By Katherine Moore

Published: Thursday, 22 September 2022 at 12:00 am

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Gravel bikes can be designed to cover everything from all-out long-distance racing to gnarly shredding in your local woods. But what is it that sets a race-ready aero rig apart from rowdier, burlier models?

Here, we dig down into the geometry details across the spectrum of gravel bikes, uncovering exactly why these off-road machines ride as they do. We also have some handy tips to help you choose the right sizing and the best type of gravel bike geometry for you and your riding.

Why is gravel bike geometry important?

Snappy or sluggish, stable or unsteady, capable or sketchy: the way gravel bikes are designed can have a huge impact on how they ride.

As the discipline has boomed in recent years, we’ve seen more categorisation within gravel too, with more models designed specifically with the demands of gravel racing or bikepacking in mind.

Ranging from aero gravel-racing machines such as the Pinarello Grevil F – which sit near the road end of the spectrum – to the steel-built Cotic Cascade, designed to take on rough, rooty and rocky trails, and wider-tyred, luggage-hauling bikepacking bikes such as the Salsa Fargo, geometry varies greatly.

Many gravel bikes, however, still sit comfortably in the middle of the category, designed to balance speed, comfort and capability over a wide variety of terrain – the Giant Revolt and Canyon Grizl are two good examples of all-round gravel bikes designed to take on most types of terrain competently.

How does geometry differ on cyclocross and gravel bikes?

While gravel bikes and cyclocross bikes may appear similar at first glance, it’s the geometry that largely sets them apart.

Because cyclocross bikes are designed with an hour or less of intense racing in mind, they feature steeper head tube angles, shorter wheelbases, higher bottom brackets and tighter tyre clearances to accommodate 33mm tyres.

These combine to give a ride that feels sharp and precise, with responsive handling to help you navigate tight corners. Larger main triangles without a sloping top tube help you to shoulder the bike more easily, which you’re more likely to need to do in a cyclocross race.

A gravel bike, on the other hand, features a longer wheelbase, lower bottom bracket and a slacker head tube angle to give a more stable ride over rougher off-road terrain. Most of these bikes tend to be designed with a more upright position in mind, more suited to longer days out exploring or even multi-day bikepacking adventures.

How to choose the ‘right’ geometry for you

Robert Smith / Immediate Media

Essentially, gravel bikes exist on a spectrum between endurance road bikes and mountain bikes: from shorter, steeper race-orientated steeds to longer, lower and slacker gravel bikes designed to be more capable on more challenging off-road trails.

Consider the kind of riding you’d like to do on your gravel bike: do you prefer mainly riding on the road with a few light stretches of gravel, thrashing around gravel race courses or pushing the limits of what you can do on a gravel bike over rocky, rooty terrain?

All of these will influence what gravel bike geometry will work best for you.

How frame size affects geometry

As you move through frame sizes, geometry measurements differ not only to account for sizing, but also to maintain the same ride feel from extra small bikes right the way through to the largest builds.

You’ll notice that some of the angles change as you switch between sizes: head tube angles tend to be a little slacker for the smaller sizes, and seat tube angles a little steeper.

Some bike brands spec the smaller sizes with 650b wheels rather than 700c to help maintain the same handling through the range, as well as reduce toe overlap in conjunction with a slacker head tube angle.

Does gravel geometry differ by gender?

Phil Hall / Immediate Media

Just like other bike types, whether there’s a need for female-specific gravel bikes depends on who you ask.

Liv is the only bike brand to make a women’s-specific gravel bike, the Liv Devote, which has geometry designed using women’s bike fit data.

Some other brands market women’s bikes that feature the same frames as their unisex bikes with different components, such as saddles and handlebars. These include the Juliana Quincy and Canyon Grail WMN.

Specialized and a growing number of other brands believe there are greater differences in bike fit data within genders rather than between them, and are now moving towards totally unisex bikes in a wider range of sizes to suit all riders, along with more customisable builds.

How can you choose the best size to fit you?

Jon Wild / Sam Andrews

Making sure you get the correct size bike for you is really important so that you can get the most out of your ride: not only in terms of comfort, but in terms of power and efficiency too.

The gold standard here is getting a bike fit with a trained professional. This can either be on a bike fit jig, which they can easily adjust to help find your perfect position, or using a bike set up on a turbo trainer.

Getting a professional fit before you part with your cash for a new bike is a wise move, and armed with your fit data you can shop around to your heart’s content knowing exactly what sizing to look for.

Bike shop staff can help you determine what size of bike you’ll need, taking measurements and allowing you to sit on or test ride bikes of various sizes.

If you already have a bike that you know fits you well, you can compare the geometry data to bikes that you’re interested in.

Remember though, to compare raw data: one brand’s medium may not be the equivalent of another brand’s medium, for example.

Gravel bike geometry measurements and why they’re important

When searching for your ideal gravel bike, you’ll find an array of geometry chart measurements to compare between models. Here are some of the most important to consider, and how they impact how your gravel bike will ride and size up.

Tyre clearance

Richard Baybutt

As your choice of gravel tyres is so fundamental to gravel bike performance, let’s consider tyre clearance first. A greater gap between fork blades, chainstays and seatstays, i.e. greater tyre clearance, is one of the defining features of gravel bikes when compared to other types of drop-bar bikes.

Gravel bikes tend to feature clearance for tyres from around 40mm up to 2.1 inches (53 mm) or more. This includes 6mm of space between the tyre and frame/fork on either side, the industry requirement to pass ISO-4210 standards.

You can physically fit slightly larger tyres than recommended, but be aware that this will affect mud clearance and could potentially lead to frame damage. Not something to risk.

