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As I understand a gravel bike, it's basically a road frame but perhaps with a slightly more upright riding position. It has wider tires with much deeper treads and a stiffer rubber than standard road bike tires. What is the theory behind these changes? I had always thought that the thicker and stiffer tires were to prevent small bits of gravel from pinching flats, but is the theory instead that the deeper treads and wider tires are supposed to increase traction? And what is the rationale for the upright riding position?

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    Re the tires, the main point is that gravel is not smooth, and a wider tire spans the gaps to produce a less jarring ride, and to provide more consistent traction. Jul 27 '20 at 23:19
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    Manufactures desire to increase n (Rule 12 )
    – mattnz
    Jul 28 '20 at 2:47
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    What is the theory behind a gravel bike? The TLDR answer: marketing, aka "LOOK AT OUR NEW SHINY!!!" Jul 28 '20 at 9:58

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GCN had a funny take on this recently: Gravel bikes exist because mountain biking is now boring. Or to give slightly more nuance, mountain bikes make it so easy to ride over difficult terrain that they remove the challenge.

Coming at it from the opposite direction, road cycling may be losing some popularity because A) roads in many places are poorly maintained due to austere public-works budgets, and B) motorists are motorists. Which creates some demand for something that you can ride on bad roads/roads that motorists don't drive on.

Gravel bikes generally are designed for a little more comfort and stability than a road-racing bike--more in line with a road "endurance" bike. Hence the more upright position and slacker angles. Some people ride gravel bikes with fat slicks—I don't think you can make any sweeping judgments about tires, other than they're fatter than road tires.

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    A gravel bike with fat slicks is basically a tourer with less luggage - the frame geometry is very similar
    – Chris H
    Jul 28 '20 at 9:29
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    @ChrisH I think the likes of 3T and Cervelo would be offended if you compared their gravel bikes to a tourer. The reality is that the gravel bike genre covers a pretty wide spectrum and is still evolving at quite a pace
    – Andy P
    Jul 28 '20 at 11:30
  • @AndyP you're not wrong there, but I wouldn't object to offending them if they noticed me. Tourers aren't fashionable, and some definitions of gravel bikes are closer to gravel-racing bikes than others, also gravel bikes are likely to be lighter
    – Chris H
    Jul 28 '20 at 11:33
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    Mountain biking has been boring from the first day on. I´ve been riding a standard Peugeot steel road frame with a slightly bigger back tyre for twenty years now. Great on gravel, fantastic on snow.
    – Karl
    Jul 28 '20 at 19:10
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    A marketing thing. Another unnecessary great American contribution the cycling world. The re-invention and transfer of the CX bike to the wide market.
    – Carel
    Jul 29 '20 at 7:34
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For me, a gravel bike is the perfect commuter bike.

Before, I had a road bike and a hybrid (normal) bike. On nice days I could maybe use the road bike to work, but mostly I would use the hybrid one because of rain/snow, having to carry stuff or whatever. However, going up the hills back home, and even the flats, the hybrid feels sooo slow and heavy. So I recently bought a gravel to replace it for these reasons:

  • It's almost as light and fast as a road bike, but a bit more comfortable / upright for normal riding.
  • Wider tires means it handles potholes, road cracks, curbs, tram rails etc much better than a road bike. And of course allow rougher surfaces like gravel.
  • It often has space and mounting points for proper mud guards / fenders, which a road bike does not.
  • There is also space enough to use studded tires during winter.
  • Can mount a luggage carrier properly, so the bike can be used to/from shops.
  • It has disk brakes that works all season. Expensive road bikes do as well now, but many don't.

So to kinda answer the question, I think its a nice trade-off between a road bike and a standard bike. What people end up using it for may be different, but it clearly covers some uses other bike types traditionally have not.

