In cross-country, more than other areas of mountain biking, a debate has begun over the optimal wheel size. 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.