They say that there are seven stages of grief. I went through all of them when I heard that plus-size tires would be the ‘next big thing’ in the bike industry. You know–shock and denial, pain and guilt, angry-as-hell muttering and throwing of crap at the wall.

Photo by Van Swae

Photo by Van Swae

But you can't stay mad forever. I mean you can, but if you do you usually wind up living under a bridge, coaching a troupe of dancing rats. So, I resigned myself to getting some answers to the following questions:

What the hell is "plus-size" anyway?
What are these bikes supposed to do well?
What are their limitations?
What kind of rider might like a plus-size bike?
Is this the end of ‘normal’ mountain bikes?
Why are we also getting new fork and rear axle standards?

Ryan Palmer, Bike magazine's gear editor, and I headed out on a cross-country journey to find those answers. It was like “The Sisterhood of the Traveling Pants” or maybe “The Fellowship of the Ring” minus the orcs and the foxy elf chick. We wound up shooting four hours of video–a mere 12 minutes of which made it into our “Blueprint” video.

Our goal with the video was to cover the broad brushstrokes. What follows are some of the more tech-oriented details–stuff that matters but couldn't fit within the video without us making some kind of three-hour epic about spoke bracing angles and legally-mandated tire clearances in France. No one–not even the geekiest of you–would have watched that crap.

WTB's Trail Blazer 2.8 tire sure wasn't the first "plus size" tire, but it kicked off the 27.5+ boom that's making waves now.

WTB’s Trail Blazer 2.8 tire sure wasn’t the first “plus size” tire, but it kicked off the 27.5+ boom that’s making waves now.


The first plus-size bike that made much of a splash was Surly's Krampus back in 2012. That 29er hardtail rocked 3-inch-wide tires, a kind of middle ground between your garden variety 2.3-inch mountain bike tire and the monstrous 4- and 5-inch wide fatbike tires. A number of other small bike suppliers, including Lenz Sport and Niner have also had 29+ models in their lines for a while now.

But the plus-size train picked up steam in earnest when Mark Slate at Wilderness Trail Bikes configured a 27.5×2.8-inch tire that could be paired with a wide 650b (a.k.a. 27.5) rim. The outer diameter on that tire/rim combo is fairly close to what you get when you run a 29×2.3 wheel and tire. In other words, you could fit this, ‘27.5+’ set-up in some 29er frames already out there on the trail.

That got people thinking.

Last April at the 2014 Sea Otter Classic festival, Rocky Mountain Bicycles showed off a full-suspension 29er (basically an Element) that wore a set of WTB's 2.8-inch Trailblazer tires paired to their 45-millimeter wide Scraper rim. Rocky called the concept bike the Sherpa, and it was clear that Rocky was playing with the idea of creating a bike that could tackle long-distance bike packing tours.

The Sherpa created a bit of buzz, which then quieted–right up until a couple of months ago. Suddenly, we started hearing that this wasn't just a Rocky Mountain one-off love child, and that a whole rash of bike companies would be showing plus-size bikes at Sea Otter 2015. More to the point, plenty of people were saying, off the record of course, that plus-sized bikes weren't just going to be overland beasts of burden–these bikes had the potential to be nimble and fun. It was, to get all Ron Burgundy-ish, going to be "sort of a big deal.”

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Right about this time, both Fox and RockShox also spilled the news that they'd soon be selling plus-size compatible forks that feature 15×110-millimeter spacing. The wider leg spacing improves clearance and reportedly boosts stiffness, which will be a better match for Boost 148 rear ends, which lo and behold, a bunch of bike companies whose names are not Trek were now considering for their own bikes.

WhatTheF&^@? When did this all happen?

Rocky Mountain Sherpa

Rocky Mountain envisions plus-size tires as more of an overland/bike-packing tire, which is why the new Sherpa is decked out in 27.5+ tires. Not everyone, however, sees it the same way.


What are plus-size bikes supposed to offer? At this point, it all depends on whom you ask. For Alex Cogger, director of product for Rocky Mountain Bicycles, plus-size bikes, like the new 27.5+ Sherpa, have a very definite, limited skill set. "What they do really well," says Cogger, "is monster truck over stuff. They really shine in loose, rubbly, crappy conditions. It's great for that. It's also incredibly stable and grippy, so for someone who's less technically skilled and is looking for some added confidence, absolutely."

