They Way We Were: Understanding Offset Frames and Wheels on Fatbikes

Frame Offset

Adventure Cycling, and in particular fatbikes. are a big part of what we do. We are commonly asked about frame and wheel offset as it relates to fatbike design.

Frames were offset when fatbikes arrived on the scene.  This offset was meant to accommodate the chain being able to pass very large tires on its way from the crank to the lower gears of the rear cassette. This frame offset was also required by the fact that there simply weren't a selection of larger fatbike-specific hubs we see today. These were the pioneers.

Figure I

shows a standard modern mountain bike condition, with a 2" tire set into a 135mm spaced frame.

Figure II

shows the interference issue that arose with the arrival of fat tires without the frame dsign to accept them.  The chain cannot pass the tire and set up in the lower (bigger) gears of the cassette.

Figure III

shows the offset frame design.  Offsetting both rear dropouts to the right 17.5mm (in the case of the

Surly Pugsley

)

The image below shows standard line (whte) of dropout structure, while the red lines indicate the offset dropout alignment of the

Surly Pugsley.

Wheel Offset

Because the frame is now 17.5 mm off center to the right, just inserting a standard/symmetrical wheel would cause the center-line of the tire to be offset. This is entirely unacceptable for safety and tracking purposes. To achieve this, many fatbike rims have parallel sets of spoke holes, making building an offset rim easier. The rime is built and dished 17.5mm to the left to achieve the result of having the tire/wheel be on the center-line of the bike

Advantages

The main advantage for this setup is that one can use any 135mm mountain bike rim on a fatbike. It also makes the use of internally geared hubs easier. In addition, if you have a fatbike with an offset fork, you could theoretically swap front for back in case of drive-train issues

Offset Fatbikes

Resources