Gravel Bike Geometry
When people ask me about cycling gear, I generally say that if I’m not thinking about it (brake levers, sleeping pad, etc.), then it’s working perfectly. I used to ride a stiff aluminum cyclocross bike for everything: short bikepacking trips, cyclocross races, road rides, and randonneuring rides (extra-long unsupported road rides that guarantee a sore bum). This bike’s gearing and cantilever brakes – the weakest of brake systems – were definitely not meant for steep hills, and the tire clearance maxed out at just 32mm. But none of this stopped me from taking it out for 2o or 120 miles. I didn’t have room in my garage or my bank account for another bike, so I chose not to think about the options in the marketplace.
As the years passed, I started to feel the differences between my old aluminum road/everything bike and my mountain bike, with its stability and strong brakes. When my trusty aluminum was about 10 years old and had seen thousands of miles, I rode it down a notoriously steep descent near my house in foggy mist. The brakes took ages to slow me down, using every ounce of my grip strength. Every tiny bump was jarring.
My neck ached at the end of the ride – and it started to swivel towards gravel bikes. Even though I was still skeptical of the difference between gravel bikes and what I already had, I arrived home that day and asked my husband what he thought about selling my old faithful and getting something that could better handle the mix of terrain near our house. Luckily he’s also a cyclist, so before I knew it he was scrolling the web with me, and we quickly found a barely-used, affordable (but still not cheap) carbon gravel bike online. After my first long ride on my new steed, I was fully convinced. My body didn’t ache. I was left wanting more miles and found myself already formulating bikepacking plans.
I’m not here to tell anyone they need a certain type of bike to have fun – far from it. I certainly had a blast before I changed up my stable. Decent tire clearance and a bike that fits are all you need to comfortably explore unpaved terrain. But if you’ve been cycling for a while and are curious about what is out there, I encourage you to see for yourself.
First, let’s ask: what is it that makes a gravel bike different? In the past, when people referred to the geometry of a bicycle, I just nodded and agreed, knowing that it referred to the angles between the frame tubes. But a larger definition of bike geometry not only includes the angles of the frame, but also design elements that distinguish one type of bike from another, optimizing it for certain terrain and riding styles.
These differences may seem small, but the more you ride, the more you notice them. I wish someone had explained some of these to me before I rode so many tense miles on my old bike. So here are some of the specifics of gravel steeds, in full bike geek detail.
Chainstays
Chainstays can only get so short while maintaining operational tire clearance. So once a bike manufacturer or builder decides what the maximum tire size should be, they then build the chainstays long enough. Shorter stays generally mean snappier handling; longer stays increase stability. For touring bikes, where stability at speed is preferred, long chainstays (430 mm or longer) contribute to a longer overall wheelbase, making the bike more stable. Gravel bike chainstays tend to range from 420 to 435 mm.
But long chainstays are not the only way to increase stability.
Wheelbase
The distance between the tires’ point of contact with the ground. The longer the wheelbase, the more stable a bike handles at speed. The wheelbase on gravel bikes is usually longer than road bikes but shorter than mountain bikes. This middle ground gives gravel bikes a combination of stability on rougher roads and at least some agility for the occasional tight turn. A short wheelbase bike won’t track as well on loose surfaces as a longer wheelbase bike. This is why full suspension mountain bikes have gotten progressively longer over the years. You don’t really need to study wheelbase on geometry charts, but this can still go into your base of knowledge (no pun intended).
Head Tube Angle
This refers to the angle of the head tube when compared to a horizontal axis. In basic terms, the slacker the head tube angle, the more stable the ride, especially at speed on loose surfaces. A head tube angle of 69 degrees is considered very slack, while one of 74 degrees, like those found on road bikes, would be considered steep. Gravel bike head angles are usually 70-72 degrees.
Head Tube Length
Generally speaking, the longer the head tube, the more upright the riding position. Road bikes have the shortest head tubes, facilitating an aerodynamic position at road speeds. A gravel bike head tube might be an inch or two longer, resulting in a more upright position that is more comfortable and helps with control on loose terrain.