"Where it begins to falls short," adds Cogger, "is for someone who is trying to push really hard in the corners and get really aggressive–that's when you get some tire roll. You'd have to make such a burly and heavy tire for it to not fold over like that, that you'd just be bolting extra weight onto your bike."

Accordingly, Rocky Mountain has positioned the Sherpa as an overland adventure bike.

Other companies, however, have a decidedly different take. The most obvious of which is Trek Bicycles. A few days ago, Trek unveiled its new Stache plus-size hardtail. The bike wears 29+ tires and has an entirely different mission statement than the Sherpa.

"We designed this bike to rail and be ridden hard," says Trek senior product manager John Riley. "It’s not meant for a beginner or a novice. This is the ultimate of the fun, play hardtail for people looking to pop off stuff, rail the bike and pick up speed. The extra floatation and traction just gives the bike more versatility than in the past."

How can the two companies be so far apart on the very idea of what plus-size bikes are good at? For starters, it's a new niche, which means that companies are approaching the puzzle from different angles and coming up with very different results. They are also using different tires and, as minor as that sounds on paper, it actually winds up making a big difference in the final product. We'll get into that later. But consider this: The new Trek Stache has the shortest rear end of just about any production bike (you can get it down to a very stubby 16 inches thanks to its sliding dropouts) yet it's rocking the biggest-diameter rear wheel this side of a penny farthing. It's not going to ride like an overland bike. It's more of a dirt jump bike with big wheels.

Here's the bottom line: What you get with really wide tires is extra floatation over rough terrain and, because the contact patch is so big, a metric crap ton of traction. But that's just the starting point; what companies do with that is surprisingly up in the air.

Whether or not the plus-size thing actually takes off will hinge on whether or not companies can produce three-inch tires that don't weigh a ton and yet withstand hard riding. Trek's 29x3.0 Chupacabra weighs less than 900 grams. Impressive. We'll see how it holds up. Photo by Van Swae

Whether or not the plus-size thing actually takes off will hinge on whether or not companies can produce 3-inch tires that don’t weigh a ton and yet withstand hard riding. Trek’s 29×3.0 Chupacabra weighs less than 900 grams. Impressive. We’ll see how it holds up. Photo by Van Swae

27.5+ Vs. 29+

Right now there are two different plus-size options out there: 27 and 29 Plus. The simplest way of thinking about it is this–27.5+ amounts to sticking a 3-inch tire on a wide (45 to 55 millimeter), 27.5-inch rim. Twenty-nine Plus, no surprise here, involves putting a 3-inch tire on an equally wide 29er rim.

Most of the buzz right now centers on 27.5+. If you were a betting man, this would be the tire you'd pick to prevail because you can already squeeze 27.5+ tires into a lot of 29er frames. From an engineering standpoint, it should be relatively easy to crank out new 27.5+ bikes. Twenty-Seven Plus is basically a squishier flavor of 29er and, yes, there's no shortage of irony there if you go looking for it.

And then there's this: 29+ tires should, by all rights, weigh more than 27.5+ tires. A bigger tire, you might guess, would require more rubber and all that. Since heavy tires are the bane of any mountain biker's existence, this should be yet another nail in the 29+ coffin.

It isn't quite so simple.

A lot of the high-volume 27.5+ tires actually have a taller sidewall than comparable 29+ tires. This makes them less stable under hard cornering than lower-profile 29+ tires and that extra sidewall rubber adds up. It's hard to believe, but there are 27.5+ tires that weigh more than some 29+ tires.

Trek suspension engineer, Ted Alsop, puts it this way, "27.5+, ideally, has the diameter of a 29×2.3 tire, but to get there, you have to give it a really tall sidewall. The bead-to-bead measurement–that's the actual width of the tire if you pressed it flat and measured from one bead to the other–is about 15 millimeters wider than a 29+ tire. Relative to the rim, the 27.5+ tire is actually taller than the 29+ tire, which is why we've found that the 27.5+ tires that we've ridden have a lot more of an un-damped, fatbike tire bounce to them and don't corner as well at lower pressures. The 29+ tire, which is actually a lower profile, shorter sidewall tire, has less of that uncontrolled bounce to it."

Chris Drewes, Trek's MTB product manager, has this to add, "It's kind of the wild west for 27.5+ tires right now. You see high-volume tires, you see tires with tons of knobs with sidewalls that are much wider than the actual tread itself. You see 27.5+ tires that weigh, literally, more than a 4-inch fatbike tire. So, it's all over the map. It's going to take some time for the market to really figure out what 27.5+ even is. What that means is there are going to be some great 27.5+ bikes coming out now and some really shitty 27.5+ bikes too."

