XD-15 ceramic bearings use a different material than all other bearings on the market and are exclusively designed for enduro applications. XD-15 is a very special nitrogen-infused steel material that does not rust or corrode. Even when in contact with saltwater or an acidic drink that you might spill, it will not break down or be affected. Dirt or sand inside does not wear them down; instead, it only polishes the races smoother. The production of this steel involves remelting a very high-quality stainless steel at the end of an electrode. This process takes place in a highly controlled remelting chamber with the introduction of nitrogen gas, resulting in a homogeneous steel with the highest fatigue strength. A super strong, solid steel, perfectly paired with Grade 3 ceramic balls. XD-15 bearings actually improve over time. While other bearings perform better on the day of purchase and then deteriorate, XD-15 bearings get better with time. This not only saves you watts but also maintenance costs. The XD-15 is the pinnacle of bicycle bearing technology. Dimensions: 17x30x7 medium/low contact LLB seal.