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Machined Rim Sidewalls
Aluminum rims are made as an extrusion, curved into a hoop and then connected together at the seam, usually opposite the valve hole.
The seam may be welded, or, in the case of a lightweight, hollow-section rim, it may be made by fitting a short metal pin into the hollow at each end. (This part is sometimes called a "biscuit.")
The seam area is never exactly perfect; there is commonly a very slight difference in width or alignment. Excess welding material is commonly ground down too far, creating a slight hollow; or the two ends of a pinned rim may be slightly out of alignment. When a brand-new rim is first used (with a rim brake), the brake may pulsate annoyingly. Generally, this goes away after a couple of hundred miles of use, as the brake shoes wear down the irregularity of the seam.
Sometime in the 1990s, Mavic introduced its "SUP ® " rims (Soudé Usine Process: “Factory Welded Process”. The French version isn't all French: it's marketeerese -- "process" is English. Mavic could expect a visit from the French Language Police any day now... see my article "Derailer, not Derailleur"). But anyway.
These rims are made with an extrusion slightly wider than normal, with extra thickness on the braking surfaces. Mavic uses a welded seam (not normally practical on a thin-walled hollow-section rim) and then, after the seam is welded, the rim is placed on a special lathe where the excess material from the sidewalls is machined away uniformly.
Other companies have copied parts of this process, using machining on both welded- and pinned-joint rims. Rims with machined braking surfaces have two advantages:
- When building up the wheel, they can often be trued to greater accuracy than traditional rims.
- They give smooth braking even brand new, with no need for a break (brake) in period as with traditional rims.
There is a potential drawback to this type of rim, though: If the alignment of the two ends is less than perfect at the weld, the machining process can leave part of the sidewall thinner than it was designed to be. This can cause the rim to wear out from braking friction locally sooner than it otherwise would.
