Tech Speak: Decoding Fatbike Hub Spacing and Drivetrain Compatibility with Wolf Tooth Components

Photo courtesy of WTC

Photo courtesy of WTC

Back in the day, fatbike drivetrains were easy. They used offset wheels with 135mm spacing and used a crank with a 100mm bottom bracket. All of a sudden 170mm centered frames came along, and before you could say fat, 190mm bikes surfaced. Fatbike drivetrain standards quickly went the way of the rest of the bike industry with enough options to leave your head spinning. While it’s a challenge keeping it all straight for your avearage consumer, it’s also a challenge for manufacturers such as Wolf Tooth Components. In order to sell fatbike products they have to know which part works on what, so the WTC crew up in Minnesota has been busy putting together a sort of “whitepaper” outlining the details of fatbike drivetrains – specifically chain lines.

If you’ve been wondering what parts you need to order for your next fatbike build, the info after the break will certainly come in handy.

title copy

Fatbike chainline optimization

  • This is not meant to be a sales pitch. We love fat biking and we own and ride all the fatbike rear hub spacings out there today so we are just sharing what we have found.
  • For consideration when you read, front ring spacing between rings is 7-8mm and the rear cogs are about 4mm apart on 10/11 spd MTB cassettes.
  • In our experience and testing +/- 3mm of a perfect chainline provides for the best functioning drivetrain

Summary

History Refresher: Different Fatbike Rear Hub Spacing “Standards”

  • 135 mm offset – the original solution used on Surly Bikes (and still being used today
    • Standard 135 hub but adding 27mm of spindle length (100mm BB – 73mm BB)
    • Optimal chainline 64mm (50mm, the standard MTB 1x optimal + ½ of 27mm)
  • 170 mm – a wider hub that was the first to offer a symmetric rear end to the fatbike world
    • Freewheel has the same 17.5 mm shift as is used on the 135 mm offset
    • Optimal chainline 64mm (50mm, the standard MTB 1x optimal + ½ of 27mm)
  • 190 mm – a wider symmetric hub used on bikes that have frame clearance for 4.8” tires (e.g. Bud, Lou, and Big Fat Larry)
    • Freewheel is now shifted out further yet by ~10mm
    • Optimal chainline is 70-72mm (a little off from the mathematical optimal, but is what we found with testing)

Standard 100mm Spindle Cranksets

  • Standard 100mm Spindle cranks are optimized for 170mm and 135-offset rear hubs
  • Standard 100mm Spindle cranks are NOT optimized for 190mm and 135-offset rear hubs
    • Most 100mm cranksets do clear the frames of most 190mm rear hub frames. This is good news as converting to 190 does NOT necessarily mean you need a new crankset!
    • The only stock solutions optimized for 190mm rear hub are the RaceFace long spindle crankset and the MWOD from Surly
    • Both solutions result in ~20mm wider q-factors

Running a standard 100mm Spindle Crankset on a 190 Rear Hub

  • Why is this important?
    • 190 mm rear hub fat bikes are becoming more common and may become the de-facto standard
    • The q-factor on fat bikes is already wide and making it wider is not an ideal solution for most riders knees and hips!
  • As noted, the ideal chainline on a 190mm rear hub is 170-172mm or about 6-8mm further outboard than on a standard 100mm crankset. The following solutions get the drivetrain within +/-3mm of the “perfect chainline”
    • 1x drivetrains:
    • For SRAM, use a BB30 short spindle single ring (chainline ~170)
    • On a triple crankset, mount the single ring in the BIG ring position (chainline ~172)
    • 2x drivetrains:
    • For SRAM – Replace the stock spindle with a BB30 short spindle 2x spider (coming from WTC in January 2014)
    • For other 2x systems – there is no solution for standard 100 mm spindle cranksets

GXP vs BB30

Chainline Difference for SDM

  • Here are X9 cranks lined up with an SDM (SRAM Direct Mount) for GXP on the left and a SDM for Short Spindle BB30 on the right
  • Note that the BB30 SS pushes the chainline out 6mm, which makes a good chainline when using an X9 or XX1 crankset on a 190 rear hub bike

GXP vs BB30 2x

Chainline Difference for 2x Spiders

  • Here are X9 cranks lined up with a stock GXP 2x spider on the left and a Short Spindle BB30 2x spider (coming from WTC in February) on the right
  • Note that the BB30 SS pushes the chainline out 6mm, which makes a good chainline when using an X9 crankset on a 190 rear hub bike (you can actually use the small ring!)

