Our friends over at Surly have just raised the bar in fatbike "fatness" by releasing the 4.7" wide Big Fat Larry tire. We know many of you are wondering if this new meat will work in a Mukluk frame and Enabler fork. Short answer is YES. Long answer is YES, BUT.....
Mukluk 3, 100mm rims, and Big Fat Larry tires...
ENABLER FORK AND BIG FAT LARRY
Lets start up front with the Salsa Enabler fork which is sold with all Mukluk and Mukluk Ti framesets, and is also available separately as an aftermarket fork.
The Enabler fork has 127mm of clearance across the fork blades around the knob area.
The BFL (Big Fat Larry) mounted to an 80mm rim has about 10mm of clearance from the knob to the fork blade and 9mm of clearance from the widest part of the casing to the fork blade.
Enabler fork, 80mm rim, and BFL...
On a 100mm wide rim, the knob clearance per side is about 10mm and the casing clearance is about 7mm. On a 100mm rim, a little tire squeezing needs to happen to get it past the front brake caliper, but it's nothing given the single digit pressures most people run.
Enabler fork, 100mm rim, and BFL...
We've only been using the BFLs on 100mm rims in the Enabler for a short time now, but have not had any problems with handling or mud clearance.
MUKLUK FRAME AND BIG FAT LARRY
Moving on to the Mukluk frame, what is shown below holds true for all current Mukluk frames out there, as well as the new Mukluk 2 and Mukluk 3 frames for the coming winter season. There were no changes to the chainstay forming and shaping between model years.
The BFL mounted to 80mm rims has roughly 9mm of clearance at the knob, and about 6mm clearance at the widest point of the casing to the chainstays.
Mukluk chainstay, 80mm rim, and BFL...
The BFL mounted to 100mm rims has roughly 9mm of clearance at the knob, and about 5mm clearance at the widest point of the casing to the chainstays.
Mukluk chainstay, 100mm rim, and BFL...
For both rim widths, tire clearance across the seatstays is ample (9mm knob, 7mm casing).
Mukluk seatstays, 100mm rim, and BFL...
Note that Joe Meiser has run the BFL in the alloy Mukluk, but he had to make some heavy drivetrain modifications to eliminate chain rub on the tire.
MUKLUK Ti AND BIG FAT LARRY
Lastly, we've got a new version of the Mukluk for 2012: the Mukluk Ti. The Mukluk Ti features Alternator dropouts for adjustable wheelbase and tire clearance, plus singlespeed capability. The clearance across the chainstays in the knob area is designed to be 118mm in the full-forward position. This number wasn't just pulled out of thin air. It was a compromise of getting the most tire clearance possible while still being able to manufacture a titanium frame with the strongest, most reliable chainstays. Depending on manufacturing tolerance, there is either very little clearance or just a slight amount of interference with the BFL at the full-forward position. However, pulling the Alternator dropout back creates more clearance.
The pictures and measurements reported below were collected with the Alternator in the full-rearward position. Our three Mukluk Ti prototype frames we measured were a little slim on the chainstay tire width spacing, so the actual production numbers should look a little better than what we are reporting here.
The BFL mounted to 80mm rims has roughly 8mm of clearance at the knob, and about 6mm clearance at the widest point of the casing.
Mukluk Ti, 80mm rim, and BFL...
The BFL mounted to 100mm rims has about 6mm of clearance at the knob, and about 5mm clearance at the widest point of the casing.
Mukluk Ti, 100mm rim, and BFL...
For both rim widths, tire clearance across the seat stays is ample (10mm knob, 6mm casing).
Mukluk Ti seatstay, 100mm rim, and BFL...
Similar to Joe's setup on his alloy Mukluk, using a BFL in the Mukluk Ti would require some cassette truncation and chain line modifications to reduce/eliminate tire-to-chain rub.
WHAT'S IT ALL MEAN?
The most important things to note and take away from this blog post as you run off to the LBS to get your super fat tires are this:
1) If you intend to run a Surly Big Fat Larry in the rear of your Salsa Mukluk or Mukluk Ti frame, YOU WILL HAVE TIRE-TO-CHAIN RUB ISSUES!
2) If you intend to run a Surly Big Fat Larry in the Salsa Enabler fork, go for it! What are you waiting for?
A COUPLE LAST THOUGHTS BEFORE WE LEAVE YOU TO YOURS
Currently, the only solution available to eliminate or reduce tire-to-chain rub is to truncate the rear cassette and not use the granny ring up front. Through a couple conversations between Surly engi-nerd Thor and myself, Salsa has learned that the Surly Mr. Whirly Offset Double (MWOD) will potentially be another future option to solve/reduce chain rub for those looking to run 4.0" tires on 100mm rims, or 4.7" BFLs in the rear on Mukluk frames. As to what this is, how it works, and when it will be available, we cannot say at this time. When we've had the opportunity to spend a bit more time with a BFL we will do another post investigating the precise drivetrain modifications needed for Mukluks and BFL tires.
I AM AN ENGINEER SO YOU AREN'T GETTING AWAY THAT EASY
What many people don't realize is that the current "standard" fatbike chainline obtained through 100mm BB, standard MTB triple crank, and a 170mm non-offset wheel (or the equivalent 135mm x 17.5mm offset wheel), when in the granniest of gear combinations, only allows adequate chain-to-tire clearance when a 4.0" tire is used on a 65mm rim. It's no mistake that the original Surly Endomorph tire and Large Marge rims were these sizes. In the name of greater floatation, 80mm rims reduced this adequate clearance down to "very litte to none". The advent of 100mm rims and the Surly 4.7" Big Fat Larry blows past this limit with wreckless abandon and creates the need for a totally new chainline to allow use of standard 8, 9 or 10-speed cassettes in combination with granny rings.
In plain English...bigger tires and wider rims don't make anything easier...except riding across soft surfaces.