Twenty-Seven Plus is the new kid on the block in the plus-size game and that means there are a lot of tires being called 27.5+ that bear little resemblance to one another. They range from 2.8-inch tires with minimal tread to monster, beefy-lugged 3.25-inch models. Take a look at the photo below. The 3.0 tire absolutely dwarfs the 2.8-inch model. Both tires are ‘27.5+,’ but they are going to make your bike ride very, very differently.

There is a flood of new 27.5+ tires hitting the market, but they vary wildly in size and shape. Both of these tires are, theoretically, 27.5+, but they are going to lead to a very different experience out on the trail.

There is a flood of new 27.5+ tires hitting the market, but they vary wildly in size and shape. Both of these tires are, theoretically, 27.5+, but they are going to lead to a very different experience out on the trail.


The most obvious potential downside to building a bike with monstrously fat tires is that, well, you're building a bike with monstrously fat tires that weigh a ton. Adding weight to a bike is rarely an awesome experience, but adding it to the perimeter of your wheel is about the worst idea in the world, like syphilis-flavored ice cream or cancer on a stick.

The poster child for big, boat anchor tires was the Nokian Gazzaloddi, a 3-inch downhill tire that was sort of the cool thing back in the day, until people came to their senses and realized that strapping a 4-pound tire to their rim was about as bright an idea as gouging their eye out with a dull spoon.

Won't this plus-size thing simply be a re-enactment of that lame trend? Not necessarily. The 26-inch Gazzaloddi tipped the scales at about 1,800 grams (3.96 pounds). By contrast, Trek's Chupacabra 29 x 3.0 tire weighs just 877 grams.

How did they do that? More to the point, won't it just fall apart if it’s that light?

The 3-inch tire incorporates Bontrager's Inner Strength casing, which the company contends improves durability. We'll see how that actually pans out in the coming months. It's a lot to ask of a casing. When the tire gets this big, it hits a whole lot more pokey, sharp stuff out there on the trail. It'll be interesting to see if tire manufacturers are able to keep weight below 900 grams and still make tires that don't crap the bed with regularity. If they can't keep the weight down, these plus-size bikes are going to join the Gazzaloddi 3.0 in the dustbin of bad ideas.

Part of the reason Trek was able to keep tire weight down on the Chupacabra is that the knobs are fairly low-profile. At first glance, it's a fairly underwhelming tire–like a Nanoraptor that retired and got sloppy fat. But out on the trail, the Chupacrabra boasts surprising traction. Not just good traction–crazy good traction.

Says Trek's Drewes, "Knob position and height are critical. We're starting to see a lot of taller and more aggressive plus-size tires. They look cool, but what we've found with this increased tire contact-patch is that you don't necessarily need that tall knob height to get outstanding cornering and climbing traction. You have so many knobs on the ground with these tires that you can get away with smaller, lighter, less-aggressive knobs. It doesn't need to look like a Minion anymore. You have to re-think tread patterns when you make plus-size tires. The lower knob height also allowed us to create a fast-rolling tire–much faster than you'd expect–but which still has a ton of traction."

If you don't have the right plus-size tire on your bike, you're not going to like it," says Trek's Alsop. "We've been riding all sorts of plus-size tires and I can say that running the right tire at the right PSI is just as important as having the right geometry."


John Riley, the Trek product manager, considers the question, but only for a fraction of a second. "No. No way. We don't see 2.2 or 2.3 tires going away. When we talk about plus stuff, we still see a world where they both live. There's no doubt about that."

"What we're going to see," says Alsop, "is wider rims across the board because you still get some of those high-volume benefits from just going to a wider rim with a 2.3 tire. That's not going away. But plus-size is cool because you get some of those fatbike benefits–the rollover, the flotation–without the drawbacks like the long chainstays, the tire bounce, the wide Q-factor, the giant hub that you are hitting your heels on."

Boost 148 is a rear axle standard that widens the hub shell, effectively pushing each flange outwards by three millimeters, which both improves the spoke bracing angle and increases tire clearance.

Boost 148 is a rear axle standard that widens the hub shell, effectively pushing each flange outwards by 3 millimeters, which both improves the spoke bracing angle and increases tire clearance.


