The mountain bike market is dominated by three different wheel sizes.
Each size has its own advantages.
26” wheels are significantly more agile, nimble and allow for a tighter turning circle. Light weight, height stability and rigidity and a compact frame geometry translate into very direct feeling power transmission and close, immediate control of the bike.
29” wheels are the champions of uneven ground. Larger wheel size results in a better angle of attack when overcoming larger obstacles. This smoother rolling action means a higher level of comfort. 29” wheels just keep on going.
The new wheel size of 27.5” (650B) is a compromise between 26” and 29” and means acceptable weight with some of the 29” wheels rolling ability, albeit on a smaller scale. Perfect for smaller riders who don't want to forgo the benefit smooth rolling, traction and comfort.