What is the theory behind a gravel bike?

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.

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.

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.

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.

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, 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.

There's an interesting discussion with Josh Poertner, an aerospace/automotive engineer turned pro cycling consultant, on finding the most efficient tire pressure for different riding conditions. Basically, high pressure tires are less efficient on imperfect surfaces because of kinetic energy lost by bouncing the whole bike up and over a small object or road crack, where a low-pressure tire can absorb most of that impact without disrupting your momentum. The rougher the riding surface, the lower the pressure needed for optimal efficiency. Wider tires can run at the lower pressures needed for efficient riding on gravel, and the aero tradeoffs are smaller at gravel speeds.

I think the more upright riding position has to do with conservation of effort and a reduced tradeoff between aerodynamics and handling. Since your not breaking any land speed records on gravel, getting max aero out of your stance isn't as crucial, but picking (and staying on) the best line often is. Raising the bars up lets a rider use flared-out drops to selectively widen their grip for better control, when needed, without getting completly horizontal in the process. I've seen some gravel riders who spend upwards of 50% of their time in the drops, vs. road cyclists who are maybe using them 10-15% of the time.

These are just some of my observations as a strictly non-competative cyclist who only recently got into gravel. I also happen to think my basic-B of a gravel bike makes the perfect everybike for all around riding. It's a great all-weather commuter, has almost identical gemoetry to some high-end randonneuring frames, and can even tackle a bit of singletrack.

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?

Gravel bike is a road bike done right. I personally think that what is currently called "road bike" should be called "racing bike", and what is currently called "gravel bike" should be called "road bike". But, you cannot change the existing terms so they had to invent a new term.

The upright riding position allows you to actually enjoy cycling without suffering from pains during and after your ride.

The wheels of a gravel bike are more likely to have the full number of spokes (36 per wheel). Such a spoke count makes a more durable wheel, and a 36-spoke wheel does not instantly go completely out of true if one spoke breaks.

The tire clearance of a gravel bike allows you to avoid the excessively narrow 23mm tires that many racing ("road") bikes must use because even 25mm tire might not have enough clearance to work with e.g. a broken spoke or some room for mud if riding on muddy roads. In contrast, you can pick some reasonable tire like 28mm or 32mm tire -- the 28mm or 32mm tires can be pumped to usage pressure in an easier manner by a pump that you can actually carry with you in case of punctures, and less likely to pinch-flat than 23mm tires. If riding in a location that has a true winter, you most likely are able to find studded winter tires that fit a gravel bike, whereas you cannot fit such tires to a racing ("road") bike.

A gravel bike is more likely to have possibility to fit pannier rack and fenders. Very useful if you actually use your bike for useful transportation like going to a grocery store instead of going for a 20km bicycle ride starting from your home and ending to your home, and then afterwards driving 10km to a grocery store by a car and driving back the other 10km.

A gravel bike is more likely to have disc brakes even though such brakes are becoming more common on even racing ("road") bikes. Very useful in the rain.

A gravel bike is more likely to have an actually useful rear cassette. In contrast to the current trend of 11 or even 12 sprockets in the rear, which makes the gear changes excessively small if you have a racing-style cassette with very small biggest sprocket (a hill ahead of me? click... click... click... oops nothing happened, oops I forgot that I have a 12-sprocket cassette with fine spacing so I must click three times more for a total of 6 times!), a gravel bike is likely to have a cassette where the biggest sprocket is actually so big that the gear changes actually do something. Not that you need a big sprocket, but it's because the current trend of 11 and 12 sprocket cassettes necessitates a big sprocket in order for gear changes to do actually anything that you can feel at your feet. Of course the best way to solve the gear count problem would be to go back to 6- or 7-sprocket drivetrains, but that cannot be done due to marketing reasons. You cannot reasonably today sell a bicycle with 6- or 7-speed drivetrain, as nobody would buy it.

A gravel bike is less likely to contain lots of carbon fiber in e.g. the frame, fork or load-bearing components. In contrast, a gravel bike is more likely to have a reasonable frame and fork material (i.e. butted high-strength chromium molybdenum steel or other similar high-quality steel alloy). You don't have to purchase that $100,000 X-ray machine and find some location in your home to install it and supply electricity for it so that you can inspect your carbon fiber components every time for damage after you fall. In contrast to carbon fiber that can suffer invisible internal damage after falling that is only detectable with methods such as X-rays, metals always fail at the surface so after falling you can just keep an eye on developing cracks at the surface of the metal. Most cyclists buy carbon fiber and skip buying the all-important $100,000 X-ray machine and only hope for the best after falling, because a carbon fiber component is expensive to replace. My approach for avoiding the $100,000 X-ray machine problem is to avoid carbon fiber.

