I explain this over an over to people, so I thought I’d write a quick blog post that I can refer to going forward: Fatbikes can do a LOT of different things by using different wheels.
-A 26×4″ fatbike tire is about 730-740mm diameter. That’s about the same as a 27.5×3″ plus tire, or a 29×2.1″ 29er tire. Which means that all three wheel sizes are roughly 29″ diameter and can be used in the same frame without causing any problems with the geometry of the bike.
-Want even bigger wheels/more volume? No problem, 26×4.8-5″ tires are about 760-770mm diameter. That’s about the same as a 27.5×3.8″ (Bontrager Hodag) or a 29×3″ plus tire (ie Knard, Chupacabra, etc). So again, 3 wheel sizes/tire sizes will fit the frame without causing problems with handling, though there is no sub-3″ option (we’ll get to that in a minute).
-There are also some possibilities (24×4″ fatbike tires exist and pair well with 26×2.1 XC tires, for example) for smaller riders who don’t want giant wheels.
So, what needs to happen when you want to do a 2 or 3-in-one bike?
-What will you really be doing with the bike? If it’s mostly a fatbike, design for that. If it’s mostly a summer bike that needs to just barely fit 4″ tires for the occasional snow foray, do that. Need super short stays? Full suspension (yes, I’m happy to build an FS fatbike/plus bike for you) vs. hardtail? Other features? Time to sit down and think about whether you really want a jack of all trades or if you’d prefer to design for one primary purpose and let others be secondary.
-Next, you need to decide what you’re going to do about front suspension (if anything). A Rockshox Bluto will fit a lot of different stuff – but not a 26×4.8 or 29×3. A Reba will do 29×3 and under. A Fox 34 will fit a 26×4, 27.5×4, 29×3, and anything smaller. Lefties, various upside-down forks, and other oddities are also possible. Or you might want a rigid fork for winter/snow, and a suspension fork for summer riding. Regardless, deciding on your front suspension requirements will dictate some of what’s necessary to make everything work.
-Now it’s time to think about chainline (the distance from the centerline of the bike to the center of your middle/only chainring) and Q factor. There are currently two fatbike “standards”:
170mm (QR) and 177mm (through axle): Uses a chainline of roughly 65mm and Q-factor of roughly 200mm.
190/197mm: 75mm chainline, 220+mm Q factor.
Some people (like me) hate ultra wide pedaling stances, but if you want to go narrower, you need to make sure you can fit in the tires you want. The easiest way to do this is to subtract 10mm from your chainline number and multiply by 2 (so max tire on a 65mm chainline bike is going to be roughly 110mm). Beyond this, the chain will interfere with the tire in some gear combinations.
-So if you’re clever, you’re thinking to yourself, “wait, why can’t I use a chainring positioned outboard to keep my q factor down?”
Thanks to the wild proliferation of chainring mounting systems and standards, yes. For example, we could take a SRAM GXP GX fatbike crank (65mm chainline, 6mm inboard offset chainring) and install a BB30 (0mm offset) ring – presto! We’ve got ourselves an extra 12mm of tire clearance and have moved the chainline out to ~71mm (which would match up well with an offset-laced 170mm hub or an on-center 190mm). Raceface CINCH cranks are even better – we could take an 83mm spindle Atlas CINCH crank (~58mm chainline, nice narrow 193mm Q-factor) and flip over the chainring to get to 65mm – so now we’re back in the 110mm tire game, at only 193mm Q factor.
REALLY hate wide stances? Take a RF Aeffect CINCH crank and flip the ring, and you’ve got a 173mm Q factor (standard XC) and 58.5mm chainline – enough to squeeze in a 95mm tire (Surly Nate/Endo/Larry, Bontrager Hodag, etc), if you want to. That would be a great match for a 150/157×12 or offset-laced 148×12 rear hub.
Raceface stuff is great for this and very versatile. Check out their Q factor and chainline chart here.
Of course, you’ll need 2 or 3 wheelsets to take full advantage of all this flexibility. That will mean some financial pain, but nothing like buying 3 bikes! You can easily have me add features like:
-Adjustable dropouts (slider/rocker) to allow different chainstay lengths with different wheels/tires as well as singlespeed use.
-EBB to allow 29+ AND 27.5+, or other combos of non-matching diameters of wheel (or just to mess with BB height and/or allow singlespeed use). Only works with 73 and 83mm shells, however, so max tire size will be in the 110mm range).
-Different rockers/swingarms for full suspension bikes to change travel/geometry for different seasons.
So, bottom line: You can have it all (or at least a LOT of cool options) all in one bike. Your significant other won’t even notice you have 3 bikes, either – just a lot of random wheels around. And what serious cyclist doesn’t have a ton of random wheels? He/she will never know!
Questions? Please let me know in the comments and I’ll update as needed.