A fatbike is a bicycle with over-sized tires, typically 3.7" or larger and rims wider than 44mm, that are designed for riding on soft unstable terrain such as snow and sand. These bikes are built around frames with large forks and stays to accommodate the wide rims required to fit these tires.
Fatbikes were invented for winter trail riding and racing in sub-arctic Alaska and simultaneously, for touring the deserts of New Mexico. Their utility has expanded to include all forms of cycling; they thrive in snow, sand, desert, bogs and mud as well as riding what is considered normal mountain biking.
The original fatbikes were normal mountain bikes equipped with the SnowCat rims, created by Simon Rakower of All-Weather Sports in Fairbanks, Alaska. Simon was involved with technical support aspects of the Iditabike (later IditaSport) race, which started in 1987. He started hand making extra wide rims for participants by welding two rims together and cutting off the middle ridge. Enthusiasts would cut and sew tire-carcasses together to maximize the size of the tire and utilize all the available space between the seatstays and chainstays; this tire and rim combination would maximize the bicycles footprint, increasing flotation on winter trails. Soon after, Simon decided to design a 44 mm rim from scratch and had it produced. SnowCats revolutionized winter cycling as they could be fitted to nearly any commercially available mountain bike.
At the time frame builders were experimenting with custom components and configurations designed to achieve a large contact patch of tire on snow. Simultaneously, in New Mexico, Ray Molina had commissioned 80mm rims, 3.5" tires and frames to fit them. He wanted the bikes for his guided desert tour business; the soft-sand of the arroyos. Rims and tires were imported to Alaska where frame builders began making small, handmade, production runs and custom-ordered frames built around the 80mm rims and 3.5" tires. Surly Bikes then released the Pugsley frame, Large marge rims and Endomorph tires. The Pugsley frame, rim and tire offerings made fatbikes commercially available in local bike shops world-wide.
As of 2013, many new brands of fatbikes have become available, including models from big box companies like Trek and Specialized.
These companies include:
- Charge Bikes
- FatBack 
- RSD Bikes (Rubber Side Down)