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The Making of a FatBike

Photo contributed by Wildfire Designs Bicycles
Everyone’s heard of the term fat tire, whether it’s in reference to a mountain bike, or a beer. But fat bike? Not so much. Imagine you wanted to ride your bike on a snow-packed glacier, or a frozen swamp, or perhaps compete in the Alaska Ultra Sport Iditarod Trail Invitational, riding your bike 1,100-miles along the dogsled course from Anchorage to Nome. No ordinary bike would do.

The fat bike concept is part of the growing sport of winter and soft condition riding. The evolution of the bike itself is one-part technological advancement, and one-part creative adaptation. Fifty-one-year-old Alaskan Mark Gronewald, who created the first fat bike in 1999, walked us through the process.

The biggest challenge was engineering a frame to accommodate the widest available rims and tires without sacrificing shifting and riding performance. A fat bike needed tires and rims much wider than those of a standard mountain bike – jumbo-sized 65- to 82-mm wide rims and 26- x 3.7-inch tires. These monstrosities required a wider chainline, otherwise the chain would rub the sidewalls of the tires. To get a wider chainline, a wider bottom bracket was needed – a 145-mm bottom bracket spindle in a 100-mm shell, to be exact.

Next was figuring how to make things come together on the rear end. A wider chainline and bottom bracket needed a wider hub at the back of the drivetrain. Back when fat bikes were born, wider hubs just weren’t available. The solution was to create a “virtual wide hub,” offsetting the rear triangle 18-mm out toward the drive side, which brought the cassette out in line with the front chainrings.

The last challenge was to bring the rim and tire back in line with the bike’s center. This was accomplished by drilling offset spoke holes toward the drive side of the rear rim, an action which gave the added benefit of a strong wheel build, since it allowed the use of equal length spokes on both sides.

Photo contributed by Wildfire Designs Bicycles
Meanwhile, in mountain biking, 29ers started taking off. The fat bike geometry was almost identical to the 29er geometry, and the industry had a collective “aha moment,” realizing a fat bike’s wheels could be swapped out for a pair of 29ers in the summer, making a fat bike a year-round all-terrain riding machine. But 29er rims are skinny, and didn’t allow for much of an offset spoke drilling. So at that time, even innovators like Gronewald just couldn’t get everything to line up using the 29er wheels.

Then technology caught up, nearly 10 years after the first build, when wider hubs became available. So out went the virtual wide hub and rear offset, and in came a plug-and-play 160-mm hub. Not only did that make the job of the framebuilder easier (less chainstay and seatstay bending), but it worked out perfectly for the chainline, and allowed the fat bike to be used year-round by substituting a pair of 29ers in summer conditions. Voila! Fat bikes were ready for the masses.

Gronewald’s version of the fat bike, which he aptly calls FatBike, has a frame made from premium-grade double-butted chromo steel. It weighs around 4.5-pounds in the largest size, and handles some of the toughest riding conditions known to man. When totally built up, the bike weights between 28- and 38-pounds, depending on the frame size and components.

Currently there are only three suitable tires on the market for winter fat-biking: the Surly Endomorph 3.7, the Surly Larry 3.8, and the Tomisea Spider. Gromewald recommends the Endomorph as a rear tire because of its widely spaced chevron lugs, which provide great forward traction and also self-clean well. He said the Larry works better as a front tire because it has more side knobs and steers better without sliding sideways as much as the Endomorph.

No matter what type of tires you run, it’s good to keep things in perspective: a fat bike is not a tank. It’s going to bog down in fresh powder or really loose granular snow. But part of the fun of fatbiking is hunting down those little pockets of firmer terrain, and learning to put together micro lines to create a doable route through seemingly impossible conditions.

For a comparison of Gronewald’s FatBike, and the three other fatties available, check out the April 9th post at FatBike Alaska.

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Jayme Otto races road and cyclocross in Colorado where she served as captain of Title Nine, an amateur woman's bike race team. Off the bike, Jayme is associate editor at Boulder-based Elevation Outdoors magazine and contributing editor at Women's Adventure. Her freelance writing has appeared in Bicycling, Backpacker, Runner's World, Running Times, Trail Runner, VeloNews, and Women's Running.
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