Squeezing a 3-inch tire into a bike's rear end quickly eats up precious real estate in the rear triangle. There isn't much room left for a front derailleur or, depending on how short you want the chainstays, even a chainring. Sure, you can make it all work if you want a hardtail with 18-inch chainstays, but then you just introduce that crappy, lumbering feel that made so many people hate 29ers for so long.

There are a couple of ways to work around the problem. You can either use a wider bottom bracket to give you the necessary tire/chain/chainring clearance or you can adopt Boost 148, a rear-axle standard developed by SRAM and Trek. Boost 148 widens the rear hub flanges 6 millimeters and pushes the chainring out 3 millimeters. Trek, naturally, is running Boost 148. It’s not alone; other companies, such as Specialized, are following suit. Rocky Mountain is going the wider bottom bracket route.

"To get enough chain to tire clearance on a tire like a 3.25, something had to move," explains Rocky Mountain's Cogger. "In our case, we chose to move the crank outboard by 5 millimeters. The big drawback, for us, that we saw in 148 was just availability. This particular bike, the Sherpa, is pitched as an off-road touring, bikepacking, overlander kinda bike. So, if you're out there in outer Kazakhstan and you experience a mechanical, you're more likely to experience a hub failure than a crank failure. You're going to want an easily-replaced hub, and 142 is going to be a hell of a lot easier to find than 148. At this point we just felt that 142 was a more practical choice for this particular bike." Finding a replacement 142 hub in outer Kazakhstan sound like a stretch, but point taken. The odds would be more in your favor than if you walked into a shop with a Boost 148 hub.

Trek is coming at it from a different angle. It sees Boost 148 as not only a way to make a stronger 29er wheel (it improves the spoke bracing angle), but also as a way to build better rear ends. "As product managers we want it all," explains Trek's Drewes. "We are always asking the engineers, 'Can we get a large chainring, short chainstays and room for wider tires?' And they always come back to us and say, 'Well, pick two.' Or even worse, 'Pick one.' And that's because those traits are all competing with each other. So, while Boost 148 sounds like a small change, it actually allows us enough room to meet the legally-required clearances of 6 millimeters from the tire to the chainstay. Boost is a critical element in solving a lot of challenges we face in trying to make bikes ride as well as they should."

"Yeah, that 'pick two' thing always sucked," adds Riley. "It was always a frustration because it was never what any of us wanted or, really, what the bike wanted. As product guys, we want it all. We want to build bikes that have short rear ends, room for massive tires and if you want to race enduro on the bike, room for something like a 36-tooth ring, if that's your thing. Boost 148 was basically us finally admitting, 'Look, there's only one way to do this. To get all those things in one package.' It's not going to be pretty. People are not going to be stoked to see a new standard, but if we are going to move forward with 29ers and with these larger tires, we have to take these chances. If we don't, we're just stuck in this box of design compromises; we can't evolve."

While you'd imagine that a 29+ bike would have to sport a sprawling rear end, Trek's Stache has one of the shortest chain stays of any production bike (about 16 inches), thanks to the elevated chain stay and Boost 148 rear end.

While you’d imagine that a 29+ bike would have to sport a sprawling rear end, Trek’s Stache has one of the shortest chain stays of any production bike (about 16 inches), thanks to the elevated chain stay and Boost 148 rear end.


No doubt, a lot of people aren't going to buy Trek's position on Boost 148. And I won't even get into why engineers didn't just go 150, the downhill ‘standard.’ Watch the video below for that explanation: they make plenty of plausible arguments. One thing everyone can agree on: realizing that your current frame and wheel may soon be rendered obsolete is a bitter pill to swallow. Alex Cogger feels your pain. "The one thing that has happened in the past couple years is that the adaptation of new technologies has accelerated incredibly," says Cogger. "It used to be that companies would hang back for a few years and think about whether to adopt some new technology. Now, companies see that something works and it's like," he snaps his fingers, 'This works? Okay, let's go. Let's put it on the next bike.' So, who knows, in a few years, everything could be running 148. Who knows?"

Can we have booth 142 and 148?

"Yes, we can," says Cogger. "But, will we? I doubt it. I think it's like the 650b tire thing. I can see that in short order the swing will be pretty hard toward 148. From a manufacturing standpoint it doesn't really make sense to make both. If some stiffness in the rear is good and you get more stiffness with 148, then there is something worthwhile to Boost spacing. And you don't have to adjust the Q-factor with 148, that's something to consider as well on a bike that's going to be pedaled long miles. I'm not against Boost 148, per se, other than the fact that it's going to piss a lot of people off. Fortunately, you will be able to get 142 hubs and wheels for awhile. I don't think it's going to change overnight."

