The size of mountain bike wheels has undergone significant changes in the history of the sport. Currently the traditional diameter of 26 inches is being challenged by the more recent 29 inch diameter (29ers). 29ers are 10% larger than 26 inch wheels and have been promoted as being easier to ride for taller riders and able to clear larger obstacles on mountain bike trails without loss of momentum or speed. These wheels make it easier on the rider's body as they navigate barriers such as rocks and logs. Until recently, it was debatable as to which wheel was better for overall performance in mountain biking.
A recent study compared the performance of elite mountain bikers as they utilized either a 26 or 29 inch wheel (Steiner et al. J Sports Sciences. 2015). The riders rode their mountain bikes on two trails, one more suited for 26 inch wheels and one for 29 inch wheels. As they rode the authors of the study collected data on power output and physiological markers such as heart rate. The authors concluded no differences in these markers between tire sizes on either course, but noted better performances using the 29 inch tire on both sections of the course. Not surprisingly, the athletes noted the 29 inch wheel was better for performance, traction, and clearing obstacles on the trail.