Mukluk Mods: Part One

Drivetrain and tire clearances are a critical clearance area on any fatbike frame. With wider rims, 80 or 100mm, becoming the norm for fatbike usage, drivetrain clearances are becoming tight. To gain clearance between the chain and tire when in the front small chainring and the largest rear cog there is a simple modification any Mukluk rider can make. If you are finding that your chain rubs the tire when in the 22T chainring and 32T rear cog this modification will help you get additional clearance between the tire and chain.  Additionally, this modification eliminates the two smallest rear cogs that I find rarely get used on my fatbike.

For this modification you'll need a Surly Spacer Kit and all of the proper tools to remove and install the cassette, along with the tools necessary to re-assemble and re-adjust the drivetrain. 

The list of tools includes:

  • Cassette Lockring Tool
  • Chain Whip
  • Adjustable wrench for Cassette Lockring Tool
  • #2 Phillips Screwdriver (for derailleur re-adjustment)

Here is a quick list of steps:

  1. Remove the rear cassette using the Cassette Lockring Tool, Adjustable Wrench, and Chain Whip
  2. Remove the two smallest cogs from the cassette and set aside.
  3. Install a 2.5mm spacer from the Surly Spacer Kit behind on the freehub body
  4. Install the remaining 7 cogs of the cassette back onto the cassette body.
  5. Install a 5mm spacer from the Surly Spacer Kit onto the freehub body.
  6. Tighten the assembly onto the freehub body using the lockring from the Surly Spacer Kit.
  7. Re-adjust upper and lower limit screws on the rear derailleur to insure proper chain alignment and shifting.  It may be necessary to adjust tension on the shift cable to tune the shifting.

When complete you'll end up with a rear cassette mounted on the freehub body that looks like this:

With the cassette moved out 2.5mm you'll gain precious clearance at the tire that looks like this:

A 2.5mm spacer behind the cassette translates to more clearance where the chain passes by the tire when in a 22T chainring and 32T rear cog. 

This modification was made using the stock X.7 rear derailleur. Different derailleurs may yield different results. For example, on my Mukluk I have run an X.9 rear derailleur. The rear plate on the X9 derailleur is aluminum instead of steel, like the X.7. In order to execute this same modification I needed to change the limit screws to a longer M4 x .7mm bolt. I have not found a rear SRAM derailleur that effectively allows for a 5mm spacer behind the cassette to provide more chain/tire clearance without modification. Both the X.7 and X.9 required modification to the derailleur itself for the upper limit screw to allow for proper adjustability.

If any of this doesn't make sense to you, we strongly recommend that you take your bike in to your local bike shop and have them help you with the modification. They can help you get it sorted and back out on the trail in no time.

This post filed under topics: Mukluk

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Joe Meiser

Joe Meiser

I've had a lot of good luck and made a series of choices to be working for the brand and in the bike industry. In 2007 I signed up for the TransIowa just to see if I could complete it. I completed it and discovered a few things about myself in the process. Adventure cycling has been in my blood ever since.


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Dan | January 25th, 2011

What a great fix! Thanks guys.

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timhMN | January 25th, 2011

Thanks for this - I think its a great idea as I noticed as well my chain rubbing on my tire. Question: I myself have had the original chain break after about 500 miles, and I’ve heard of others who have had broken chains as well. Do you think that part of the reason for that might be bad chain lines when using the smallest chainring, and do you think this fix will also help with that?


Joe | January 25th, 2011

Tim,  The Chainline on the Mukluk is no different than the chainline on an MTB like our El Mariachi.  It is just moved out to account for the 100mm BB and 170mm rear hub. 

We too have heard a few instances of chain breakage on the Mukluk.  I can assure you it has nothing to do with the drivetrain spacing.

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BRENDAN | January 25th, 2011

I’ve noticed that mine does not rub. Could the rubbing be due to variations in rim offset and couldn’t a small change to the centerline of the rim during the build of the wheel be a simpler solution?


Joe | January 25th, 2011

Brendan, There are a few variables that can play into the chain rubbing on a fatbike.  Wheel/Rim centerline, frame alignment, crankset specification and installation, and tire fit could all play into it. 

The stock Mukluk build does have clearance with the tire.  This is a modification to get additional clearance.

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timhMN | January 26th, 2011

thanks for the clarification, joe.

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Tom | January 26th, 2011

Isn’t the spacing gap different between the 11 or 12 and the 12 or 13 on shimano cassettes?  The indexing would not line up if that were true.  Or am I wrong?


Joe | January 27th, 2011

Tom, The spacing between the cogs on a cassette is consistent.  This will not create a problem with indexing.

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Nicolai | January 27th, 2011

Yo, awesome bike! will it be able to purchase in europe?


Joe | January 27th, 2011

Nicolai, we have distributors in several European countries if you are interested in a Mukluk.

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Seth | January 27th, 2011

I’m getting ahead of myself, but I would like to see a Mukluk Mods post about racks and fenders.

Have any data on the blue paint you guys use? I would like to paint some custom fenders to match. Color matching at the local hardware store never turns out well.

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Nicolai | January 29th, 2011

Cool, what distributors is it?

Craig Smith | February 17th, 2011

Hmmm, very interesting, Thanks.
The article mentions a 32t rear cog, and the Salsa spec page has a 36t cog. I know specs are subject to change (My bike has a FSA stem, a Salsa is spec’d) but will a 36t make a difference?
My bike looks like it has about the same or more clearance than the modified photo above.
This tight clearance was something I noticed when I first got the bike, and I’m aware, careful, and listen for possible chain-tire rubbing.
I will also go count the teeth on my rear cog, it sure looks like a 36.
Thanks again, Handy, useful info.

Kid Riemer

Kid Riemer | February 22nd, 2011

Seth - The number for the powdercoat paint is YS-PC-20192

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DamnitMan | February 28th, 2011

not for nothin…but you really only need to remove one cog and replace it with one spacer to improve chainline…keeping the 11-t locking cog installed helps preserve the structural integrity of the cassette / hub assembly.  Long story short…you only need to put one spacer behind the largest cog…

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