 The 26-inch wheel size that was settled on with the inception of the current age of mountain biking has all but died off at the top level of cross-country racing as 27.5 inch (650b) and 29 inch wheels are much more popular now.650b wheels are 584 mm diameter wheels, and approximately 27.5" with a mounted tire.650b bikes are becoming ever more prevalent, while 29" bikes, or "29ers" experienced rapidly gaining popularity in the mid to late 2000s, and having some success in cross-country racing, but more so in marathon cross-country.