Fat Bike Drivetrain Dimensions
When you think about different size fat bikes you probably think about tire size first: 26x4” fat, 26x5” fat, and, now, 27.5x4" fat. Although tires are the main consideration, other bike dimensions must be adjusted in order to accommodate certain tire sizes.
When fat bikes first came out, 26x4” tires were the only option. These first fatties were built with a 100mm bottom bracket shell for tire clearance along with an offset hub (asymmetrical) configuration that allowed for 135mm rear hub spacing (also called over locknut dimension or abbreviated as “OLD”) since wider hubs did not yet exist. Eventually 26x4” fatties received a rear hub spacing of 170mm for a standard 10mm axle and 177mm for 12mm thru axle. This wider spacing allowed fat bikes to have symmetrical spacing, ditching the asymmetrical spacing, which was kind of a pain in the ass.
To get even more float and traction than the 26x4” fat bikes would provide, the 26x5” fat bike was created. The wider tire, however, required wider rear hub spacing so that the chain would not rub on the tire. The standard rear hub spacing for a 26x5” fat bike is 190mm for a standard 10mm axle and 197mm for 12mm thru axle. This 20mm addition to rear hub spacing also required a 20mm addition to the bottom bracket shell and crankset spindle width to get the chainline correct. Initially this additional bottom bracket shell spacing was handled by a wider crankset and two 10mm spacers (one on each side of the bottom bracket shell). Later on, the option of a 120mm bottom bracket shell was provided in order to get rid of the spacers and provide a bit more real estate in the rear end of the bike for shorter chainstays.
At Pornstar we typically build our 26x4” and 27.5x4" fat bikes with a 100mm bottom bracket shell and 170/177mm rear hub spacing. Similarly, we typically build our 26x5” fat bikes with a 120mm bottom bracket shell and 190/197mm rear hub spacing. However, with an expanded selection of fat bike drivetrains and tires on the market--especially with 1x11 and 1x12 drivetrain options--it is possible build a 26x4+” (maybe even 26x5" depending on the tire) fat bike with a 100mm bottom bracket shell and 170/177mm rear hub spacing.
If you’re interested in Pornstar’s Fill It, Chubby! frame or a fatty Fluxilingus (love that alliteration!), we’ll work with you on the design to get the setup that is best for you. If you’re buying a fat bike frame or complete bike elsewhere, just make sure you understand which spacing your fat bike has so you can purchase the correct components for your new bike. As always, feel free to drop me an email at firstname.lastname@example.org.