing reducing fatigue strength ~50%, and says that's accounted for in the design.

The blogger references: "Aluminum and Aluminum Alloys
By J.R. Davis & Associates, Joseph R Davis, ASM International" and provides a link to page 472 where a table gives actual measured changes in fatigue strength, 2-4 mil depth, 6061: 105->40 (MPa), 7075: 150->60 (MPa).

The accompanying text says:

"Anodic coatings are hard and brittle and will crack readily under mechanical deformation. This is true for thin as well as thick coatings, even though cracks in thin coatings may be less easily visible. Cracks that develop in the coating act as stress risers and are potential sources of fatigue failure of the substrate metal."

Of course this is what Jobst has been saying for years in the context of rims. Most of us had little trouble accepting the logic.
.

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