A gravel bike is less likely to be excessively lightweight and emphasizes durability over light weight. This is very useful, because an excessively lightweight bike fails in no time, perhaps in a dangerous way.

Some time in the past cyclists wanting a reasonable bike had to buy a drop bar touring bike (oops, it has a too low bottom bracket), or a cyclocross bike (oops, it has too low handlebar position). No need for that anymore, as you can purchase a gravel bike with neither of these problems.

About the only problem with gravel bikes is that the preinstalled tires are gravel tires (they have to be, otherwise you couldn't name the bike "gravel bike"). You can fix this problem by removing the gravel tires and installing high-performance slick tires, if you mainly ride on pavement.

I recommend selecting gravel bike as your only bike. If riding often on roads having soft (muddy? snowy?) surface, of course you might benefit from a second bike (MTB? fatbike?) with even thicker tyres.

The other answers address one theory of gravel bikes. Many road bikes have 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, but mainly on tarmac. Many gravel bikes extend this design paradigm by taking endurance road geometry and adding more tire clearance and hardpoints. This enables riders to tackle long stretches of unpaved surfaces. This is attractive for reasons already discussed.

(NB: many current endurance road bikes have enough tire clearance to enable you to ride confidently on hard packed dirt or limestone. In fact, many older endurance road bikes can do the exact same thing. In general, the narrower your tires, the more you have to actively pick your line through an unpaved road.)

However, an emerging design paradigm for gravel bikes draws from the mountain bike side of things. Global Cycling Network summed up the overall argument in a video titled Gravel Bikes Exist Because Mountain Biking Is Now Boring. Basically, modern MTB geometry and suspension design has evolved to the point where bikes' capabilities far exceed the demands of less technical off-road terrain, which is probably the majority of off-road terrain (i.e. you are over-biked in most places). Or, as Dave Rome wrote for Cyclingtips:

On the other side, mountain bike terrain has only gotten more challenging over the years, and bikes have progressed to keep pace with this. In turn, a mountain bike suitable to many tailored trail networks can make riding simpler terrain, such as gravel roads, rather dull and slow-feeling.

Some gravel bikes are influenced by MTB design

Some gravel bikes have geometry closer to modern MTBs. These gravel 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).

Where does suspension come into play?

If we are talking about taking design elements from the MTB world, some readers may wonder why I omitted suspension. To my knowledge, suspension is still rare among gravel bikes, and where it exists, it does not currently appear to be a major differentiating point between MTB- and road-oriented gravel bikes. Many riders seem comfortable relying on their tires for suspension, at least on the terrain that gravel bikes frequent.

The Chamois Hagar has no suspension, aside from its massive tire capacity (700x50). 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.

Suspension systems on gravel bikes will probably evolve, so I might have to revisit this article in a few years.

Q1: Riding position

A1:

Do you know the riding position of a so-called gravel bike and a regular road bike could be 100% identical?

Let me show you my real case.

I just have a s-works diverge (so-called gravel bike) and a giant advanced tcr (so-called regular road bike).

I have carefully studied their differences which include geometry. Here is part of my study work.

And, let's put them together now and check it again.

They are almost 99% identical. Surprised? But, too see to believe, right!

The so-called gravel bike has slightly higher handlebar but it is under the range of fitting and adjustment. Actually, the riding position of my diverge (so-called gravel bike) is more aggreesive than the tcr (so-called road bike). If you like to, they can be adjusted to 100% identical easily.

Q2: The tires which gravel bikes use.

A2:

The key characteristics of a so-called gravel bike (here is the diverge) might include:

1. It is much more comfortable and enjoyable riding experience.
2. Slightly longer wheelbase.
3. It is also an adventure machine.
4. It is as fast as the regular bike or even faster. (I have multiple new PR with the diverge)
5. It can not replace the XC and DH MTB. For technical off-road, I will wish I ride a MTB or a specific purpose bike.

It is quite similar to the relationship of a SUV and a regular car.

Thus, the so-called gravel bike usually uses wider tires (>=38C).

But, some of your knowledge about gravel bike might be not so correct.

The thread of gravel tires is not necessarily deeper. There are gravel tires without thread (e.g. Gravelking)

Check here to find the thread of a gravelking plus tire: https://www.panaracer.com/lineup/gravel.html

Ok, it is my study work. It might not be your expected answer but I wish it will be helpful.

Final words and it is an important reminder.

The upper case is just only one of gravel bikes which are recoginized by the gravel players' community. Actually, there is not a single kind of gravel bikes but many many kinds.

That's the reason I use "so-called gravel bike" but not "gravel bike" in the article.

Cheers