Bottom Bracket Height
This affects center of gravity. The lower it is, the more stable the bike will feel, which is generally what we want on gravel roads, loaded touring, or both. A low center of gravity helps us get our weight back when descending on rougher terrain, but it also means the pedals have less clearance for pedaling through rocky terrain. Most of the time, we aren’t pedaling through technical or off-camber terrain, so this is not a major concern with gravel bikes. A bottom bracket height of 260-280mm, including the volume of your tires, will put you where you want to be. For contrast, traditional cyclocross bikes, designed to bunny hop barriers and pedal through hairpin turns, have high bottom brackets (280-310 mm).
Top Tube and Stem Length
The length of the top tube should coincide with your correct bike size and is one the more important considerations when choosing a bike. Once you have the right bike size, you can easily exchange the stem to dial your position even more, depending on if more of your height is in your torso or your legs. The proper top tube/stem length will prevent you from stretching out too much, so you can keep your elbows bent and shoulders relaxed. Your arms (and legs) are like shock absorbers, and you’ll feel much more in control if they are bent and ready to absorb whatever comes your way, rather than stretched out and rigid.
Tire Clearance
Gravel bikes come with ample tire clearance for 40mm or larger tires. I love wide tires so much that I get a warm, fuzzy feeling when I look at mine! The rougher a route is, the wider the recommended tire is, within the limits of the frame design. With wide tires, there is more surface area and volume to absorb the bumps of the road. You could also consider using smaller 650b wheels to get even more tire volume if you want it.
Components
- Lower gearing, meaning easy gears for going up steep hills with weight
- Wider or flared drop bars, for increased stability
- Disc brakes apply braking force to a rotor mounted near the middle of the wheel instead of directly on the rim, and are increasingly common on gravel bikes. I personally love this trend. Rim brakes, which are another name for cantilever brakes, are certainly capable of stopping you, but disc brakes come with some huge advantages, including:
- Far more braking power.
- Disc brakes perform far better in wet terrain. (Rim brakes must first displace water from the rim before biting on the surface, hence my many harrowing descents on my old bike.)
- The power delivered by disc brakes is more consistent and predictable.
- Since disc brakes are not in direct contact with the wheel, they work fine even if the wheel is slightly out of true and won’t wear down the surface of the rim.
- Disc brakes also allow for better tire clearance (Yay! Wide tires!).
- Despite these benefits, some cyclists still prefer traditional cantilever brakes because of the disadvantages of disc brakes:
- They typically weigh more, cost more, and take more effort to adjust.
- Maintenance depends on whether you have hydraulic or mechanical disc brakes.
- Hydraulic disc brakes shouldn’t need much daily maintenance because they are sealed, but when something does go wrong, it’s not an easy field fix. If there is an air bubble trapped in the line, hydraulic disc brakes won’t work properly and must be flushed out, or “bled,” which isn’t as simple as adjusting a caliper on mechanical disc brakes or rim brakes.
Frame Material
Today, gravel bikes are built out of carbon, aluminum, steel, and titanium, so you have choices! To read more about the differences between materials, check out this Adventure Cycling blog post. The good news is that gravel bikes of all materials are reinforced to be stronger in certain places and are often designed with loaded touring in mind. Some even have strategically squared tubes to accommodate serious bikepacking gear.
Bikepacking Considerations
More and more gravel bikes are coming with mounts for front or rear racks or cages and with weight-bearing in mind. But if the one you are eyeing doesn’t, don’t worry! There are bag and rack manufacturers out there making gear for bikes with no mounting points, and there are also plenty of DIY mounting techniques. A good overview of bikepacking bags is here and a list of gear manufacturers can be found on here.
After thousands of miles on my carbon gravel bike, I can confidently say it has been a fabulous bike for a variety of situations – dirt, pavement, gravel, loaded and unloaded. It performed on the most extreme terrain I dared to take it, including rugged singletrack and a month-long bikepacking trip through Spain and France.
But though I love my carbon gravel whip, I’m still a believer in “ride what you have.” Don’t turn down an adventure because you don’t have the “right bike.” Does it have two wheels and no serious mechanical issues? Great, go for it. Remember, people were riding penny farthings around the world before any of us were born. But now, hopefully, you know more about what’s out there and can make an informed decision to upgrade in the future. If you’ve read this far, you’ve already got the most important ingredient for any bike trip: a sense of curiosity and adventure.
