Jim Beam writes:
>>> I can kind of agree with these statements. I had 517's that showed stress marks (very fine
>>> cracks) at the eyelets. I freaked out. I ordered new 519's to get a stronger rim and kept riding
>>> until they failed. They didn't fail. I replaced the rear when the freehub died on a night ride
>>> since I had another rim ready to go. I am still running the front. I am now 3 years into these
>>> things. I could put a new freehub on the 517 and ride it more. I just haven't.
>> This is all old hat and has been aired here often. Anodizing causes rim fractures that initiate
>> from fine surface cracks occurring in the hard and brittle anodized surface. Even before any
>> spokes are tightened, these cracks are already there, caused by insertion and crimping of
>> eyelets. If the wheel is heavily stressed in use, these cracks grow to the extent that whole
>> sections of rim break out. When the anodizing fad began and rims were more heavily anodized, some
>> rims split circumferentially in the middle of the braking track, leaving two interlocked hoops,
>> the tie still mounted on the outer part.
>> If bicyclists would not have such a desire for aluminum rims that don't look like aluminum, aka
>> dark non shiny colors, then we wouldn't have this expensive source of rim failure.
> With the greatest of respect, the hooping cracks are extrusion problems. This is evident from
> metallographical examination.
Flattery will get you nowhere! metallographical? Wow!
By whom? The anodizing was the only difference, the extrusions at that time being available in shiny
aluminum and black hard anodized. I took samples of each and have the dyed and polished cross
section photos done by a metallurgical laboratory at Alcan that show cracks propagating from the
anodizing into the base metal in the anodized version and no cracks at all in the plain version of
the same rim, the MA-2 and MA-40, the latter having cracked circumferentially. Both rims had
substantial use. The plain one was bent in a crash.
> Yes, you are correct that cracking anodizing /can/ initiate fatigue, but it's not commonly what we
> see in practice - for rims at any rate. Anodizing raises compressive surface stresses, and this
> helps mitigate crack initiation. /All/ aluminum is anodized to some degree, just by exposure to
> air. The color which the anodizing has been stained is not relevant.
The way you say that one might guess that you work for Mavic. The natural oxide on a polished rim is
possibly 0.01mm thick at best and has so little strength that it cannot but craze on the microscopic
level under stress. That is, the skin is so weak that it cannot effect the base metal. Besides, the
structure of that surface is not the same as anodizing.
Jobst Brandt
[email protected] Palo Alto CA
