ASPEN - "I haven't ridden that trail yet this year, but we could try it if you're up for a little adventure," Bill Boughton said as he sat waiting for me near the top of Smuggler Mountain, his dog Millie panting by his side.
I paused. We'd been riding for nearly two hours, and I had managed to stay reasonably close to Boughton, though I suspected he was going easy on me.
We'd covered a popular route, climbing Smuggler and descending into the Hunter Creek Valley, with a single twist: It was mid-January, and the ground was covered with snow.
As he stood waiting for me, Boughton leaned on the bars of his fat bike, a rig equipped with 4-inch-wide tires designed to plow through the fluffy stuff. He'd agreed to show me the ropes of this young but increasingly popular sport and suggested we ride something that offered the "giggle factor" of a fast descent.
"What the hell," I said, nervous but not wanting to cramp my guide's style. "I'm in no hurry. Let's give it a shot."
We dropped into the narrow trail, following a 10-inch-wide band of packed snow in the center that was flanked by drifts of deeper powder. Within seconds, Boughton was out of sight, and I struggled to navigate the snowy singletrack as it descended through the woods.
Suddenly my front tire drifted off of the packed trail and augured into a snowbank, throwing me over the handlebars. I landed with a poof in a foot of snow, shaken but unhurt. In the summertime, such a fall certainly would have drawn blood, but this one had merely fogged my glasses.
Softness seemed to be the norm - minutes before we had been speeding downhill toward Hunter Creek, on trails whose stumps and rocks were smoothed by the snow into a slick, fast carpet of white.
I rose and brushed myself off. This, I decided, was fun.
Fat bikes have been around for years, though in Aspen their popularity has mainly been limited to a circle of gung-ho bike racers looking to stay fit all winter long.
For such seasoned cyclists, the sport offers a way to rediscover the joy of riding a bike. And as curiosity about fat bikes has grown recently, many of their early adopters have become evangelists, spreading the fat-bike gospel to all who will listen.
"I remember the first time I got on a fat bike, I was riding the Ditch Trail in Snowmass in about 10 inches of powder and just laughing my ass off," said Eric Skarvan, who owns the local adventure company Sun Dog Athletics and recently started offering fat-biking lessons.
"The last two winters, I haven't been skiing as much because the fat biking has been so good," Boughton added.
"People have been riding these bikes for a long time," said Randy Tuggle, a dedicated fat biker who sells the bikes at the Gear Exchange in Glenwood Springs. "But no one really took notice, and over the last two years it's just exploded. These bikes are no longer a novelty - it's a true functional, fun, fast race bike."
Along with Gear Exchange owner Darin Binion, Tuggle leads fat-bike night rides up Sunlight Mountain Resort twice a week, and riders also flock to the Prince Creek trails outside Carbondale. Upvalley, the Maroon Bells, Smuggler and Hunter Creek are popular routes, and Ute City Cycles, the biggest retailer of fat bikes in Aspen, is now organizing a weekly fat-bike ride.
Boughton recently rode up to the Independence ghost town on his fat bike, but the snow was too deep there to go any farther.
"My next goal is to ride Little Annie's Road and come down Spar (Gulch)," he said, referring to the run on the front side of Aspen Mountain. "We'll see how that goes."
Even for the seasoned mountain biker, it takes time to adjust to the width and weight of a fat bike. The upside of that girth, though, is a supremely stable ride.
"The stability you get with those fat tires makes it feel like you're on a dirt bike without a motor," said Dave Carter, manager of Ute City Cycles. "We ride the same trails that we would ride on full-suspension mountain bikes in the summer."
Fat bikes don't excel in all snowy conditions - they certainly won't float in a field of untracked powder. In general, hard-packed snow is best.
"You typically have to wait for two or three days after a big dump before you ride," Boughton said. "And you learn to read the snow. Anywhere people have taken snowmobiles is a good place to ride."
Dressing for a fat-bike ride also remains an inexact science, as the sport is characterized by periods of high exertion followed by cold and fast downhill stretches.
"You generate a lot of heat on those climbs, and it gets very cold on the descents," Skarvan said.
Fat bikers favor multiple polypropylene layers, shoe covers, neck and ankle gaiters, spare jackets, insulated water bottles and sometimes even ski pants. Even with all that gear, Boughton said, "Fat biking can sometimes be cold and miserable."
