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:
- Remove the rear cassette using the Cassette Lockring Tool, Adjustable Wrench, and Chain Whip
- Remove the two smallest cogs from the cassette and set aside.
- Install a 2.5mm spacer from the Surly Spacer Kit behind on the freehub body
- Install the remaining 7 cogs of the cassette back onto the cassette body.
- Install a 5mm spacer from the Surly Spacer Kit onto the freehub body.
- Tighten the assembly onto the freehub body using the lockring from the Surly Spacer Kit.
- 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.