What is the theory behind a gravel bike?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

Here's an alternate take. Think of performance road bikes, endurance road bikes, all-road bikes, adventure gravel bikes, and hardtail MTBs as forming a spectrum of bikes. Each of the identified bike types is a discrete place on that spectrum. As you go toward hardtail MTBs, you can comfortably navigate more and more technical terrain - tighter switchbacks, rougher surfaces, short steep climbs, short steep descents.

Each bike has an operating envelope. I can ride my older performance road bike with 25mm tires on many dirt roads in the Midwestern US. When there's a layer of loose gravel, that becomes actively unpleasant and a bit sketchy. Many other older performance road bikes had even narrower operating envelopes.

For a modern endurance road bike, which can often take 32mm tires, even a bit of gravel is comfortably in the performance envelope, at least for many riders. You could complete Unbound on some endurance road bikes, like the Trek Domane with 38mm tires. It might not be the most comfortable, of course.

Or consider an adventure oriented gravel bike. This is at home on a lot of terrain that would have challenged old MTBs. In terrain where you need a full-suspension MTB, it will definitely be outmatched. If you put slick tires on it and ride it on tarmac, it will also be outmatched.

Thus, what type of bike is optimal for you depends on your local terrain and on your preferences. Endurance road bikes were mainly designed for tarmac, but their operating envelope has expanded. The operating envelope for gravel bikes spans paved and unpaved roads. Different styles of gravel bikes emerged pretty early on, and some gravel bikes' operating envelopes overlap with hardtail MTBs. In 2024, a lot of new bikes have been called all-road bikes; those bikes' operating envelopes span endurance road plus the roadier gravel bikes.

juhist's answer may overstate some points, but he is correct that performance road bikes are designed for relatively narrow performance envelopes. They're designed to go really fast on tarmac in an aggressive riding position. And many non-racing cyclists might want to forego performance road bikes.

I'll discuss how some categories of bike have changed.

Endurance road bikes

These are still optimized for tarmac, but as stated above, they can actually handle some off-road scenarios. Many people do the Belgian Waffle Ride in the US on road bikes.

They tend to have more upright riding positions than performance road bikes. I haven't comprehensively surveyed them, but they will probably tend to have trail figures in the low 60mm range, a bit more stable than performance road bikes which I think are in the mid 50s. Endurance bikes' wheelbases will probably average a bit longer than performance road bikes. Their carbon layups or tube selections are probably oriented more towards comfort and less towards stiffness. Current generation ones can take 32mm tires, often more. Of course, some performance road bikes are creeping towards clearance for 32mm.

All-road bikes

This term isn't well-defined. It may originate with Jan Heine, an engineer and bike enthusiast who has some opinions that may be controversial, but who has been on the right side of a number of developments (especially wider tires on road bikes). They're endurance road bikes with more tire clearance, sometimes up to 40mm.

This BikeRadar article gives an opinion on the term. They probably come stock with road groupsets (e.g. Shimano 105), and not gravel groups (e.g. Shimano GRX), but I'm not 100% sure on this part. For example, Shimano's GRX 800 group with the 48/31t crankset would be fine gearing for these bikes. Typical compact gearing (50/34) is a bit high with 40mm tires for off-road use.

In 2024, a number of companies are releasing bikes that would fit this category.

Road-oriented gravel bikes

In the past, if someone just said "gravel bike" without any other qualifier, they might have meant a gravel bike that was like an endurance road bike. More recently, some gravel bikes have emerged that are basically designed for racing.

I'd expect trail figures about mid 60mm, and clearance for about 45mm tires in modern bikes. These can definitely be used for bikepacking. The ones that aren't gravel racing oriented can still be raced. You can expect three bottle cage mounts and a top tube bag mount. All-road bikes may not have these.

Adventure gravel bikes

These will have longer wheelbases than the former category. Some of them may have MTB-inspired geometry, i.e. very high trail, long TTs, short stems. If so, they'll be more capable in MTB-style terrain. That said, suspension is still rare on gravel bikes. You should expect tire clearance of 50mm for 700c wheels, and a bit more for 650B. These may add even more mounts than previous, e.g. mounts on the fork blades. If you showed this type of bike to a traditional roadie from the 2000s, typical reactions might include dismay, derision, or both.

Evil Bicycles' Chamois Hagar is one bike like this. From an older Cyclingtips review, the manufacturer 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.

Adventure gravel bikes vs hardtail MTBs

At some point, you'll really be better off on an MTB. I have almost no MTB experience, so I can't really say what that point is. I can tell you that I have ridden a very road-oriented gravel bike on some local MTB trails, and it's definitely not optimal, but I can stay on the trail. Suspension and flat bars are the major differentiating point here.

Suspension exists for gravel bikes, but it has very limited takeup right now. I don't expect a lot of gravel bikes to incorporate it. It does have major advantages for traction - keeping the wheels in contact with the ground - on very rough terrain. But most gravel bikes don't need suspension, and suspension does add maintenance requirements. You get enough suspension from the tires.

Answer 7

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

Answer 8

My very short take: there was a gap in the market coverage for mid-range and high-end bikes for generalist bikes that can be comfortable on "bad roads" but still maintain some performance on tarmac. The road segments focused on tarmac, the MTBs went further into the technical stuff. But for many people, there's a secondary network that is not technical enough for MTBs and too rough for road bikes. The segment gap couldn't be addressed from the MTB side, because it departed from there (hence the "gravel bikes are the new 90's MTB), and it came from the road segments. Ironically, on entry level bikes, this use-case was covered (the so-called "dual sport" bikes).

Of course some will say "the geometry is very close to touring/cyclocross", but touring bikes are rugged bikes meant to be bring you reliably in remote areas (so no carbon frames, no fancy transmission, no hydraulic disc brakes, only durable tires,... only proven stuff) and cyclocross are meant to be raced on cyclocross tracks, not for long straight "groads" and often lack mounting points that you may need to attach accessories for (very) long rides — and being a UCI-race discipline, cyclocross was also limited by the rules and necessities of that discipline (agressive geometry, max 33mm tires, higher bottom brackets, lower comfort requirements). On non-racing segments, marketers prefer nowadays to use gravel instead of cyclocross for a do-it-all drop bar bike, that being said.

Answer 9

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.

Answer 10

I't only a marketing hype.

Gravel can be identical to road bikes. You can put a bit wider tires on a road bike, like 28C to make it more comfortable.

Answer 11

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.