This article is part of the Fat-bike 101 series and is intended to help new fat-bike owners, and potential fat-bike owners, with some of the questions they may have about fat-bikes. In addition, it is a chance for experienced riders to add comments to help our new brethren enjoy all that fat-biking offers. Consider these articles as conversation starters, not exhaustive explanations!
Fat-Bike 101 Frame Types
If you go back in history far enough you will find all kinds of unique bike designs trying to solve the same basic problem; how to get fatter tires to work without the chain catching on the rubber! Since this article is not intended as a history lesson we will leave that discussion for another time and, hopefully, from people more qualified to really dig into the root of fat-bikes.
So, that said, fat-bikes have pretty much taken two paths to the solution for getting the mechanical bits around the big tires. They are, in fact, quite similar but with some obvious differences.
Offset fat-bike frames move the rear hub to the right in order for a standard MTB cassette to clear the big tire.
The tried and true Offset design, like the popular Surly Pugsley, relies on a frame that has the rear end offset 17.5mm to the right to move the cassette/gears out so that when you are running your chain on the little/inner chainring in the front and the big/inner cassette cog on the back that there is still ample clearance for the chain not to rub on that big, whompin’ 26 x 4.0 tire. Now that you’ve moved the hub/gears over you’ve got a wheel that is offset too! Caddywhompus won’t do for a bike that needs to go in a straight line from time to time though. That is why you will find that in order for the center of the wheel to be on the centerline of the bike the wheel must also be built offset; this time to the left to compensate for the right offset of the frame. Very clever and it works quite well at solving the drivetrain problem while also giving you a bike that tracks straight and true.
Fat-bike wheel built offset to the left so it is centered in an offset fat-bike frame.
You may notice that some offset fat-bikes also include an offset fork with the front wheel built on a 135mm single speed rear hub and offset the same 17.5mm. The reasoning here is that fat-bikers tend to go on adventures where there is little to no support and if there is a failure in the rear drive system be it cassette, derailleur or whatever, you could swap wheels end for end and have a single speed drive wheel to get you home.
A major advantage of the offset design is that it uses rear hub axle spacing common to many mountain bikes of 135mm. This gives you a wide range of choices on which rear hub you choose from basic standbys like the Shimano XT to high-zoot hubs like Chris King as well as the most compelling reason to still consider an offset fat-bike, the ability to use an Internally Geared Hub like the Shimano Alfine line, the Rohloff Speedhub or the Nuvinci N360. All use 135mm rear axle spacing.
If you are considering an Internally Geared Hub (IGH for short) for your fat-bike you pretty much have to use an offset frame. Salsa tried an adapter system for their 170mm Mukluk a while back but have since dropped the idea from the Mukluk line and we haven’t run across any other workable solutions.
IGHs offer several compelling reason to consider them for your fat-bike drive system including the fact that all the gears are enclosed, there is no derailleur to break off and the ability to use a Gates Carbon Drive belt system instead of the more conventional chain drive. When you consider that many fat-bike owners ride in snowy winter conditions as well as sandy-beach summer conditions IGHs make for a very clean and reliable drive system.
Centered in the frame like traditional bikes symmetrical fat-bikes most often use a 170mm hub spacing.
Referring to a fat-bike as “symmetrical” means that the rear triangle is symmetrical, like most traditional bikes, with equal spacing of the right and left dropouts from the centerline of the frame. Using a conventional 135mm mountain bike hub on a symmetrical frame would not allow the chain to clear the tire in the low-gear range so the space between the rear dropouts needs to be wider. A rear axle spacing of 170mm is the emerging “fat-bike standard” though other rear axle spacings have been used for fat-bikes most often from smaller builders including 165mm, 160mm and 150mm. Originally, available hubs drove this difference as builders adapted hubs from tandems to fat-bikes. The 170mm dimension got a push towards standardization when Fatback owner Greg Matyas began specing them on Fatbacks back in 2008. With the explosion of fat-bikes more hub makers are offering wider hubs and the 170mm width has become the defacto standard for symmetrical fat-bike hubs.
A traditional mountain bike rear hub is 135mm and, as stated above, is often used offset 17.5mm to the right on offset frames so to keep the wider spacing on the right but make the rear triangle of the frame symmetrical you’d need to add in 17.5mm to the left side as well so 135mm + 17.5mm +17.5mm gives us the 170mm rear hub!
So why didn’t fat-bikes go Symmetrical from the get-go? There are a couple of reasons why symmetrical fat-bikes did not take the world, immediately, by storm. Lack of available parts was at the top of the list. Only recently have a good selection of 170mm hubs made it possible for frame builders to spec this width hub.
How wide can you go? The Surly Moonlander upped the ante!
The Surly Moonlander upped the ante even more for the fattest of the fat. Using an extreme offset of 28mm this unique fat-bike was designed to use 26 x 5.0 tires mounted on 100mm rims to give your fat-bike a HUGE footprint and by doing so disperse weight over an even bigger area that “conventional” fat-bikes.
Finally, some of the Alaskan contingent are working with symmetrical frames again but with an even wider rear hub that measures in at 186mm. This rear triangle spacing allows for a symmetrical rear wheel to be built up with enough clearance to accommodate a 100mm rim, 26 x 5.0 tire, and full 2×10 drivetrain.
What is next?
190mm hubs? Up to 160mm rims? Crystal balls are notoriously mirky but as the fat-biking market matures we should continue to see more choices for frames and components to build your dream fat-bike from. Which one is right for you? Hard to tell, each riders needs are unique but if you want a fat-bike I am sure you will be able to find one that suits your needs.
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