The concept of slapping some really wide tires on a mountain bike to enable the bike and rider to cover more terrain is pretty simple, but the practical implications are a little less so…
Geometry – FatBike roots in 29er set ups?
Arguably the thing that first helped modern FatBikes really emerge as design possibilities was the 29er revolution. The larger diameter 29er mountain bike wheels (roughly 29 inches in diameter instead of the 26 inch standard to that point) forced manufactures to really figure out what tube lengths and angles worked best to get the appropriate ride characteristics out of a bike with more rubber. Though FatBikes are technically 26 inch wheeled, the tires are so large that they bring the outer rubber diameter to pretty much the same measurement as an average 29er tire. In other words 29ers helped us understand how to design the length of a large wheeled bike, but how about the width?
Obviously the rear triangle and the fork have to be spread farther apart to accommodate the wider rubber of a FatBike, but that’s just the beginning. The much larger issue is chain clearance and chain line (resolving the former goes some distance towards helping with the latter). If you use a conventionally dished rim and standard rear hub, and a ‘normal’ bottom bracket / crankset width, the bike’s chain will rub egregiously against the wide tire. In fact with such a setup you can’t even shift into many of the bike’s gears because the smaller (inner) chainrings and larger (also inner) cassette rings are well inboard of the outer edge of the tire. The solution is of course to push the crankset and the cassette further out from the bike and the edge of the tire.
Crank and chainrings
Until you get REALLY fat most FatBike manufacturers push the chainrings further out by simply using a 100mm bottom bracket instead of the usual 73mm. The bb is still centred on the frame, so the crankset and chainrings are effectively pushed out an additional 13.5mm from the centre line. Depending on the type of bottom bracket you can also use spacers to offset the crankset a little, giving you another millimetre or two on the bike’s drive side.
Some set ups require not only a wider bottom bracket but also some creative crank engineering to get the chainrings even further away from the bike’s centreline or lose the innermost ‘bail out’ chainring to accommodate super fat tires, assuming your frame has the tire clearance necessary.
Surly, the 1st manufacturer to bring FatBikes closer to the mainstream, solves the other half of the equation by dishing their FatBike rims to the left, thereby pushing the standard 135mm rear hub and mounted cassette to the right of centre relative to the rim. Then they actually make an asymmetric rear triangle that bends to the right (that is, the drivetrain) side of the bike. The net effect is that the hub and cassette are pushed to the right but the offset dish of the rim brings the tire back in line with the centreline of the rest of the bike. Surly may be moving away from this design though with the release newer models that use an even wider rear hub and spacing, like Salsa.
Salsa opts to use a more conventional rear triangle design, simply making the whole thing wider, and then uses a 170mm-190mm rear hub to push the cassette further right and outside of the plane of the tire.
The front of the bike is much more simple. Essentially Salsa and Surly have made wider than usual front forks that accommodate 135mm hubs, so the tire can fit between the fork legs and the hub is wide enough to span the gap between the fork legs.
It’s actually a little more complicated than that because Surly produces 3 different forks for the Puglsey that allow for slightly different set up (one actually allows the front and rear wheels to be interchangeable), and depending on FatBike set-up you also have to know details like whether you need to use a front or rear brake calliper (in some cases you use a rear calliper even on the front of the bike), but you get the general idea. You also have to consider whether you have a quick release or thru axle compatible frame and fork if you’re making conversions, like switching to 29er wheels for the summer.
In simple terms that’s about it really. There are certainly other FatBike design considerations like how to save tire weight on such large hoops, how to mount front derailleurs far enough outboard to fit properly around the chain, what the ‘best’ rim width is, and so on, but we’ll save those considerations for another page.