Sliding dropouts allow you to run the Stache with a range of chainstay lengths--from short to crazy-short. Photo by Van Swae

Sliding dropouts allow you to run the Trek Stache with a range of chainstay lengths, from short to crazy-short. Photo by Van Swae


Perhaps the real question is less "Why do plus-size bikes exist?" and more "Why is everyone suddenly unveiling these bikes right now?" How did the bandwagon get rolling so damn fast?

"I think there are definitely brands out there right now," says Cogger, "that felt the pinch of being caught behind on trends, first on 29ers and then on 650b/27.5, then fatbikes and so now they are keeping their ears to the ground and reacting as fast as they can so they don't miss the boat again.”

Cogger knows what he's talking about. When Rocky Mountain showed off the Sherpa at last year's Sea Otter event, the bike attracted as much, if not more, attention from other bike companies as it did from consumers.

"There were project managers and engineers from practically every brand sniffing around our booth last year," says Cogger. "We had to literally shoo some engineers out of the booth because they actually had tape measures out and were measuring the Sherpa. So, yeah, there were a lot of people in the industry who got marching orders last year that sounded something like, 'Go make that. It exists. Make it.'

Trek's global mountain bike manager Riley agrees. "There are definitely people who are going to take the effort and do this plus-size thing right and there are companies that won't," he says. "The problem with what I have seen with both 650b in general and now plus-size tires is that there is this 'I don't want to be left out' mentality that is a holdover from people feeling like they missed the boat on 29 a few years ago. Now no one wants to miss a trend, so they fill their line with anything. I wish there was more effort being put into building bikes that matter instead of bikes that fit the latest trend. And I'm sure people will hear me say that and say, 'Well, aren't you the pot calling the kettle black?' but at the same time, I think it's clear with this bike and others, like the Remedy 29, that we are putting a lot of effort into building bikes that we think are right, that aren't like the other bikes out there. And, yes, I do think there will be some plus-size bikes out there that are just checking the box, so to speak. And, yeah, that's frustrating."



Whenever a new niche emerges, we try and categorize it. Put it in its little box. Label it as ‘good,’ ‘bad’ or ‘lame.’ The truth is that we rarely know the actual potential of the thing that we're looking at. There was a time when suspension seemed like a crap idea, a crutch for losers who couldn’t ride. Disc brakes were "more than you needed.” I know. I felt that way about those things at the time, not to mention 1 1/8-inch forks and all manner of things I now cherish.

I am not (so hold off on the internet forum hate for a second) saying that plus-size tires are the equal of these innovations. For all I know, plus-size bikes might wind up being a horrible idea. It also might morph into something cool. Time, engineering evolution and your own choices at the bike shop will give us the verdict. My job, however, isn't to immediately pronounce a new niche or product as ‘awesome’ or ‘shit.’ That would be premature and egotistical. We editors are supposed to chronicle what's actually happening out there and to try and foster an honest conversation about it.

As for the new fork and rear axle standards, am I stoked about them? No. Not at all. Honestly, it bums me out. The rate at which things become outdated is accelerating at a pace (and I say this while looking at my iPhone, my laptop and my bike) that is truly disheartening. Then again, I can see the merits of Boost 148. I don't think it's a ploy to rob me of my paycheck–it makes engineering sense–but it's still tough to swallow.

Which brings up a larger point, something that I hope people ponder beyond this matter of chubby tires: What are you willing to stomach when it comes to innovation?

We all want our bikes to improve, but when does the benefit exceed the cost and vice-versa? In other words, how much better does a new standard have to make our actual experience on the trail for us to be okay with the fact that we are going to have a hard time finding replacement parts for the bikes we already own?

I don't have an answer to that question and if I did, it'd probably be different than your answer. We all weigh this cost-benefit thing differently. I have a feeling, though, that the rate at which components now go defunct is going to bite some of the bike industry in the ass. How willing are you to upgrade your fork or wheels, for instance, if you are always wondering whether those parts will even fit next year's bike? Bike components are pricey; I'd personally want some assurance that they'll fit the next frame I'm going to buy a year or two down the road.

But, hey, maybe that's just me.

More on plus-size bikes:

Blueprint: 29+ and 27.5+ Bikes – Innovation or Industry Scam?

First Look: Specialized Fuse and Ruze 27.5+ Hardtails

Introducing the Rocky Mountain Sherpa