Tire Clearance4.8” Tire on a 170mm Hub

Common question – “Can I run a 4.8” tire on my 170 rear hub bike?” -> Answer is “Maybe”

  • Our Setup Pictured – carbon Beargrease with Surly Lou tire (4.8”) on a 80mm rim and a BB30 SS (Short Spindle) DM (Direct Mount) ring
  • Using BB30 SS DM ring is the only way to make the chain have appropriate 4.8” tire clearance with a 1x DM setup (chain was almost touching with a GXP ring)
    • The chainline is outside the optimal range by ~3-4mm
    • Mounting a 104 BCD ring the big ring position will have a similar tire clearance and chainline
  • Tire clearance to the frame is VERY tight on this frame
    • Note that this frame was not designed for this size tire
    • Clearance will vary from 170 frame to 170 frame by model

Other Links to Fat Bike Chainline and Tire Clearance References.

Thanks to Wolf Tooth Components for putting this together!

 

Comments

Eastman - 01/13/14 - 9:05am

See also: http://salsacycles.com/culture/tech_talk_understanding_drivetrain_rear_tire_capacity
for an *excellent* presentation about fatbike drivetrains.

King County - 01/13/14 - 9:48am

Nice breakdown

gringo - 01/13/14 - 10:55am

Your use of the word standard reminds me of something….I have been in the bike industry for a long time and the single most irritating thing / word / phrase is ‘standard’. If you are simply throwing a bunch of ideas at the market to see what sticks it’s not a ‘standard’.

I am talking to all the pompous Product Managers who push this crap on consumers and self righteous journalists who are automatically experts on said ‘new standard’ after reading a press release from Company X.

All that ranting aside, this is a nice overview.

tom - 01/13/14 - 11:06am

I like watching all of my friends suffer on 1x fatbike drivetrains in the snow. They don’t brag much about weight savings and simplicity after the ride. Long live marketing hype!

George - 01/13/14 - 11:54am

Manufacturers are building rings with hard pack riding or racer boys in mind. The average rider is going to suffer in a real snow with existing product. Someone needs to come out with rings built for the broader community or 1x is going to be a huge failure. Have these guys ever been out in a 4-6″+ snow with 5″ tires and tried to push a 28-30t+ or even a 26t ring? The big cog for the rear cassette may help, but IMHO the front ring size needs to be addressed.

JP LaMere - 01/13/14 - 12:35pm

This is great reference material! Good news of the day is that our LaMere fat bike with 170mm rear end fits the 4.8″ Bud and Lou on our carbon tubeless 80mm rims no problem, even a little room to spare, but this is using a RaceFace crankset with the ring flipped over….have not tried yet with Sram XX1 crankset, probably it would rub in the granny…..so Wolf Tooth guys please make a Sram XX1 chainring replacement that can flip over to gain additional clearance, or do you guys already have that?

Thx, JP

Brendan - 01/13/14 - 1:05pm

@George, you asked about our backgrounds. Thanks for asking!
So here is a quick summary:
Dan – Iditabike last year, 4-5x Arrowhead 135 (think 60lb bike)
Me (Brendan) – Raced and Ridden fat bikes since the 2nd year of the Pug.
Mike – races and rides fat bikes for 4 years

So collectively, we have put 1000′s of mile in the conditions you describe, actually many miles in worse conditions with loaded bikes, which is why we are pursuing the 42t cog.

Note that the 26t ring with 42t rear is a pretty low gear, and the same as (with 3% of) a typical smallest gear on a 2x combo of 22t ring front 36t cog rear.

We agree that 1x is not for everyone, which is why we also discussed 2x in the summary.

