XD-15 ceramic bearings use a different material than any other bearing on the market and is exclusive only to Enduro bearings. XD-15 is a very special nitrogen infused steel material that will not rust or corrode. Even running in salt water or any caustic nutrition drink you can spill, it will not be degraded or affected. Even dirt or sand inside will not wear it out; this will simply polish the races smoother. The process to produce this steel involves re-melting a very high grade of martensitic stainless steel from the end of an electrode. This occurs in a highly-controlled re-melt chamber while introducing nitrogen gas, producing a homogenous steel, free from course carbides and giving ultimate fatigue resistance. A super tough and strong steel perfectly mated with Grade 3 Silicon Nitride ceramic balls, XD-15 bearings start out great and get even better with riding over time. While other bearings are best the day you buy them out of the box and degrade from there, XD-15 bearings improve over time, even years from now. We even offer a lifetime limited warranty to the original owner. You will not only save watts, you will save maintenance time. XD-15 is the pinnacle in bicycle bearing technology.