Tech Talk - Understanding Drivetrain & Rear Tire Capacity

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.

This post filed under topics: Beargrease Fatbike Mukluk Pete Koski Snow Biking Tech

Pete Koski

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 hanger. Outside of work and riding, I enjoy kiteboarding, traveling, and watching hockey.

samh | October 9th, 2013

This does a nice job of clearing up some things I found to be a bit mysterious, Pete.  Thank you.

Doug D | October 9th, 2013

This is excellent information, presented so that many more people will understand it.
It also shows quite nicely why I have chosen to remove some of the cogs from my cassette to make it clear the tire rather than moving the chainrings further outboard.
My Mukluk Ti is a 1x6 in the winter with a bit of dish in the rear wheel to make the chain clear the Lou on Clownshoes.  I am not going to claim this is the best setup, but it is a legitimate choice for those handy enough to reassemble cassettes in new sizes.

JayP | October 9th, 2013

This is an amazing article - great information for the people trying to make decisions with all that is out there. You guys rock and THANK YOU for putting this together and also for doing your homework!

Scott | October 9th, 2013

Great article Pete!
Can someone now answer me this:
What other (if any) cranks out there have the 76mm chainline critical to the 190mm rear end?

Or, are the 190mm manufacturers mostly ignoring the drivetrain “rules”?

MattS | October 9th, 2013

Rohloff.

Brian | October 10th, 2013

Great info!  As a prospective fat bike rider this helps fill in some gaps and makes for planning a build a bit easier!

bsimon | October 10th, 2013

bookmarked

Hamilton | October 10th, 2013

Thanks for the great information. Scott’s question certainly piques my interest, hopefully you can reply to that here.

PK | October 10th, 2013

I know of two cranksets with chainlines compatible with 190hubs: the Surly O.D. Moonlander, and the RaceFace Next SL with a direct-mount ring.  It is very possible that there are others, or more on the way, I’m just not aware of any at this time.

Doug | October 10th, 2013

So, let’s assume a 2014 Muk 2.  What I’m seeing from the Surly tire/rim chart is that a Lou 4.8 on a Holy Rolling Darryl will just not physically fit in the rear frame - period.  True?  The numbers I get out of that chart for Knob Shoulder width and Casing width are 116mm and 111mm - both exceeding the 110mm number in the article above.  I just want to verify that.  Thanks!

samh | October 10th, 2013

Nah, that’s what sandpaper is for, Doug!

PK | October 10th, 2013

Doug,
In the paragraphs right after the Capacity chart, what I am trying to say is that we design the C/S and S/S capacity to be larger than the drivetrain capacity.  There are minimum requirements for tire clearance and space between rotating parts that we must adhere to as frame designers.  The physical distance between the C/S and S/S is actually larger, but the 170mm drivetrain is what actually limits the tire/rim size to the 108-110mm ball park.  A Lou on a Darryl may fit in the frame with the dropouts all the way back, but the clearance between the tire and the stays may not be large enough for me to officially endorse or recommend that combination, as that is not what we designed the frame for. And lastly, but most importantly, unless some type of modifications away from a “proper” 170mm drivetrain are made, the chain and tire will be interfering with each other.

Doug | October 10th, 2013

I am struck by your statement: “...the clearance between the tire and the stays may not be large enough for me to officially endorse or recommend that combination…”.  Seems to me that the HRD/Lou combo either physically fits or doesn’t fit the rear triangle, and if it does fit, there are clearance measurements when the dropouts are all the way back.

And much like the rim/tire measurements, these clearance measurements for a ‘14 Muk 2 would be very good to know.

Also - assuming a 2014 Muk 2 with the HRD/Lou rear combination, if it does physically fit, and it clears by some number of millimeters, and if one were to, say, remove the two smallest cogs from the cassette and offset the cassette with spacers, would that allow for complete chain clearance and proper alignment?

Believe me - I’m not trying to be a pain - rather, I’m trying to learn about your products so I can make a good buying decision.  Thanks again!

PK | October 10th, 2013

It’s not about whether the tire physically fits in the frame or not,  Salsa (or myself representing Salsa) cannot endorse a product we make or other products used with products we make - that falls outside the bounds of the intended design scope and/or violates a regulation.  In this case the “EN 4.10.2 Wheel/Tyre Assembly - Clearance” regulation states a minimum tire clearance of 6mm between any part of the frame or fork. Since Salsa sells bikes world wide, this is a regulation we choose to play by.

Secondly regarding removing cogs from your cassette - this may lead to chain-to-tire clearance, but not proper alignment.  The parallel links of your derailleur are designed to move the cage and pulleys along a certain path that has a predetermined alignment to the imaginary “cone” that your cassette makes.  Truncating the cassette and moving it outboard shifts the imaginary cone in relation to the predefined path of the derailleur.  Similar to skewing chainring to cassette alignment laterally, this skewing of the derailleur to the cassette cone at some point, will have negative effects.  Putting a 36t cog where a 28T or 30T usually sits is not likely something the drivetrain maker intended or designed for, whether it works in practice is another thing - but it certainly is not “proper” from an official capacity.

