The last big breakthrough in mountain biking was the creation of 29-inch wheels – bikes equipped with them, while slow and unwieldy, could pancake everything in their path, and made newer riders confident enough to saddle up and take on steep, rocky downhills. Now there's a new sweet spot: the 27.5-inch wheel. These have plenty of leverage over roots and rocks, but are lighter and have a shorter wheelbase than 29-ers. This means riders can be more playful with them, and so can mountain-bike manufacturers, who are changing geometry to accommodate nimbler, easier training. We pedaled hard to find the best of this new breed.