We constantly get asked what size tires fit in our fat bike frames. The easy answer is four-inch tires on 82mm rims. This is what the Mukluk and Beargrease frames are designed around. Usually the next follow-up question is: What is the biggest tire I can run? The answer to that is: It depends.
The Beargrease and Mukluk frames are designed around a 170mm-wide rear hub to facilitate fitting the wider rims and tires that make fat bikes… well…fat. The 170mm hub spacing isn’t an accident. It has roots in the original Surly Pugsley frame design (editors note: Correction. Mark Gronewald of Wildfire Designs in Alaska began using the 135mm offset rear triangle design on his bikes in 1999. The Surly Pugsley became the first widly-available fat bike and also used the 135mm offset rear triangle design.)
When Surly was designing the Pugsley, they knew they had to design a frame that would allow the chain to pass by a Surly Endomorph tire on a Large Marge rim. This combination of rim and tire measured out to about 91mm at the widest point. Surly also knew that they were using a mountain bike triple crankset mounted on a 100mm bottom bracket, which has roughly a 65mm chainline. This positioned the granny ring at 56mm from the frame’s centerline.
Through some simple math and sketches, it was calculated that if a 135mm standard mountain bike hub was offset by 17.5mm towards the driveside of the bike, and the chain was placed in the smallest (or lowest) gear combination (biggest rear cog, smallest front chainring) that the tire capacity produced by the chain around the critical area of the tire was around 94mm. This accounted for a few millimeters of clearance and alignment tolerance. At the same time, the alignment of the cassette to the chainrings was reasonable, not that much different from a regular mountain bike, so shifting performance and drivetrain wear would not be affected. This all seemed pretty reasonable, and thus, the Pugsley was born and fat biking for the masses changed forever.
Let’s get back to the 170mm rear spacing thing. If we look at the rear end of a Surly Pugsley frame with 135mm spacing offset 17.5mm to the driveside, and calculate the symmetrical equivalent, we come up with 17.5 + 135 + 17.5 = 170mm. This is why most fat bikes today use a 170mm-wide rear hub. It results in the same cassette alignment as the original Pugsley, which we know offers decent space for fat tires and rims, and also has an adequate chainline.
As fat bikes have continued to evolve from that initial Pugsley point in time, we have found that wider rims and/or wider tires offer performance advantages in certain situations. This has caused the total width options of the tire/rim combo to swell from the 91mm Endomorph/Large Marge setup to new setups that easily measure 100mm in total width, and even as wide as 121mm! Those are massive changes!
Considering the original triple crank allowed for only a 94mm-wide max tire capacity, one can deduce that—at even just 100mm wide—these new tire and rim combos will now be occupying the same space as the chain in the lowest (or most inboard) gear combination.
Fortunately, 2 x 10 cranksets became popular in the skinnier-tire bike world, and at about the same time, crank makers started offering these cranksets in a fat bike-compatible width. 2x cranksets have an advantage over triple cranks in that the innermost chainring sits roughly in the space between where the granny and middle rings are on a triple. This in turn means that when a 2x crank is used, the innermost gear combination is now also a few millimeters wider.
This is good because it means we can now squeeze a little bit bigger tire/rim combo into the fat bike than we could before with a triple. SRAM happens to be one of the crank makers that started offering fat bike cranks. SRAM also makes full drivetrains. Using their knowledge in drivetrain and shifting performance, and the fact that fat bikes have the rear cassettes positioned on a 170mm hub, they centered their fat bike crank offerings on a ~66.5mm chainline. This is a subtle but critical detail. This alignment results in a drivetrain on a fat bike that has identical shifting performance and wear life as that of a properly designed, standard mountain bike. From a drivetrain maker’s perspective, this is the most important thing. From a fat biker’s perspective, this is also advantageous because now we’ve moved from the triple crank setup which had a granny gear at 56mm and yielded a 94mm tire capacity in the lowest gear combination, to a 2x crankset which has the inner chainring at around 62mm and yields a tire capacity of about 104mm. That’s about a 10mm increase in the total allowable tire/rim combination that can be used, simply by updating the crankset design.
Lastly, the 1x drivetrain has come onto the scene. Although one could always remove the granny and outer rings from a triple crank to make a 1x crankset, this also limited the gearing range of the bike. A limited gearing range is generally not an advantage for a bike that can see many different surfaces, and many different surface conditions, in the same ride. Newer, dedicated 1x crankset and cassette designs are allowing for smaller chainrings and wider range cassettes—both advantageous for fat bikes. Additionally, using only one chainring means that the lowest (innermost) gear combination is even a little wider than that of a double, resulting in even more tire capacity!
So finally, to answer the original follow-up question: What is the biggest tire I can run on my Mukluk or Beargrease? Answer: It depends on what crankset you are using. Below is a list of cranksets designed for use with 170mm hubs (the size the Mukluk and Beargrease frames use). Each cranksets’ innermost chainline is listed, and what the resulting largest tire/rim combo is (Tire Capacity) that will still allow full use of the drivetrain:
Surly offers a handy chart on their website, as does 45NRTH, that lists total widths of various rim and tire combinations. By comparing the total allowable tire/rim combination above for a given crankset, you can determine what tire and rim combinations will result in a fully functional drivetrain on your Beargrease or Mukluk. Any combination wider than the max allowable size will result in chain to tire interference.
