By Tom Jow
Fat bikes. Everybody has seen them. If you haven't, you will soon. They are those bikes with those funny looking ultra-fat tires. These bikes evolved out of necessity in order to be able to survive, and conquer the Idita Sport ultra marathon mountain bike race during the winter in Alaska. Few people outside of Alaska knew about these bikes until a Minnesota bicycle company named Surly designed the venerable Pugsley. Now, fat bikes are entering another wave of popularity. Mainstream bicycle companies such as Trek, Specialized, Kona, Norco and others have all entered the market. Custom brands have upped the ante by building frames with ultra-modern materials such as titanium; custom parts makers now offer lightweight cnc aluminum hubs, carbon rims and tubeless tires. This availability of custom frames and parts enables bike owners to upgrade their current bike, or for some, piece together a new or used frame with some leftover parts from the garage. As with any frame up build or upgrade, it is important to be sure all parts are compatible with each other. However, not only are some parts in the garage incompatible with fat bikes, some new fat bike parts are not compatible with older fat bikes. So there can be a conflict between some new and old standards.
Starting with the frame, it all begins at the bottom bracket. Unlike the 68/73mm bottom bracket shell of a standard mountain bike, a fat bike uses a 100mm shell (same as some downhill bikes). Conveniently, bottom brackets of all types are available for that size. Phil Wood makes a high end unit for square taper cranks and both FSA and Truvativ offer Isis Drive bottom brackets. Portland, Oregon based manufacturer Chris King has bottom brackets to fit modern through axle cranks made by Race Face and Surly. Cranks are also available from e.13, FSA and SRAM.
At the rear of the frame, the standard hub width is going through some changes. The early fat bike frames used standard 135mm rear hubs. In fact, current Surly frames still do. However, the newer frames are being built with 170mm or 190mm rear axle spacing. This change in axle spacing also represents a new design of the rear triangle. On the older frames, the only way for the chain to clear the tire in the low gears was to move the frame and drivetrain to the right and the tire to the left. Now, with wide hubs the both the frame and wheel can be built with a stronger asymmetrical design.
It's the wheels that really define the fat bike. Over the years both tires and rims have gotten wider, and in some cases lighter. The tires, having more than a half dozen makers, range in size from 3.7 inches to a whopping 5 inches wide; tubeless or with tubes. It is these wide profiles that provide the flotation and traction for riding on sand, snow and ice. To support these fat treads, rims are available in widths ranging from 65mm to 100 mm. Surly, one of the fat bike rim originators continues to make lightweight aluminum rims which are compatible for both asymmetrical and symmetrical lacing. Stan's No Tubes will be coming out with a tubeless rim at the end of the year. As in all of cycling, the new gold standard for rim material is carbon fiber, with manufacturers Borealis and Fatback leading the charge. And with tires and rims this size, it can be easy to overlook something like hubs. For 135mm frames, any good mountain bike hub will do. For the newer 170mm and 190mm standards it is necessary to look at custom parts. Top of the line hubs of either size are made by Paul Components, Phil Wood, and White Industries. Joining these stalwarts of manufacturing is Alaskan frame and component maker, 9zero7. So, when purchasing wheel parts, remember that rims to be used on older frames must be compatible with an offset lacing pattern to match the offset of the frame and/or fork.
Just as there was a beginning, fat bikes are now at a new beginning. Their popularity is growing rapidly. Consequently, the development of bikes, components and accessories grows with it. Honestly, it is difficult to keep up with. Nevertheless, even though they look different, the fat bike is just that, a bike. The frames are made of the same materials; steel, aluminum, even carbon fiber. The drivetrain components are mostly the same, and the brakes are all the same too. The wheels are their defining component and everything is designed to work around them. And even their design is undergoing changes. This makes now a great time to get into fat bikes.
Got a bike question? Email Tom at [email protected]