The big news in mountain biking is the sudden emergence of new bikes with larger tires: three-inch treads that split the difference between the standard 2.3-inch mountain bike tire and the four- and five-inch treads you’d find on a fat bike.

Called “plus-size,” this isn’t really a new niche – companies such as Surly, Niner, and Lenz have put gigantic tires on some of their 29ers for years – what’s changed is that numerous big brands, including industry giants Trek and Specialized, are scrambling to introduce plus-size bikes, too. What’s also significant is the palpable sense that this trend is being driven not just by the desire to innovate, but to not get left behind. More than a few brands have lost sales as the hot bike has flipped from 26-inch wheels to 29 to 27.5 to fatbikes. Nobody wants to get left holding the bag.

All of the major suspension manufacturers have introduced forks designed to fit the mid-fat tires, too. And several tire companies are now producing plus-size 27.5-inch tires – some of which you can fit into 29er frames already on the market.

The obvious question is, why? What do these bikes do? What are their limitations? What kind of rider might benefit from all this? And do you really need a different bike or is this just the industry changing for change’s sake?

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Rocky Mountain's Sherpa overland bike.

Rocky Mountain’s Sherpa overland bike.

MASSIVE TIRES EQUAL MASSIVE GRIP AND STABILITY
Anyone who has ridden a fatbike can attest: The massive tires provide outstanding grip and stability. Plus-size tires, while narrower than true fatbike models, offer some of those same benefits without the extra weight and cumbersome handling of the typical fatbike. For that reason, some companies, such as Rocky Mountain Bicycles, are pitching their plus-size models as ideal off-road touring machines.

Alex Cogger, the director of product for Rocky, said, “They really shine in loose, rubbly, crappy conditions. [They’re] also incredibly stable, and grippy.”

Other companies, however, say the bikes are for expert riders. Trek Bicycles recently launched a new version of their popular Stache hardtail with 29+ tires.

“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 fun, play hardtail for people looking to pop off stuff, rail the bike, and pick up speed. The extra flotation and traction just gives the bike more versatility than in the past.”

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So, which is it, stable overland machine or aggressive play bike? Those are two very different beasts. The niche is too new for a definitive answer.

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SHOULD YOU CARE?
Mountain bikers pedaled blissfully along for more than 30 years on the standard 26-inch wheel, but the past decade has seen the demise of 26 and the rise of both 2-9 and 27.5-inch wheels. These required new frames and forks, a costly upgrade. Not surprisingly, many riders are skeptical of the sudden explosion in plus-size bikes. How does this new tire size fit into the scheme of things? Is it any better than what you are already riding?

Plus-size tires are neither better nor worse – whether they’re right for you depends on your own terrain and riding style. If you want to ride in the snow, you need a true fatbike, with at least a four-inch tires. On the other hand, if you want to do long overland tours on poorly maintained trails (or have considered a fatbike for the desert), the extra flotation afforded by three-inch tires will provide a big improvement. But as for driving current bikes extinct…not gonna happen.

Even though bike companies are working to give these plus-size bike different personalities, the massive tires dictate their primary character: They give your bike a very planted, staid feeling. Some people are going to love that extra sense of security. Others will feel as if someone just pushed the “mute” button on their bike.

“A lot of folks seem to be pigeonholing plus-size as a spinoff of fatbikes,” says Sean Estes of Specialized. “They are positioning it as more of an adventure or comfort thing, not so much about performance. The way we see it, 29 is the fastest, 650b is the most nimble, and 6fattie [Specialized’s name for plus-size] offers the most control.”

UPHILL BATTLE
Whether or not 27+ and 29+ bikes gain mass acceptance remains to be seen. They do, however, have a few hurdles to overcome if they are going to appeal to riders who demand the same nimble handling already offered by regular 27.5 and 29er options.

First and foremost: the new plus-size tires will need to be light. That’s no easy task: when you enlarge most materials, they pack on the pounds and rubber is no exception. This puts plus-size bike manufacturers in a bind. Plus-size tires demand wider rims, but the best way to kill the buzz of riding a bike is adding rotational weight. Heavy wheels have inertia that takes tremendous effort to overcome. What’s more, the further you place that weight from the hub itself, the less your bike feels like a Ferrari and the more it feels like it dump truck. For that reason alone, plus-size tires should face a lot of skepticism right out of the gate.

Some of the 27+ and 29+ tires hitting the market weigh nearly as much as a fatbike tire. If you strap that kind of weight to any bike, it’s going to crush your inner unicorn. There are, however, some impressively light plus-size tires hitting the market now. Bontrager’s Chupacabra 29 x 3.0-inch tire, for instance, weighs less than 900 grams, which puts it on a par with plenty of 2.3-inch tires. I’ve ridden the Chupacabra and can attest that it rolls faster than any fat tire deserves. But here’s some advice: If you are interested in giving plus-size tires a spin, pick wisely. The wrong tire will make a pig of this kind of bike.

Specialized's Fuse 27+ hardtail.

Specialized’s Fuse 27+ hardtail.

A TIGHT SQUEEZE
Another obstacle that plus-size bikes need to overcome? Awkward frame geometry. Simply put, it’s difficult to squeeze a massive tire into the back end of a bike without having the tire eat the seat tube or front derailleur. In the past, most companies have dealt with the problem by adding inches to the bike’s rear end, but while lengthening a wheelbase can add stability in high-speed, wide-open conditions, it also tends to make a bike difficult to maneuver in tight turns. For touring bikes, a long wheelbase isn’t much of a problem. For high-performance trail bikes, the sluggish handling would be a deal breaker. It’s hard to enjoy the hallowed act of “threading the needle” when you’ve blown a corner and t-boned a tree.

Still, a few companies are already engineering solutions to that problem. Specialized’s 27+ Fuse hardtails sport very short (16.9-inch) chain stays. Trek’s Stache are 16 to 16.5 inches, via sliding drop-outs while using the even larger 29+ wheel. What’s enabling these is the Boost 148 hub.

Boost 148 is a hub and drivetrain standard co-developed in 2014 by Trek and SRAM. The new design widens the rear hub flanges by six millimeters and pushes the drivetrain out three millimeters. A few millimeters of doesn’t sound like much, but it’s enough to get tire clearance without having to resort to long chainstays. Boost 148 also makes for a stronger wheel by improving the bracing angle of the rear wheel spokes.

That’s the good news, but if you already own a mountain bike, it has a 135-millimeter or 142-millimeter rear end and Boost 148 is not compatible with it. Now, if Boost 148 was simply a standard for plus-size bikes and only plus-size bikes, this wouldn’t be a big deal. But most industry insiders expect Boost 148 to become the standard on every new bike, even the ones sporting regular tires.

“I can see that in short order,” says Rocky Mountain’s Alex Cogger, “that the swing will be pretty hard towards Boost 148. From a manufacturing standpoint it doesn’t really make sense to make both. If some stiffness is good and you get even more stiffness with Boost 148, there is something to be said for Boost…I’m not against Boost 148, per se, other than the fact that it’s going to piss a lot of people off.”

Vernon Felton is a contributing editor to Adventure Journal. He lives in Bellingham, Washington.