Subject: Anodized vs. Non-anodized Rims
From: Jobst Brandt
Date: April 20, 1998
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 cracked often.
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 brake pads. 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.Jobst Brandt
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