As companies scramble to get their best designs to market in the exploding segment of fat bikes, a whole new array of dimensions are being offered. Tire size, hub spacing, bottom bracket width, are all being hammered out to find the sweet spots of functionality.
Here’s a quick overview the situation:
Tire width on fat bikes falls into one of two nominal labels: 4-inch or 5-inch.
So called 4-inch tires were the first of the huge knobbies, offered on classic models such as the Salsa Mukluk and Beargrease. Actual dimensions are typically labelled 3.6-3.8 inches and are mounted on 60-80mm wide rims. This size is now considered a year-round, all terrain performer. Light and narrow enough for general trail riding while still having enough flotation to venture onto the beach or navigate packed snow conditions.
5-inch tires are the latest development and are intended for maximum flotation over extreme terrain: deep sand, snow, and whatever loose, rocky, leaf strewn course you plot. Trail or no trail, these behemoths will conquer whatever your legs can push through. Actual sidewalls are labelled 4.6-4.8. Tires of this size are ideally mounted on rims 80-100mm wide. Featured on Specialized Fatboy, Salsa Blackborow, Moots Frosthammer, and Framed Minnesota XWT.
With wider and wider tires and rims, frames and forks have had to swell to provide necessary clearance.
Up front, the original 135mm width is still prevalent on bikes with standard quick release front hubs such as the Mukluk and Fatboy. Rigid thru axle forks use 142mm wide, 12mm axles, just like the rear of a standard MTB. The Rock Shox Bluto suspension fork has brought about the burlier 150 x 15mm thru axle format. Several rigid bikes from Salsa, Rocky Mountain, and Framed are spec’d with the same spacing to facilitate an easy exchange between forks for different seasons and riding environments.
Out back, 170mm quick release or 177mm thru axle hub spacing is found on 4-inch tired bikes. 5-inch bikes run on either 190mm QR hubs or 197mm thru-axles.
Luckily many hub manufacturers are offering a variety of end caps and axle options to convert between the standards should you choose to alter your setup or buy a new frame or fork. Most front hubs will convert to any of the 135-150mm standards. However, rear wheels have less flexibility. 170mm QR and 177mm thru axle are generally interchangeable, as is 190mm QR and 197mm thru axle, however 170mm cannot interchange with 190mm.
The 100mm bottom bracket is nearly standard on fat bike frames at this point. Both threaded and press fit shells are common and there are now a wide variety of cranksets available, from heavy duty to ultralight carbon. However, the resulting chainline may still not be optimal on the widest platforms. As such, Salsa and Surly have introduced an even wider 132mm shell for the best possible gear alignment.
If you’re looking for One-Bike-to-rule-them-All, consider a 4-inch tired platform such as the Mukluk, Beargrease, or even the ground breaking Bucksaw. On the other hand, if barrier beaches, knobby prints through untracked snow, and generally going where no bike has gone before dominate your day dreams, then perhaps a 5-inch fattie will be needed.
As always, experience is best. Stop by, chat with our staff and take a ride on each. We’ve got the the best fat bike stock in Rhode Island.
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