There are two note worthy rim makers in Japan, Araya and Ukai. They are note worthy because each of them has had their rims sold as original equipment on new bicycles and at one point both had their rims sold in the US as after market replacement parts. Both are well known for excellent quality and workmanship, however, at the moment, only Araya seems to have rims available in the US for after market replacements.
Ukai's participation in the United States parts distribution system dropped off beginning in the fall of 1987, after the G7 meeting drove the value of the dollar down and Japanese goods edged their way toward double the Yen price. Ukai is reported to have been the maker of the Specialized and also made the Ritchey rims that are sold on new bikes as original equipment. The Ukai manufactured Ritchey rims can be identified bt their welded rather than sleeved joint.
Right: Sectional View of Araya joint.
Both of these rim makers sell their goods through importer/distributors who have to make a value judgement in terms of what they think will sell to retailers in a reasonable period of time. The importer/distributor then places an order through a Japanese export trading company for delivery 90 to 120 days in the future. If they predict wrong they will have an overstock, tying up their capital. By nature, these organizations, when it comes to bulky items like rims, buy very conservatively which leads to occasional stateside shortages of either models, colors, or drillings.
The RM-20 has been in Araya's rim family for several years and has always been a worthy rim for mountain bike riders. The RM-20 is a double wall designed rim that places the cross-tie wall closer to the spoke seating wall than other rims of its kind and the inner bear width is wider than any of Araya's other rims. Its made of aluminum alloy, though its alloy composition isn't publicly known. The Box shaped rim doesn't use steel eyelets, but is extruded with a thickened spoke bed to help prevent spoke nipple pull through failure.
Unlike the other Araya rims we sell, the RM-20 has a flat sidewall, not concave. Once the rim is rolled into its hoop shape the ends are TIG welded together. The extra aluminum from the weld, the "flash" as it's known, is ground away from the interior, and the exterior three sides of the rim are machined and then polished leaving a smooth surface for the brake pad contact. The only evidence of the weld is a slight, very slight, discoloration of the joining aluminum wire used to fuse the two ends, but the smoothness of the rim makes it almost imperceptible.
Post polishing, it's drilled for the spoke and valve holes, then the rim is anodized in one of two colors, Clear leaving the rim Silver, or Hard Anodized to the customary Grey color. We sell the RM-20 in the 26" diameter both the Silver or the Grey finish with either 32 or 36 holes. We also sell the RM-20 in the 24" diameter, when available in the States, in Silver only with either 32 or 36 holes. The RM-20 comes typically drilled for a Schrader valve.