December 14, 2015

The Fat+Plus+Skinny do-it-all FAQ

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IMG_4770

83mm spindle/180mm q factor fatbike in action! Sarah can add 27.5×3 or 29×2 wheels if she wants to in the summer.

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.

To whit:

-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.

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197mm rear end and wide Q-factor for Chuck, who likes his 4.8″ tires. 29×3″ would work and fit great too, as would 27.5×4″.

-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?”

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SRAM XODH (83mm) cranks with 0mm offset ring/60mm chainline and 95mm Surly Nate.

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.

 

24 Comments on “The Fat+Plus+Skinny do-it-all FAQ

Feldy
December 14, 2015 at 3:11 pm

This post reads suspiciously like a recent email thread … And I think you mean 157×12 and not 150×12, unless you’re talking about a downhill bike from 2003.

But now that I’ve been “that guy” I’ll end my comment by being amazed at the crazy algebra-based captcha-like device.

Walt
December 14, 2015 at 3:13 pm

Heh, yup. Suspiciously similar!

157/150, yeah, yeah. I’ll fix that.

mikeetheviking
December 15, 2015 at 11:41 am

I love this idea. I really appreciate you compiling all this info here. Thanks!

Spiff
December 16, 2015 at 4:13 pm

“though there is no sub-3″ option (we’ll get to that in a minute)”

Did I miss the part where you got back to that? Is there no 29xsub-3″ option because the BB ends up too low, i.e. messes with the geometry too much?

I, for one, love versatile bikes! My next mountain bike will probably be of the 29/27.5+ variety.

Walt
December 16, 2015 at 4:15 pm

I wasn’t clear enough in the EBB part at the end, I guess. If you need to use different diameter wheels, that’s one way to deal with the BB height issues.

Walt
December 16, 2015 at 4:16 pm

Hence the sub-3″ option for the 760mm diameter wheels only exists if you do something to compensate for the BB height issue, since there’s no 30.5×2″ tires we can buy.

JK
December 19, 2015 at 10:09 am

What do you think is chainstay length limit with 83mm BB, 26×4, 12×177 and ~65mm chainline?

Walt
December 19, 2015 at 10:21 am

Around 400mm.

Ben
December 19, 2015 at 12:08 pm

Great site! First time here. Do you know if the boost reba will clear a 3.8 Nate?

Walt
December 19, 2015 at 12:09 pm

It will not. Max tire about 80mm is the limit on that fork.

Ryon
December 23, 2015 at 11:46 am

Walt,

Do you know what something like a Bridger on a Scraper rim would measure width wise, and if that would clear the Reba? I really like the clearance on the Fox 34 Boost, but would prefer the shorter axle to crown of the 100mm travel Reba Boost. Thanks!

Walt
December 23, 2015 at 9:22 pm

I’d guess between 70 and 75mm. Should fit great, the fork is officially designed for 27.5+, after all.

Bud
January 1, 2016 at 10:42 am

Most people fail to realize the geometry difference that changing fat tire p.s.i. makes. The effective BB drop on a fat bike going from 12-15 p.s.i. to 2-3 p.s.i is probably close to as much as changing wheel sizes from 29 to 26. When dropping down to very low p.s.i on a fat bike for soft terrain conditions, you easily lose 3/4′-1″ of BB height maybe more.

Yet bike makers still insist on making fat bikes with your average 12″-12.5″ effective BB height at high tire p.s.i., which IMO really sucks and is way too low at the p.s.i. fat bikes are often used, giving the bike a sub 12″ BB. I built my own custom fat bike frame with an effective 14″ BB at high p.s.i. and it handles awesome.

I think people are making a big deal out of something that isn’t really, worrying about trying to keep wheel size exactly the same when changing wheelsets, as far as bike handling goes. The main concern is having too low a BB. I would say just build the bike BB around the smallest diameter wheelset you will run and then make the bike able to fit larger diameter wheels if you want that. Who cares if your BB raises by an inch as long as both wheels are matching sizes, the bike will handle just fine.

So to summarize, with a fat bike, you already have two different handling bikes in one bike depending if you’re running 2-3 p.s.i. or 12 p.s.i. due to drastic BB height change, in addition to handling characteristics of soft terrain and soft floppy tire wheel feel.

Wade
March 19, 2016 at 6:23 pm

The common thread in the production bike area is absurd BB drop with no consideration for peeps letting the air out of the tires and getting squishy for the soft stuff. Another missed issue with the lowering of the rock crawler is that peeps are gonna take them out rock crawling. Makes a hill of sense. A great geo for me shall have a BB drop of not more than 20mm, preferably 0mm drop.

I have no custom frames that are more than 0 drop and they handle amazingly and are as playful as a stock trials bike and as trail worthy as any of the most expensive production bikes on the market with no pedal strikes that were not caused by operator error.

Glad that I’m not the only one understanding “dirt geometry”.

Thanks, Bud

27.5 + SS goofy question
January 28, 2016 at 8:00 am

[…] From Walt The Fat+Plus+Skinny do-it-all FAQ […]

Dimitri
April 12, 2016 at 8:12 am

Great thread. It may be time to come around for my second waltworks! I have been rifding my 29er less and less as I have been riding my fatback year round. intrigued by the idea of a do-i-all frame. Will a 29 pike fit 27.5×3.0 on a +rim (hugo or scraper), or is a fox34 a better option?

Walt
April 12, 2016 at 8:23 am

There are several flavors of Pike that will fit 27.5+ OR 29+ now. The Fox 34 is a great option as well but there are now at least a dozen good options for plus tires in all sorts of travel/configuration.

Dimitri
April 12, 2016 at 10:57 am

thank
i own both a 2016 fox 29 34 and a 2015 pike 29- both are 15×100 not boost. wondering if they have the clearance for a plus wheel

Mark
May 16, 2016 at 8:40 pm

Walt: great info! I am curious about a rear wheel with a 190mm hub and ‘regular’ mtn rim, something like 25mm internal width. Reason being is that my 907 can accept a 29er fork that I have, along with a regular old 135mm hub and Stans arch rim. This rear wheel that I wonder about would make the bike a 29er hard tail, albeit with a wide q factor. Most peeps I ask say the rim is too narrow

Walt
May 16, 2016 at 9:05 pm

Lots of people do exactly that, Mark. Go get a rim and a hub and lace it up.

Chris Yould
October 22, 2016 at 6:40 pm

I know this is an old thread, but maybe someone is still listening.
Wanting to largest tire I can put on a 27.5 plus rim on a 29″ Fox factory 32 100 non boost.
Bike is a Spot Rocker SS

Walt
October 25, 2016 at 10:41 am

Get out your calipers and start measuring. 2.8 should work fine. I would guess a 3.0 is a no-go, though.

Dave
March 21, 2017 at 5:41 pm

What are my chances of lacing 26in 65mm rims on 110/148 boost hubs? Or would 50mm rims work better for mounting 26 x3.8 Hodags on to a 29ner frame?
Great site by the way!!

Walt
March 21, 2017 at 5:44 pm

I’d use 50mm rims with the Hodags. They are really 3.5″ tires, no need for full fatbike rims.

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