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  • When you say commuting, how far do you mean? Presumably quite far if using a road bike on nice days?
    – thosphor
    Jul 29 '20 at 10:03
  • @thosphor Using the road bike vs the hybrid was for me more about pleasure than strictly saving time. Depending on where I'm biking it's mostly about 3-8 km each way, so not so much that it makes a huge dent in time using the road bike really.
    – Matsemann
    Jul 29 '20 at 10:36
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    Can't upvote enough. Basically Gravel bikes are one bike that lets you do almost anything you want.
    – James
    Jul 30 '20 at 10:08
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My theory and it is just a theory. In the U.S. at least there has been a proliferation of unpaved bike trails due to the conversion of abandoned railroad rights of way to bike paths. They are for the most part either flat or have a slight grade. Most of these trails are too uneven for a conventional road bike. At the same time they are not much of a challenge for a mountain bike and are mostly an access point to more technical trails. So now we have a reason to buy yet another type of bike. With a gravel bike the gearing allows much higher speed than on a mountain bike. You're on a route that is pretty much guaranteed to be free of motor vehicle traffic so you can basically go as fast as you're comfortable riding. The larger tires allow better traction on loose surfaces while increasing comfort. So while the riding environment is similar to cyclocross the two frame geometries are different in a sense like a Tri bike and a road bike, functionally the same but tweeked for a more specific type of riding.

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    “With a gravel bike the gearing allows much higher speed than on a mountain bike” – well, mountain bikes come with very different gearings. Newer 2×10 or 3×8 mountain bikes generally have all the range you could ever wish for; it's the 1× hype that has sacrificed some of this flexibility. (But with a sufficiently big chainring, 1×12 gets you very fast too). And downhill bikes have fast gears allright, just no light ones – but they're unsuitable for speed on the flat for other reasons, not to speak of longer distances. Jul 28 '20 at 8:34
  • Too uneven for a conventional road bike is a sales pitch really. On (slick) 28mm tyres I can get over most stuff short of red MTB trails unless it's too muddy for the wheels to grip. On coarse gravel/dirt the speed is the same as my hardtail with gravel tyres.
    – Chris H
    Jul 28 '20 at 9:32
  • @ChrisH But a good-sized rut in the gravel can still cause a nice pinch flat with 28s. Which, if you're going fast enough, causes you to lose control badly enough to pitch you into a thorn bush... :-/ Jul 28 '20 at 10:04
  • @AndrewHenle my only ever pinch flat was on tarmac, a pothole I couldn't see in time to do anything about. This timelapse that I've posted before doesn't show the 10" dropoffs on that ride (and I think I was on 32s then), but I did some heavily potholed gravel byway yesterday on 28s
    – Chris H
    Jul 28 '20 at 10:49
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    @VladimirF that's a different matter. CX round here at least is often more mud than gravel, needing some grip, and road race frames when that rule came in were designed for skinny tyres like max 25mm, or maybe slick 28s at most. CX also prioritises speed while I may have to pick my way over stuff.
    – Chris H
    Jul 28 '20 at 17:30
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I think you are asking two separate questions:

  1. What's a gravel bike and why are they popular?

  2. Why are off-road tires designed they way they are? (Which applies equally to mountain bikes and hybrids)

Gravel bikes have longer seat stays and more relaxed steering geometry than road bikes, which makes them a little more stable on loose surfaces. They have wider tires that allow riding on rougher surfaces. There are many reasons why they are popular - filling a gap between road bikes and increasingly heavy and complicated mountain bikes, suitability for touring or bikepacking and suitability for general riding with increased comfort are some of them.

I think you are perhaps a bit confused about off-road tire design. Off road tires do not have to be stiffer but they do have to be tough to deal with more flexing and distortion. A thick tread is not totally necessary either, slick low pressure tires will actually grip pretty well on dry gravel. Lower pressure increases the contact patch side and does increase traction on loose surfaces.

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  • I don't know about theory but one my nastiest falls was coming around a curve into some deep loose gravel. If these bikes can prevent that then they are a good idea in my book.
    – JimmyJames
    Jul 30 '20 at 20:37
  • We don't have a lot of 'gravel' roads in the UK, but does gravel generally mean a non-smooth but solid surface, e.g. it's rough and bumpy but the surface stays where it is; or does it genuinely mean there is a covering of loose gravel on the ground, like a shingle beach?
    – Wilskt
    Jul 31 '20 at 18:53
  • @Wilskt en.wikipedia.org/wiki/Gravel_road. Gravel roads are constructed with loose gravel but the particles are angular and compacted to form a solid road surface (there will be some loose material on the surface of course). The term is also used for poorly maintained tarmac with loose material and compressed dirt where no gravel has been added. Jul 31 '20 at 19:44
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What is called a "gravel bike" is really a new word for a cyclocross bike, but with maybe a tad less aggressive geometry. And cyclocross goes back pretty much to the beginning of bicycling, as the first races were held on unpaved courses and no roads were paved yet, or at best cobbled. The original road bikes looked a lot more like today's cyclocross bikes, only a pound or three heavier.

