Subject: Machined rims
From: Jobst Brandt
Date: January 26, 2003
Just wondering if it really makes any difference. Some manufacturers don't even advertise whether the sidewalls are machined; others do. Velocity for example, makes both, but I believe they're the same price. What gives? Just marketing hype?
What you hear and read is mostly marketing hyperbole, but machining rims has its reason, and it isn't for your benefit. If you inspect a machined rim closely, you'll find a surface that looks as though made by a thread-cutting tool. The purpose is not to get a flat braking surface, but rather to produce a series of fine grooves to prevent brake squeal on new bicycle test rides.
The machined grooves, about the texture of LP vinyl record grooves, can be felt by running a fingernail across the rim. These fine grooves usually wear off on the first braking descent in wet weather, the condition that causes rim wear in the first place. Even anodizing, which is a hard ceramic, whether thick or thin, is more durable than the machined rim. However, anodizing is not the solution to wear, because it degrades braking. Anodizing being an insulator that overheats brake pads and causes brake fade.
The claim that machining is for purposes other than suppressing brake squeal is far-fetched. For instance, rim joints have been made with no perceptible discontinuity almost as long as aluminum rims have been made. Unfortunately, some people in marketing believe that rims will separate if not riveted (or welded) and introduced riveting that usually distorts rim joints. Fortunately, that rims were made for many years without rivets and had flawless joints proves otherwise.
In practice, machining solves the new-rim squeal problem at the cost of a rim wall of unknown thickness. It also adds a bit of sparkle to the new product by giving rainbow reflections in showrooms. Mavic, for instance, has rims listed as having "CERAMIC2", "SUP, "CD", "UB", MAXTAL", all features that substantially increase cost over plain aluminum rims that were offered at about 1/4 the price not long ago.
The web site explains that "CERAMIC2" is an insulator that improves braking even though the rim is "UB" machined, ostensibly for the same purpose, before ceramic coating. This is a tipoff, because without special brake pads, this feature overheats pads causing them to wear rapidly while degrading performance. Not mentioned is that its main purpose is to reduce rim wear in wet and gritty conditions.
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