Hopefully that sheds some light on why I answered your question(s) the way I did.  There is a limit to what myself or Salsa can actually say or endorse regarding things like tire fit, etc.

Doug | October 11th, 2013

OK - thanks.

Snippy | October 11th, 2013

Folks… don’t get bogged down or ramble. Fit what you can and ride. Middleburn Duo is my choice… I’m sure the designers have a fair idea what they’re talking about… trust them.
Snip

Doug | October 12th, 2013

I was fine until I read Grampa Snip’s post.  Now I’m temporarily annoyed.  Any other homespun advice for us kids?

DG | November 14th, 2013

If you can clear a tire 107.4mm with a 170mm rear setup and a 64.9 chainline,  why then do you say that you can only clear 109mm with a 190mm rear/76mm chainline combo, am I missing something?  If you move the rear cassette out 20mm and the chainline out 11mm, then I would expect at least an 11mm+ improvement in clearance.

PK | November 14th, 2013

DG - You mis-read that part. A 109 tire capacity is the result of pairing a 190mm hub to the 65mm chainline crank.  Using the wider hub and narrower crank not only results in skewed chainlines, but poor tire clearance, assuming the reason someone would use a 190 hub in the first place is to clear 5in tires.  The ~76mm crank is a proper pairing for a 190mm hub.

Additionally, switching to from a 170 to 190 hub only moves the cassette outboard half the overall change = 10mm. Moving the cassette out 20 and only moving chainrings out 10 would result in 10mm of chainline mis-alignment.  That’s something drive train makers want to avoid.  If the cassette moves out ~10mm, then so should the chainrings to maintain proper alignment.

dg | November 14th, 2013

Perfect, thanks, now I get it!

Jason Carter | December 24th, 2013

Great article, but still difficult to get the techy stuff for a lay person.  Bottom line is that a 190+ lb. person really needs a 4.8/100 tire/rim to get enough float for a snowy ride on about 2 passes of a snowmobile trail (really we need a table on float or patch width using clysdale weights).  So that’s what I did on a 2012 Muk - clowns+MWOD+Lou+lost the 11 and 12 cogs (not that I needed them anyway)  But it still didn’t work and I pared down a lot of nobs from the tire - but it still was not very good because it rubbed when I stood up or swayed from side to side.  So, back to the BFL which works fine with this set up (although it adds a bit more side slip which I don’t like).  Anyhow, for an XL bike user, getting into a 190 setup with frame capacity to go to the next generation of fat tires can’t come fast enough!

Sean Ralph | January 12th, 2014

I am running Lou on an HRD out back on my Mukluk Ti using a 2014 RF Turbine in a 1x10 config with 11/36 cassette.  Mounted the 30t narrow wide chaining on the outside of the crank spider.  no issues with chain tire interference and sufficient clearance on the SS and CS.  It might not be recommended but it works.  Just waiting on a OneUp 42t adaptor to get a little more low end gearing.

Mark Gronewald | January 13th, 2014

Contrary to what you wrote, Surly did not design the 135 offset rear triangle. That had already been in use since 1999 by Wildfire Designs.

Kid Riemer | January 16th, 2014

Thanks for the clarification Mark. I’ve put a note into the article to correct this error.

Also a note of thanks, the Wildfire Designs frame bag I bought from you many years ago is still going strong and continues to serve me well.

Mark Gronewald | January 17th, 2014

Thanks for correcting your very informative article, Kid. Glad the frame bag is still working. Cheers from Alaska.

Alex Oliver | February 24th, 2014

Hi from me, Alec, from Bonnyrigg over in Scotland.  Just took delivery of mukluk 2 with the gold frame.  First ride on Saturday 24 feb.  60 plus miles over a couple of days On very muddy terrain.  Loving the bike.  Build quality and details are the best I’ve seen on any bike.  Everyone I meet looks on and I have to stop a few times just to tell people about the bike.  Cheers again.  Alex

Chris | March 21st, 2014

Hi. I’m trying to figure out a replacement fork for my 2014 mukluk 3 in either steel or carbon to reduce chatter. Besides the 600\$ makwa—is there another fork on the market? Thank you.

Marque | July 26th, 2014

A very useful article, indeed!

One thing though. The chain line given for the MWOD inner cog is 64.0 mm. However, this is in contradiction with Surly, who state that it’s 64.8 mm, and therefore almost exactly the same as their OD cranks: http://surlybikes.com//uploads/downloads/Surly_Crankset_Dimensions.PDF. My understanding is that ideal chain line for 170-mm spacing is 65 mm, so it makes sense that Surly shoot for this.

The only reason I spotted this is that I’m currently agonising of the best 180-mm+ single-ring cranks to get for my first fat bike build, so millimetres of additional tyre clearance have suddenly become hugely important to me;)

Sara | September 10th, 2014

Very nice blog post. I definitely love this website.

Thanks!

Spovegas | September 13th, 2014

Superbly written, highly informative article, and fantastic comments discussion.  Thank you!

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