This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Mon, 20 Apr 1998 15:31:32 PDT
Dark anodized rims were introduced a few years ago as a fashionable
alternative to shiny metal finish, possibly as a response to non
metallic composites. Some of these rims were touted as HARD anodized
implying greater strength. Hard anodizing of aluminum, in contrast to
cosmetic anodizing, produces a porous ceramic oxide that forms in the
surface of the metal, as much as 1/1000 inch thick, about half below
the original surface and half above. It is not thick enough to affect
the strength of the rim but because it is so rigid, acts like a thin
coat of paint on a rubber band. The paint will crack as the rubber
stretches before any load is carried by the rubber. Similarly,
anodizing cracks before the aluminum carries any significant load.
Rims are made from long straight extrusions that are rolled into
helical hoops from which they are cut to length. Rims are often
drilled and anodized before being rolled into a hoop and therefore,
the anodizing is already crazed when the rim is made. Micro-cracks in
thick (hard) anodizing can propagate into the metal as a wheel is
loaded with every revolution to cause whole sections of the rim to
break out at its spoke sockets. In some rims, whole sidewalls have
separated through the hollow chamber so that the spokes remained
attached to the inner hoop and the tire on the outer one. In
contrast, colored anodizing is generally too thin to initiate cracks.
As an example, Mavic MA-2 rims have rarely cracked except on tandems,
while the identical MA-40 rims, with a relativley thin anodizing, have
Anodizing is also a thermal and electrical insulator. Because heat is
generated in the brake pads and not the rim, braking energy must flow
into the rim to be dissipated to the atmosphere. Anodizing, although
relatively thin, impedes this heat transfer and reduces braking
efficiency by raising the surface temperature of the brakes. When
braking in wet conditions, road grit wears off anodizing on the
sidewall, an effect that improves braking.
Anodizing is not heat treatment and has no effect on the structural
properties of the aluminum.