I’ve found a few racks sold that are specific to fat tired bikes, but they all share one thing in common: they’re expensive. Really expensive. Regular rear racks most often do not have the space that would allow a 4+ inch tire to fit and actually be able to spin. I managed to find the closest thing possible to a perfect fit in a “normal” rack, the Transit TD-1 from Performance, for right around $30. Not only does this rack fit well, it’s beefy enough that it looks at home on a fat bike.
Putting a rear rack on a current Mukluk is further complicated by two issues. First, is the fact that the Salsa Mukluk has rear spacing that is 170mm, instead of the typical 135mm spacing of nearly every other bike sold in the western world. There are advantages to this wider spacing but fitting a rack is not one of them. The second is that there are no lower braze-on mounts for a rack (or fenders, or anything else) with Salsa’s Alternator dropout system. This rack gets around both of those issues pretty well.
The rack is marketed as being compatible with bikes that have disc brakes. They manage this compatibility by providing long mounting bolts and thick aluminum spacers that are about an inch long to push the mount points for the rack far to the outside on a 135mm spaced frame. This makes the rack just about as wide as the outside of the Mukluk’s dropouts (without the spacers). Since there are no braze-ons for a rack you need to replace the top pivot bolt on the Alternator dropout with a longer bolt; a disc brake mounting bolt works perfectly. Fortunately, I had several of those kicking around in my collection of bike parts. The only hacking that had to be done was reducing the length of the aluminum spacer. I took about 1/3 inch off using my dremel tool and a hack saw. The dremel is probably one of the most useful power tools ever made. Seriously, it’s infinitely awesome. Get one.
Once both spacers were trimmed and filed smooth, it was simply a matter of replacing the pivot bolts.
The only other modification I made was purely optional. Since I planned on using this rack with loaded panniers, I wanted to lower the rack as much as possible over the wheel. I need a lower center of gravity for any unusually heavy loads I might be carrying, like bringing my lunch to work with me. I did this by drilling out an additional hole in the leg of the rack about one inch above the uppermost hole. I started this using the Wonder Tool (dremel) and finished the hole off with an appropriately sized regular drill bit. This allows the rack to set down a bit lower but still have plenty of clearance for snow and ice. See below: