Wide tires/rims mean that in order to use a derailleur at the back end of a Fat Bike you need to either offset the drivetrain to the right [ie. Surly Pugsley & Moonlander] or use a very wide 170mm rear hub [ie. Fatback & Salsa Mukluk]. To me the obvious solution for a bike that will be used in the snow/sand/mud is an IGH where the shifting is internal and not exposed to the elements which also means the chain doesn’t have to move side to side so clearance with the tire is not as much of a problem.
Offset Fat Bikes like the Pugsley use a 135mm dropout spacing so an IGH works just fine. However, bikes like the Mukluk shown above that use a 170mm dropout spacing don’t allow for an IGH since nobody is making 170mm IGHs.
Salsa and Fatback are offering adapters that reduce the rear spacing of their frames from 170mm to a standard MTB 135mm. That gives you the option to use an IGH and if you want to run a 170mm derailleur hub you can do that as well.
Fatback is talking about offering a 170mm frame with sliding dropouts. Currently their adapter will only work with the aluminum Fatback frames. Those frames are available without sliding dropouts, but they are prototyping an aluminum sliding dropout Fatback. I’m not sure when that will be available.
A few notes:
- from what I can tell you’ll have to use an IGH that accepts a QR [ie. Rohloff @ $1600] to use a 170mm to 135mm adapter. I also have to confirm that OEM2 torque support/Monkey Bone setup would work with the adapter.
- all IGHs require some method to resist rotation of the shell under high pedal loads [Rohloff uses a torque arm, Alfine uses non-turn washers, etc..] I haven’t seen anyone make this work with the Salsa adapter yet. It might be feasible, but it could be a deal breaker if not so it’s worth noting the uncertainty.
- the chainline of an IGH needs to be straight and lined up with the single ring up front. I know that the 17.5mm Pugs offset is no problem, the Moonlander uses 28mm offset [I'm not sure how easy it is to get a good chainline there] and with the Salsa adapter you’d have an offset of 35mm which may be a significant issue as you don’t have that many ultra wide BB/crank choices.
- 9:zero:7 offers frames in 135mm and 170mm dropout spacing and has confirmed that a 135mm AL fat bike with sliding dropouts is on the menu for next season
- so if you want to use a bolt on IGH [ie. Alfine 8 or 11] you’ll need to stick with a 135mm dropout Fat Bike
- so your options for an Alfine’d Fat Bike without using a chain tensioner are Surly’s which use 135mm horizontal dropouts
- you can use an Alfine in a 135mm 9:zero:7 frame with a tensioner
Taken together I don’t think a 170mm wide fatbike and an IGH make much sense. You lose so much of the benefits and there are so many hassles unknowns that if you really want an IGH in a fatbike you should stick to a 135mm offset build.