Although it’s been around for more than a decade now (unless we’re counting old school MTB… let’s not), gravel is the only cycling genre that’s still expanding, and pretty rapidly, at that. As a weird middle ground between road cycling and mountain biking, it’s still finding its identity while pushing limits both towards the racy aero, and the capable adventure ends of the spectrum. But with suspension and fat tyres becoming increasingly common in gravel, we’re finding that it’s beginning to straddle the line between gravel and MTB. So, what’s stopping modern gravel from morphing into MTB?
When does a gravel bike become a mountain bike? I was asked a question along those lines when talking at the Adventure Travel Show + Gravel Adventure Expo in London a couple of weeks ago, and while it didn’t have me stumped, it’s a question that’s stuck with me since. And let’s face it, with suspension and mountain bike-sized tyres being specified on a range of new bikes now, like Pinarello’s latest Grevil MX, where does the gravel line stop?
There are answers, however. Mainly, those are in the intentions of how a bike is designed to be ridden, and to a point, how a bike is built. Although the latter point is increasingly hazy. But let’s take a dive into each of those, and perhaps have a bit of a comparison.
The devil is in the details
Close your eyes and picture a mountain bike. It likely has front and rear suspension; it may be a hardtail. It’ll have large knobbly tyres, too… That’s kind of like a gravel bike, right? Especially when the Trek CheckOUT exists, and the Niner MCR of old.
But put one of these bikes next to a cross-country bike, and not only do they look different, but we’ve gotten to a point where the devil is in the detail. That’s because, despite Life Time’s recent ban, drop handlebars on mountain bikes are certainly a thing.
Shimano GRX 12spd chainring, by Shimano
Let’s dive a little deeper into a gravel bike’s typical specification. It’ll have a large, 40t (at least) chainring, or a 2x setup. That kind of ratio will help you and the bike maintain higher speeds over what gravel bikes generally ride over – flatter, smoother, and faster terrain. And although some gravel bikes are specified with mountain bike drivetrains, we’re now seeing 13-speed cassettes. No, they’re not designed to increase range, as you might expect of an MTB-focussed cassette. They’re designed to minimise the jumps between gears, offering subtler changes, a more suitable cadence and more efficient transfer of power.
On a mountain bike, you’ll find 28t to 38t chainrings. Yep, smaller. That’s to help tackle steeper and punchier climbs. Then, there’s rarely clearance to bolt on anything larger.
The next key component that sets a gravel bike apart from a mountain bike is its tyres. Yeah, we are seeing mountain bike tyres being used in gravel, but look at the tread patterns being chosen. They’re often minimal, super low-profile and fast-rolling choices. Instead, gravel riders are choosing mountain bike tyres more for their larger volume, taking advantage of the greater comfort and traction they provide.
Schwalbe G-One RX Pro Edge.JPG, by Matthew Page
But even then, gravel bike tyres, or the mountain bike tyres gravel riders are choosing, aren’t as wide, nor as aggressively treaded as common mountain bike tyres. That’s because the balance of rolling resistance and traction lies in a slightly different area where a lesser rolling speed provides more of a gain over all-out traction. Plus, gravel (generally) isn’t as unpredictable and ever-changing as what’s common on a mountain bike trail, where larger knobs and aggressive tread patterns sacrifice rolling resistance in return for dependable grip.
Geometry choices dictate what a bike can and can’t do
I’ll admit, this subheading isn’t exactly accurate. You can ride a bike equipped with a 70-degree head angle down Fort Bill, but would you want to? I bet it wouldn’t be a fun time.
But geometry is another area that differentiates modern gravel from modern MTB. In fact, geometry bears an incredible weight in all genres of cycling and plays a key role in how they’re split. But again, this is another area in which brands are pushing the envelope, for better or for worse. For example, the Marin Gestalt XR gravel bike is equipped with a 67.5-degree head tube angle, which is a full degree slacker than Canyon’s Lux World Cup mountain bike.
Of course, there are many, many other measurements to consider, but generally speaking, gravel bikes are shaped to excel on flatter and less technical terrain at a higher average speed, while mountain bikes are shaped to provide support and stability for steeper tracks, both up and downhill. That latter point also correlates to a bike’s specification, like chainrings, as I mentioned before.
So, when does a gravel bike become a mountain bike?
We’re not at the point where mountain bikers really need to worry about gravel kicking down the door and putting an end to the party. Although a surprising amount of similarities are being struck between the two genres, a gravel bike isn’t going to be as effective where mountain bikes are designed to tread, and mountain bikes aren’t going to be as fast on what’s considered ‘gravel’.
Conversely, you can tweak the specifications of each bike to be better at tackling each other’s terrain, to a point, as there are limitations such as tyre and chainring clearance, and suspension fork correction to consider. This is where that Dogma XC-like Pinarello Grevil MX gets particularly interesting.
Yes, you can rip a gravel bike down some light singletrack, but it’ll be nowhere near as confident as a purpose-built MTB. And you can ride your local gravel loop on an XC bike, but you’ll run out of gears, and you’ll have some extra rolling resistance and weight to contend with. But neither of those is wrong, or sacrilege of any kind. It’s riding bikes after all, and that’s what we’re all here for.
That said, it’s all about the combination of geometry and componentry. If a gravel bike came kitted with a 120mm suspension fork, a 64-degree head tube angle and 2.4in wide tyres, then we’d have a very valid argument to call that bike a mountain bike. However, for what gravel riding currently is, that doesn’t make sense, as it would really depart from what gravel is all about.
Although lines are being blurred, there are still lines defined by the riding style that each bike encourages. So while they’ll likely get even more similar, the definition of gravel, and what it is, will stop the genre from becoming mountain biking, and vice versa.