Originally Posted by
Trakhak
Dark/hard-anodized rims have a tendency to crack at the eyelet, whether it's single or double. Jobst Brandt detested hard-anodized rims. From
this page:
"
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, it 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."
Photo shows what looks to me like a double eyelet.
Brandt's bias against anodizing was as puzzling to me as his bias against helmets as not having any benefit, when he was a mechanical engineer so should have understood that helments spread the impact energy over a much longer time interval, drastically reducing the impulse or peak loading. Just like auto seat belts do. But I digress...
Hard anodizing was a big advance for rims. There's a current thread on aluminum bits embedded in rim brake pads, that's what happens when you wear thru the hard anodizing into the softer aluminum. The micro porosity of the hard anodizing provides a superior brake surface (until the invention of "ceramic" coatings), however the aluminum oxide of the anodizing is by definition a ceramic, a metal oxide, and it's as hard as knife sharpening stones, at least Rc60. I think there may have been more issues with early anodizing, as I've never seen any flaking. I don't know what is the elastic modulus of aluminum oxide, versus aluminum.
That may be a double socketed rim, but I have my doubts, that may be just the head of the spoke nipple. Here's the thing: The deep socket cuts the stress by at least half, perhaps more as the rim of the socket has way more area than near the rivet, and is pulling on the outer wall, not the inner wall like the rivet and bottom of the socket. Each 10% reduction in fatigue stress, roughly doubles the fatigue life. 50% reduction (from tying into both inner and outer rim walls) equals 2^5 or 32X increase, which is a LOT. So in our example, I wonder about proper contact of the socket rim to the outer rim wall, because if properly fit, the load at the inner wall and rivet is drastically reduced.
I have massive amounts of old hard anodized aluminum cookware, from Commercial Aluminum Cookware Company, the forerunner of Calphalon, these are industrial strength, and they get heated over a wide temp range, and I've never seen the anodizing layer crack, nor on my old Mavic rims.
I'm on my third set of rims on my road race bike. The first two sets each lasted about 4 years and both failed via cracks at the spoke holes. The current set, double-socketed, is going on 25 years with no cracks.