Additionally, when we designed our Beargrease and Mukluk frames, we made sure the clearance around the chainstays and seatstays was large enough to handle the largest combination possible listed above without issue (~110mm or less). If your desired tire/rim combination allows for a fully functional drivetrain, then it will also fit between the stays.
Let’s use the Surly Nate tire mounted on a 100mm rim as an example: This combination results in a 108mm total width according to Surly’s chart. If we are running a SRAM X9 2x crank, we see from the list above that the max allowable size is only 103.8mm. This means we either need to use a narrower rim to pull in the tire casing, or find a tire that measures smaller when mounted to a 100mm wide rim. However, if we really want to stick with a Nate on 100mm rim, we could also switch to a SRAM XX1 or RaceFace Next SL fat bike crank, as this pushes the innermost run of chain outboard, further opening the drivetrain’s tire capacity up to greater than 108mm. This combination should be okay, as long as frame alignment, wheel dish, rim extrusion, and the tire casing are in tolerance, and even has a 2.5mm buffer added in for good measure.
There are several other tire, rim, and crank options out there for fat bikes. Hopefully more charts like Surly’s will pop up, helping people select tires and rims based on crankset. At a minimum, the Surly chart can used to estimate where other tire/rim combinations might fall in terms of total width.
So what if I want to run a 4.8″ tire on the rear wheel of my bike? Some of the larger tires (4.7–4.8″) are designed for use on 100mm-wide rims. These tires measure wider than 110mm depending on the tire and rim models used. They are so wide that the chainline of the crankset must be increase from 66.5mm to about ~76mm to physically get the chain around the tire. This increase in chainline skews the alignment of the chainrings to the cassette too much to use a 170mm-wide hub. The ramps, pins, and gates on the chainrings and cassette are simply not designed to function at these chain angles. Shifting will be poor, auto and ghost shifting will occur, there will be a tendency to drop chains along with an increased rate of drivetrain wear. Each of these problems are all things drivetrain makers carefully design out of our favorite bicycle drivetrain systems these days. To correct these misalignment issues caused by the fatter fat bike tires, the rear hub width also needs to be increased to bring the cassette back into proper alignment with the new 76mm chainline. This results in about a 190mm hub width.
This newer width also happens to be a symmetrical equivalent that is based on the offset Surly Moonlander and Surly O.D. crankset design, which happened to be the first complete frame and crankset designed around those 120mm wide tires. Yes, the Surly guys do their homework, eh?
So long story short—in order to have a fully functioning drivetrain and adequate tire clearance for tire/rim combinations that are wider than ~110mm, the frame should be designed around a ~76mm chainline and 190mm back end. This is why we do not recommend using the current 4.7 and 4.8″ tires in our Mukluk and Beargrease frames. With a 170mm rear hub and a 66.5mm chainline crank, it physically does not work, and using a crank with 76mm chainline misaligns the cassette and the chainrings so much, that we cannot recommend or endorse this type of setup.
Along the same lines, a frame with a 190mm rear hub should never be paired with a 66.5mm chainline crankset, as this will also result in a drastic misalignment of the chainrings to the cassette. Yes the cranks will turn and the gears will go around—but the misalignment will result in poor drivetrain performance and premature wea – fingernails on a chalkboard for the drivetrain designer. Additionally (and most notably), a 2x crank paired to a 190mm cassette only produces a tire capacity of ~109mm which completely fails to achieve the intended goal of the system in the first place: clearing 4.8″ tires mounted on 100mm rims which are roughly 120mm wide!
What’s up with the 177mm thru-axle hub on the Beargrease? Mountain bikes with rear thru-axles use a 142mm wide rear hub, which is simply a 135mm hub with 3.5mm extensions on either side to interface with the frame. Those extensions make axle installation easier. The Beargrease Carbon is also designed around a rear thru-axle hub, so a 170mm hub plus the 3.5mm extension on each side comes out to 177mm in total with. Really it’s no different than a 170mm in terms of chainline or tire capacity.
Fat bikes, and their drivetrains, continue to evolve, and standards seem to slowly be forming. If this all seems confusing and a bit mumbo jumbo, rest assured that the 2014 Salsa fat bikes are all spec’d to have proper performance without drivetrain/rim/tire compatibility issues.
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Hi, I'm Pete and I am a product development engineer for Salsa. I like all kinds of riding from commuting to dirt jumping. I think flat pedals make you a better bike handler, that the thru-axle is vastly superior to the quick-release for off-road applications, and that moving through the world on bicycle allows one to see things they might not otherwise. I suffer daily from hunger-induced anger, also known as [i]hanger[/i]. Outside of work and riding, I enjoy kiteboarding, traveling, and watching hockey.