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ABOUT "XD-15" BEARINGS
XD-15 High Nitrogen Steel Alloy is 3.5 times more corrosion resistant than 440C Stainless Steel. In the past, ceramic hybrid bearing manufacturers were faced with a choice in bearing race material: 1) The longer fatigue life of SAE52100 bearing steel when paired with the extremely hard ceramic balls, or 2) The corrosion resistance of 440C stainless steel for use in harsh environments. If they opted for the longer fatigue life of 52100 bearing steel, the intrusion of water would always be a threat to the races. If they opted for 440C Stainless, corrosion would become less of a threat at the expense of longevity. The XD-15 steel not only combines the benefits of both, but EXCEEDS them on both counts. Yes, much greater corrosion resistance than 440C and longer fatigue life than 52100 bearing steel.
XD-15 High Nitrogen Steel Alloy has a 40 percent higher load rating than SAE52100 bearing steel. Ceramic hybrids made with 52100 steel races, while having a longer fatigue life than stainless, were still subject to premature wear when used with ceramic balls. Of course the main claim of ceramic hybrids was less rolling resistance, resulting in greater speed and a more efficient transfer of the rider's output to actual forward momentum. Wouldn't it be great to get this superior rolling quality while actually increasing longevity of the bearing? Now you can. XD-15 has an extremely fine grain structure which dramatically reduces the risk of failure under mechanical loading, lowers noise and vibration, and makes a perfect interface with ceramic balls. Weak spots in bearing races due to grain structure flaws in traditional bearing metals are not present in XD-15, which has a homogeneous structure of finely dispersed carbonitrides, as opposed to large carbides concentrated at grain boundaries (which are vulnerable to corrosion and mechanical stressing).
Grade 3 pure silicon nitride (SI3N4) ceramic balls are the best available. These made-in-USA balls are Hot Isostatically Pressed (HIP) for a pore-free microstructure with micron-sized grains. Reduced bearing friction from ceramic balls is a function of smooth surface finish. The balls used in the XD-15 bottom bracket boast a surface finish of < 0.10 micro inch (< 0.003 micron).
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