I personally don't like the name "gravel bike". Gravel is the stuff they dump along the train tracks near my house to deter mountain bikers, or the stuff you decorate your yard with if you live in places like Las Vegas, but I rarely encounter gravel out on the trails here in northern California. Maybe some gravely stuff mixed in with loose dirt intermittently, but there's nothing too fun or challenging about riding on a layer of gravel as there is a natural dirt or muddy path.

In theory, I doubt you will be able to ride one off road without encountering mostly dirt. The style of bike is ideally suited for trails. They should call them "fire road bikes" or something more realistic if "cyclcocross bike" is too worn out for the marketers.

I would call the bike I commute to work on a gravel bike because I built it with a steeper stem to bring the bar up so I can see traffic easier, and a single, low gear, 38/18t. But it started life as a 'cross bike. A gravel bike might be ideal as a commuter bike, especially if you can outfit it with panniers, lights, etc.

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    CX is a race discipline. You wouldn’t find an all-day or even a multi-day ride on a CX bike nearly as comfortable as one on a gravel bike.
    – MaplePanda
    Jul 28 '20 at 7:44
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    @MaplePanda: My travel bike is a Focus Mares cyclocross from 2008. My longest trip was over 3000km through Scandinavia and took more than a month (all nights spent in a tent). Can’t complain about the comfort. Today I’d get a gravel bike with supercompact crank set and long chain stays for better handling with luggage.
    – Michael
    Jul 28 '20 at 8:49
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    It is like a cyclocross bike but comes with features meant for longer more stable rides potentially with lugage and definitely with two bottles.. Not for ultra twisty chicanes of a racing course but for eating some miles on reasonable trails/unpaved roads. I do not mind to jump of my gravel bike, shoulder it over some stairs/a creek, quickly jump on behind the obstacle and I like watching CX races but I like the more stable feel of the gravel bike for most rides. does it necesarily need a new name? Probably not, but it makes more clear what one is buying.
    – Vladimir F
    Jul 28 '20 at 17:28
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    @Michael Of course there's nothing saying you can't ride long distances on a CX bike, but you'd certainly be better off on a gravel bike if you had that choice.
    – MaplePanda
    Jul 28 '20 at 18:13
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    @MaplePanda In the US at least, many frames billed as CX had more of an endurance road geometry, like road-oriented gravel bikes. CX bikes were often raced in the US season, and then used for commuting or possibly touring at other times of the year. You are absolutely correct if you're referring to a CX race bike, which I think is basically a criterium-specific bike with bigger tire clearance and maybe minus the water bottle bosses.
    – Weiwen Ng
    Jul 29 '20 at 0:02
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The other answers address one theory of gravel bikes. In the past, many road bikes focused too much on on-tarmac performance, at the cost of general utility and comfort. Endurance road bikes married high-performance capabilities to a relatively upright position, enabling many cyclists to enjoy a fast and comfortable experience. They could accept relatively wide tires, but they were still limited off-road.

Initially, gravel bikes were basically bikes with endurance road geometry but with much more tire clearance, e.g. for up to 40mm tires. They also tended to have more equipment mounts than endurance road bikes, e.g. mounts for fenders, racks, top tube feed bag (aka bento boxes), and 3rd bottle cage mounts under the down tube are more common on gravel bikes than on endurance road bikes. Road-like gravel bikes are perfectly fine for long stretches of dirt roads. There are a lot of this type of road near where I live in the Midwestern United States. However, road-like gravel bikes are not as capable on very rough terrain with large chunks of gravel or on terrain approaching singletrack. In my experience, you can navigate some singletrack on a road-like gravel bike, but it isn't ideal. Everyone's use case varies depending on where they live and their own preferences.

At the time of writing, gravel bikes are starting to become more specialized. One branch may be referred to as "all-road bikes", and it hews closer to the endurance road bike + wider tire paradigm. Another branch, which some may call adventure bikes, borrows design ideas from mountain bikes.