@JP, you could have just asked at the race this past weekend ;) The BB30 offset 1x rings will add tire clearance but will not create an ideal chainline. See the section “4.8″ Tire on a 170mm hub” above.

Cheers!
-WTC Brendan

rich - 01/13/14 - 1:14pm

@Eastman, you mean like the link already posted at the bottom of the article? ;)

Salsa Cycles - 01/13/14 - 2:02pm

I feel the need to make a few comments here -
-The Beargrease Carbon frame is not designed to take the Lou tire, and we highly recommend against it. All of our bikes are engineered to all EN safety standards, which include a 6mm tire clearance envelope over the entire tire. Our tire recommendations are based on this, and fudging to something larger than we recommend is dangerous, as these standards were developed by a group of engineers from many large cycling companies. Clearances smaller than standard can lead to debris getting stuck and stopping the wheel.
-Salsa engineers spent countless hours with Sram engineers to cooperatively develop fatbike drivetrains that work. From the initial fatbikes, one of the biggest requests we had from users was to have fatbikes that work as seamlessly as a normal mountain bike, and that is where we spent our time. In all of that, proper chainline is the crucial element to making the drivetrains work properly. These offset rings seriously change the chainline, by over 10mm, when an acceptable variance is typically plus or minus 1mm. This will cause severe chain wear, and since it is to the outside, it will cause the chain to drop from the largest cassette cogs, and/or chainsuck into the chainstay. Do not use this ring on any bike with a 170/177mm rear hub. The only acceptable place for this ring from an engineering standpoint is a 190mm rear hub bike with a 100mm BB shell, as that is the only theoretical place to use a Sram crank, the offset ring, and have the proper chainline. Since all Salsa bikes are engineered around the 170/177 hub, this ring should never be used on a Salsa bike.
-The same applies for the offset spider. However, having two rings will further compound the issue, as the outer ring now has up to a 13mm improper chainline.
- It is impossible to engineer a proper operating drivetrain with a 170/177mm rear hub that will allow the 4.8 tires. For more info on that, consult our linked blog post. Simply put, two objects cannot occupy the same space.
- The caveats listed by Wolftooth above say most of this, but we need to be clear that these “solutions” will void your warranty with Salsa and Sram products, and may lead to unsafe conditions such as the tire jamming, or the chain falling off under load. Salsa highly recommends that you do not try these things.

Thanks!
Tim@Salsa

Wolf Tooth - 01/13/14 - 6:48pm

Hey Tim, we agree with you! Some of our info. could be worded better. We will revise it in the future.
– Most 170 mm hub frames cannot run a 4.8″ tire. Just because it does not actually rub does not mean it is safe to use.
– Even if a 170 mm hub frame has enough tire clearance for 4.8″ tires, there is no way to get both good tire-to-chain clearance and a good chainline.
– Our offset rings and spider were specifically designed for 190 mm hub bikes with 100 mm bottom bracket shells (and for conventional MTBs with BB30 short spindle cranks). Don’t use them on fat bikes with 170 mm hubs.
– Use of any aftermarket drivetrain component may void your drivetrain and frame warranty.
Mike

Mark Gronewald - 01/13/14 - 11:04pm

Great article. Just want to clarify that the 135 offset design had been in use by Wildfire Designs since 1999, so Surly was not the originators of that standard.

esc8engn - 01/14/14 - 10:40pm

Sheldon Brown would be proud. : )

Mindless - 01/15/14 - 3:49am

Yeah, yeah, do not use this, do not use that, your warranty is void.

It works. It lasts just fine. Chain line is fine it really depends on what rear cog is used most as far as chain wear. Just CYA type advice from manufacturer.

josh - 08/29/14 - 6:28pm

1x is awesome if you cant push a 28 or 30 in the snow prolly need to consider retiring.. if you go way smaller ur just gonna be spinning the rear wheel and digging a hole.. ran a 32 1×10 all winter in michigan and we had a record year.. and i by no means am super power grinder..

Post a comment:

Comment sections can be a beautiful source of knowledge, conversation and comedy. They can also get pretty ugly, which is why we've updated our Comments Policy. If your comment isn't showing up or suddenly disappears, you might want to check it out.