This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Sun, 26 Jan 2003 19:57:48 -0800 (PST)
> 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 it's main
purpose is to reduce rim wear in wet and gritty conditions.