Adventure gravel bikes and MTB design

Some gravel bikes are borrowing design factors from MTBs. These bikes tend to have long top tubes, short stems, and slacker head tube angles. The slack head angle makes for very large trail; all else equal, a bike with greater trail will be more stable than a bike with less trail. The shorter stem counterbalances the stability somewhat by making it easier to turn the handlebars (your bars have to traverse a shorter arc to turn the wheel the same angle).

This arrangement may benefit riders on rough terrain. From Cyclingtips' recent review of Evil Bicycles' Chamois Hagar, which is very much inspired by MTB geometry, Evil Bicycles claims

more stability on loose terrain, more confidence in tricky situations, more speed overall, and — listen up, big-footed riders! — an end to toe overlap.

James Huang, the reviewer, described the Chamois Hagar as very stable at high speeds, and he said that riders could simply dive the bike into a corner with loose gravel without fear of the front wheel sliding out. He and a co-reviewer described the bike as very stable on descents. One downside was that at low speeds, the bike didn't hold its line very well on technical climbs; this is a consequence of an aspect of bike geometry called wheel flop.

Riders who expect to encounter rough terrain and singletrack might consider an MTB-inspired gravel bike over a more road-inspired one. Furthermore, personal preference and cycling background might also play a role. Mountain bikers who do indeed find most terrain boring on their MTBs might be more comfortable with a MTB-inspired gravel bike, whereas road cyclists may be more comfortable on a road-inspired gravel bike.

The MTB-inspired gravel bike segment appears to comprise the minority of gravel bikes as of 2019 and 2020. Cyclingtips identified the Evil discussed above and the BMC Urs as being distinctly on the MTB end of gravel geometry, with the Santa Cruz Stigmata being neutral on the endurance road-MTB geometry spectrum. Cyclingtips later reviewed the 2020 Specialized Diverge, and they felt it was also on the MTB side of the geometry spectrum. To my knowledge, the Lauf True Grit discussed by GCN is likely an MTB-oriented bike.

In contrast, the Cervelo Aspero and the Salsa Warbird are two examples of more road-oriented gravel bikes, although the bikes handle very differently. The Warbird is very stable, perhaps more like a road touring bike. The Aspero is nimble (i.e. lower trail, more along the lines of a typical endurance or performance road bike).

Adventure gravel bikes vs hardtail MTBs

As the terrain gets rougher and the proportion of paved roads you traverse goes down, you will at some point be better off on an adventure gravel bike than on an all-road bike, and then you will be better off on a hardtail MTB than on an adventure gravel bike. Your own skill and preferences will probably influence those transition points. If you are uncertain about which bike to choose, you could consult friends who ride similar terrain as your target.

Suspension and gravel bikes

Suspension forks are rare among gravel bikes at the time of writing. Consider that at the time of writing, many gravel bikes can accept tires wider than 40mm, and some can take nearly 50mm tires. That's already quite a lot of suspension.

Nonetheless, some manufacturers are designing short-travel suspension systems for gravel bikes. The BMC Urs has an elastomer-based rear suspension, with very short travel (~20mm). The Specialized Diverge has a 20mm travel, oil-damped front suspension system. The Niner MCR has full suspension (40mm front and 50mm rear travel) and a geometry based on old hardtail MTBs. That suspension system is tuned more for frequent small bumps like you would face on gravel than for the big hits one might encounter on singletrack. Additionally, short-travel suspension seatposts, stems, and forks are available aftermarket.

At the time of writing, it seems safe to say that suspension is the dividing line between MTBs and gravel bikes on the more adventure-oriented end. The gravel bikes that have suspension have very little travel compared to MTBs.

What divides endurance road and all-road/gravel bikes?

On the roadie end of the spectrum, the dividing line is probably that performance and endurance road bikes usually can clear 32mm tires at most (there are some exceptions), whereas gravel bikes can usually clear at least 40mm tires. It should be noted that road bikes - even older ones on narrower tires - can traverse dirt roads. The more loose gravel there is, the more careful you need to be. If you are mainly on tarmac, you likely don't need more than 32mm tires.

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  • With the update, it should be noted that both Fox and RockShox have released gravel-specific suspension forks: Fox 32 AX and Rockshox Rudy. Other manufacturers may soon follow suit.
    – MaplePanda
    Nov 11 '21 at 2:05
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