While manufacturers of road bikes continue in the quest to get lighter and lighter, that's not the case with mountain bikes. In fact, fat is all the rage when it comes to the off-road machines. Fat-tire bikes, with tires as wide as four inches and more, are designed to be ridden through the snow and really sloppy terrain. The newest grouping of mountain bikes features tires a step below those behemoths, but wider than traditional tires. Why the emphasis on the big wheels? The larger tires need less air pressure, which provides better cushioning on the trail. They also create more contact with the ground, giving you better traction while climbing, cornering and descending.