In my first post, seen here, I gave an overall view on “fat bikes” and why they matter to 29″er aficionados. In this post, I will go over a few of the oddities of fat bikes that are necessary to make them work. Those big, big tires cause a few things to be quite different from the typical mountain bike.

birthday ride 008

Due to the extra-wide nature of the tires and wheels on a fat bike, several evolutions occurred to make drive trains compatible with the wheels. The biggest challenge is how to get a full range of gears without chain/tire interference. There are a couple of ways used today to solve this conundrum. Let’s take a look at them and dissect the technical aspects of each.

interbike2010 173

The Offset Frame/Fork Type: Essentially, what needs to be done is to get the cassette in the rear further outboard to help clear the tire, while doing something similar with the crankset, all the while maintaining a workable chain line. The “Offset Type” fatbike does this by offsetting the frame 17.5mm to the drive side of the bike. This sticks the hub out further from the center line of the wheel/bike and allows the chain to clear the rear tire, even when using the lowest gears available. This requires special offset drilled rims.

Up front, the crankset is offset further from the center-line of the frame by using a 100 mm bottom bracket. But instead of an asymmetrical set up, the bottom bracket is centered on the center-line of the frame. This gets the crankset in a position to maintain a workable chain line and clear the big rear tire in the lowest gears. Typically an “E Type” bottom bracket mounted derailleur is used for front shifting duties. (However, a new product made by Problem Solvers is now available to employ the use of direct mount type front derailleurs as well.)

With the drive train and rear wheel all pushed over to make accessing the drive train’s complete gearing range possible, the last thing designers had to do was to get the front fork to line up with the rear end. To do this, the front fork is also offset, just like the rear end of the frame, by 17.5mm. To get those big wheels in and out of the fork when necessary, designers found that spacing the front end at 135mm, like the rear end, allowed for easier wheel removal. Now with this set up, it is also possible to swap front and rear wheels. This allows for several possibilities in setting up the bike with different gear ratios on cassettes, or to set one up as a single speed wheel as a bail out measure in case of derailleur failure.

The Offset Type fat bike is offered by Surly Bikes in their Pugsley model and Chain Reaction Cycles AK with their 9:Zero:7 model.


The Symmetrical Type: While setting up offset wheels in an offset frame and fork was one way to solve the puzzle, Greg Matyas of Speedway Cycles and others developed a symmetrical rear ended fat bike that still allowed for full access to the rear cassette without interference from the fat rear tire. How did they manage this? They simply added width to the rear hub until the cassette was in the proper position. This amounted to adding 17.5mm to each side of a 135mm O.L.D. hub for a grand total of 170mm.

A Phil Wood 170mm OLD hub in back of a traditional 135mm OLD single speed hub.

The wider hub kept everything “on center” so no offset wheels or fork is necessary. However; unlike an Offset Type fat bike, the wheels are not interchangeable. This may not be a big deal to traditional mountain biker types, but some fat bike aficionados are not big fans of the lack of a fail-safe for extreme conditions where failure is not an option. Supporters of the Symmetrical Type will point to the dishless wheel builds as being stronger, which isn’t the case with offset fat bike wheel builds. Other than that, the Symmetrical Type only differs in that the front fork is not offset. The width between the drop outs on these forks is still 135mm.

Purveyors of Symmetrical Type fat bikes include the aforementioned Fat Back Bikes and Salsa Cycles, which makes the Mukluk model I have shown at the top of this post, amongst others.

mukluktwo 006

Rims And Tires: Obviously, the focal point of these bikes are the wheels. With the wide rims and nearly 4 inch wide tires, they certainly dominate and define the look of a fat bike. Tires for these monsters are only available from two sources and all are made by the same manufacturer in Asia. The most popular models are from Surly Bikes and have curious names like “Endomorph” and “Larry”. I won’t get into the details on the “why” of the tires since Surly does an excellent job of explaining it already. (Check the Endomorph page here for a great treatise on fat tires for fat bikes.)

Obviously Surly and others wanted a really wide rim to stretch out those early down hill tires and the subsequent Endomorphs to maximize the flotation effects. Early attempts at wide rims were usually done by attaching two mountain bke rims side by side, or by crude, single walled aluminum extrusions that were extremely rare. Now days, wide, light-ish rims are the norm. Surly has sold the “Large Marge” rims for some time now, but single walled, drilled out rims are the hot ticket these days. Rims like the Surly “Rolling Darryl“, or Chain Reaction Cycles “Flat Top” rims are usually modified extensively by riders seeking the lightest, widest rims possible. This is a common practice in trailsin bikes as well. Rim liners are used to prevent the tubes (which are also especially large), from poking out and offer the rider another way to personalize the look of their wheelset by using colorful ribbons, duct tapes, or reflective tape which shows through the rims’ “windows”.

Visual impact aside, the end result is a wheel with a very wide tire capable of running extremely low pressures, allowing riders to tackle terrain and conditions that would stop a normal mountain bike dead in its tracks. This includes, but is not limited to, snow, sand, loose rocks, and more. Of course, there are some drawbacks. Weight is an issue that can not be overlooked. The wheels and tires are quite heavy, weighing far more than traditional mountain biking fare, but supporters will tell you that once you get those big wheels turning, they like to keep turning, much like a flywheel. Add in the way they can roll over difficult terrain, and the fun factor stays high.

mukluktwo 003

Typical fat bikes will weigh in from just under 30lbs to the 40lb range, depending upon materials and drive train choices. I have not accurately weighed my Salsa Cycles Mukluk yet, but it falls into the low 30’s for weight. Not terrible, but you can feel it at first. Oddly enough, that feeling seems to go away after a couple of rides. Must be all the fun I am having. Speaking of the Mukluk, I will introduce my bike that I have dubbed “The Snow Dog” in my next post, and give you my First Impressions on what it is like to pilot one of the “monster trucks of the cycling world”.

Note: Salsa Cycles did not submit this bike for review/test. This is Guitar Ted’s personal bike. He will strive to give his honest thoughts throughout the test.