Although the Fat Bike was born in snow they have proven useful in sand and other environment where greater floatation is desirable.
The most popular tires for the modern production bikes born of Alaska are made by Quality Bicycle Products under the Surly name. They are 4" wide! This requires a frame with much more clearance for the huge tires. Putting a Fat Bike build together requires attention to a lot of details not found on normal Mountain Bikes.
The asymmetrical Fat Bikes like the original Surly Pugsley have a 135mm rear spacing and a 135mm front spacing designed to use a rear hub in the front. The logic behind this is that if your rear hub fails you can swap the front and rear wheels and be back in business. This design causes some challenges in getting wheels built, installing racks and with brakes. So keep this is mind if you are building a bike up from a frame.
Last year during the North America Birkebeiner cross country ski race a skier who was unable to ski the race wanted to go along and cheer on his friends. He and a few compatriots rode their Fat Bikes on forest roads to intersect the ski race course. (Jerry Wrights Story) This was the birth of the new Fat Bike Birkie. Scheduled for March 9, 2013 two weeks after the Birkie ski race organizers expect to get 200 or more Fat Bike riders to ride the first ever Fat Bike Birkie.
So come on up to the Northwoods of Wisconsin in March to ride the Birkie on your Fat Bike!