A definitive history of fat biking is hard to pin down, but the sport has its roots in cold northern winters. Mark Gronewald, of Alaska-based bike-maker Wildfire Designs, is widely credited with building the first fat bike, and Minnesota-based bike company Surly produced the first mass-market bike, the Pugsley, in 2005.
The innovations that distinguish fat bikes from their conventional cousins in the mountain-bike world reside mainly in the wheels: Fat bike tires can be up to 5 inches wide, compared with 2.25 inches for typical mountain-bike treads.
To increase traction, the tires are inflated to anywhere between 3 and 9 pounds per square inch, which is about a quarter of the pressure common in summer mountain-bike tires.
Such fat tires require wider rims, along with frames whose forks and bottom brackets can accommodate the wheels. To avoid pedal contact with the wheels while riding, the spindle connecting the bike's cranks also must be wider.
Because of their girth, the bikes are significantly heavier than conventional mountain bikes.
"The stock Surlys are coming in at between 35 and 37 pounds," said Ute City Cycle's Carter. His shop sells a titanium frame made by Carver Bikes, of Maine, which he says can bring the bike down to around 27 pounds.
Today, Surly dominates the fat-bike market along with sister company Salsa - both firms are owned by the bike-supply company Quality Bicycle Parts. However, a suite of boutique bike-makers also has sprung up. The Alaska-based company 9:Zero:7 makes an aluminum snow bike, and Carver makes a titanium frame.
For now, other than doing some welding of your own, there's no cheap way to get into the fat-bike game. A Surly Pugsley starts at $1,500, and a Carver titanium fat bike with nice components could run upward of $5,000.
Recently, there have been rumors in the fat-bike blogosphere that bike behemoth Specialized might enter the market. That could mean the introduction of an entry-level bike priced at less than $1,000, which would surely push more people to try the sport.
While recent advances in fat-bike technology have been impressive, gear improvements aren't the only thing driving increased interest in the sport. In western Colorado, recent weather has certainly played a role.
During the 2011-12 ski season, area snowpack was well below historical averages, and the same has been true this year. With less powder comes a decreased interest in skiing, particularly among local powder-hounds, who then hunt for something else to do.
"We are looking at getting into (fat biking) for next winter," said Morgan Vail, manager of Ajax Bike and Sport in Aspen. "With two years in a row without a lot of snow, we're looking for other ways to pay the rent, and this is one of them."
"We've been getting increased demand for the bikes, and we want to carry them for rental and sale," said Ed Garland, owner of Aspen Bike Rentals. "We're getting some 9:Zero:7 bikes in any day now."
Ute City Cycles has been carrying the bikes for nearly four years, longer than any other Aspen shop. Skarvan has an agreement to use Ute's fleet for fat-bike lessons, and shop manager Carter says fat bikes represent between 50 and 60 percent of Ute City's sales during the winter.
In the past year, though, Carter says fat biking has become so popular that it's become hard to find parts for
"We go to order parts, and they're not available," he said. "They are already out of stock on a lot of wheels until next year."
Charlie Tarver, owner of The Hub of Aspen bike shop, has been riding bikes in the snow for a long time. In fact, he nearly died in the winter of 2002 when he crashed trying to set the world speed record on a specially designed bike at Snowmass Ski Area. He reached 98.65 miles per hour before the accident.
Even before that, though, Tarver and his friends would weld mountain bike rims together and run two tubes beneath extra-fat tires, using the rigs to ride up Independence Pass or over Vail Pass during the winter. In 1987, Tarver used a similar setup to compete in the Iditabike Race in Alaska, a 274-mile winter ride that some say marked the birth of the modern fat-bike movement.
These days, Tarver's shop seems to be the only one in Aspen without plans to stock up on fat bikes. He carries the Surly Pugsley but has only one in stock, and he says the bulk of his revenue comes from selling high-end road bikes year-round.
Perhaps it's not surprising, then, that Tarver doesn't ride fat bikes himself - he's content with an ordinary mountain bike to get around town, and he still likes skiing. (The backside of the "Open" sign on his shop door says "Gone Skiing - You Should Too!")
Still, even Tarver sees the appeal of this emerging sport.
"Just like fat skis are better for powder than narrow skis, fat tires are better for snow than narrow ones," he said. "It's going to be more fun."