The bulky rubber eases through muddy hairpins and floats across lengthy sand traps. I track smoothly through rock gardens and over intestinal tangles of roots. I haven’t had this much fun on new gear since I first strapped on fat skis and pointed them into fresh powder. Aren’t these rigs supposed to be piggish and slow? Weren’t they built only for snow?While the origin of fat bikes is widely debated, with deep ties to Alaska and the desert Southwest, it’s here, in the Minneapolis suburbs home to bike makers Salsa, Surly, and their parent company, Quality Bicycle Products (QBP)where this particular design has grown from an obscure novelty to mountain biking’s newest big thing.

The fattie phenomenon might well have stayed confined to the winter-sports wonk-o-sphere were it not for the efforts, about a decade ago, of Dave Gray, a designer at Surly and a self-described garage tinkerer. In 2005, Gray produced the Pugsley, a squat machine painted purple and kitted with bulbous four-inch tires—the first mass-produced fat bike.

“When I saw the Pugs, it blew my mind,” says Nick Johnson, a product-launch coordinator at QBP and one of my riding companions on the river-bottom trail. “It was like a human-powered monster truck. I had that feeling you get when you’re a little kid: This bike is freedom.”If necessity is the mother of invention, then the need around these parts was for a bike that could roll over the Midwestern snowpack, which can linger for five months or more. At the time, Johnson had been working as a bike messenger in downtown Minneapolis, skidding and squirreling around icy roads on an ill-suited, skinny-tired cyclocross bike. The Pugsley was an epiphany. The fat tires floated on the hardpan and plowed through new pow. The bike worked with winter, not against it.