Wider tyres have a really significant effect on gravel bike capability, so gravel bikes designed with more technical terrain in mind will tend to have greater tyre clearances, as will bikes designed for bikepacking. For racing gravel bikes, you can generally expect to find more slender tyre clearances.

Allied Cycle Works

When it comes to designing bikes, achieving a wide tyre clearance for the rear wheel can be a real challenge, because it impacts the drivetrain. Some brands have tackled this by going 1x only such as Genesis’ Fugio 30. Others, such as 3T with the Exploro RaceMax, have adopted a dropped chainstay, or alternatively raised, as on the Allied Able. The Specialized Diverge features a slimmed-down, reinforced chainstay.

In some cases, where tyre clearance is limited, brands may spec 650b wheels to allow you to use wider, higher-volume tyres with the existing frameset, or you might opt to invest in a set of these yourself to give you more tyre options.

Head tube angle

Richard Baybutt

Head tube angle is another key element of gravel bike geometry.

Mountain bikers will be familiar with this term, with the degree of slackness (or steepness) having a great impact on the ride.

The head tube angle determines how far in front of you the front wheel is, which affects stability. All else being equal, the slacker the head tube angle (lower number), the further in front the wheel will be, which gives a more stable ride over rougher terrain and down steeper slopes.

For this reason, gravel bikes at the MTB end of the spectrum tend to have slacker head tube angles for tackling technical terrain (typically just below 70 degrees). Racing-orientated gravel bikes feature steeper angles between 70 and 72 degrees, more akin to road and cyclocross bikes, to help retain sharp steering.

For comparison, endurance road bikes tend to overlap a little at the higher end of this range (e.g. the Trek Domane 56 measures 71.9 degrees), while race-focused road bikes are steeper (the Trek Madone measures up at 73.5 degrees).

Head tube length

Russel Burton / Immediate Media

Head tube length affects the height of your cockpit, and while this measurement is fixed, it can be adjusted by adding or removing headset spacers.

Taller head tubes tend to give a more upright position, although other factors can also affect this, such as fork length.

Stack is a better figure to refer to if you want to compare how upright or aggressive a bike is, and is explained below.

Bottom bracket height

Russel Burton / Our Media

From the ground to the centre of the bottom bracket, this measurement tells you how high the bottom bracket of the bike is, though it’s worth remembering it will vary according to wheel and tyre size too.

The bottom bracket height affects the position of the rider’s centre of mass: a lower bottom bracket will give a more stable ride over rougher ground, on steep descents and during hard braking.

However, a lower bottom bracket will also make pedal strikes more likely, which is an important consideration when tackling roots and rocks off-road.

Bottom bracket drop

Russel Burton / Our Media

This measurement is not affected by wheel and tyre size, because it is the vertical difference between the centre of the bottom bracket and a horizontal line between the hubs. Effectively, it describes how low the bottom bracket is in relation to the rest of the frame.

Again, a greater drop, and hence lower bottom bracket, will give increased stability, at the cost of increased risk of pedal strikes.

Wheelbase and chainstay length

Measured between the hubs of the wheels, the length of a gravel bike’s wheelbase will determine how it feels to ride.

Shorter bikes tend to feel more responsive, and are more common for racing, while gravel bikes optimised for more testing terrain tend to have longer wheelbase measurements, giving a more stable ride.

Chainstay length is one of the major factors determining wheelbase measurements. Typically, the greater the tyre clearance, the longer the chainstays will need to be to accommodate it: which in turn contributes to a more stable, planted ride feel.

Standover height

Salsa Cycles

Typically measured from the ground to the midpoint of the top tube, the standover height describes how far off the ground the top tube is.

There’s a wide range of designs in the gravel bike market here, from relatively high, straight top tubes of aero race bikes such as the Ridley Kanzo Fast to the slanted top tubes of more MTB-like gravel bikes such as the Evil Chamois Hagar.

For example, in a size medium these measure at 841mm and 671mm respectively, with a more middle-of-the-road Trek Checkpoint at 789mm.

Sloping top tubes are often favoured because this enables designers to use more exposed seatpost for the same sizing, which gives a more comfortable ride with enhanced compliance.

While a sloping top tube will give you a little more clearance as you stand over the bike and more manoeuvrability, this will also mean less space in the frame’s main triangle. If you’re planning on going bikepacking with your gravel bike, that’s something worth bearing in mind because it’ll limit real estate for fitting a frame bag.

Top tube length

Russel Burton / Our Media

The top tube length is one of the main indicators of sizing, measured from the headset to the seat tube. This number will be affected by whether the top tube is sloping or not, so the measurement ‘effective top tube’ is often used, which takes a level, horizontal measurement that isn’t affected by slope.

Top tube length has a big impact on the reach of a bike: too short and you’ll feel cramped on the bike; too long and you’ll be overreaching for the handlebars. Both are likely to cause discomfort and limit your bike’s handling potential.

Because many gravel bikes tend to opt for a more upright position, you’re likely to find a shorter top tube and taller head tube compared to a road race bike, which tend to put you into a longer, lower and more aerodynamic riding position.

As with head tube length, reach gives a clearer view of how one bike compares to another and is a better figure to compare when deciding between bikes.

Reach and stack

Russel Burton / Our Media

The reach is the horizontal distance between the bottom bracket and the centre of the top of the head tube, and the stack is the vertical distance between these two points.

Bear in mind that some manufacturers will include the headset in these measurements, so make sure you check how they’re measuring.

If you already have a bike that fits you well and know the reach and stack figures, you can use this to easily compare sizes of different models.

Remember there are other ways to adjust fit beyond this though, such as headset spacers, differing stem lengths, short or long reach bars and the relative size of different shifters.

Other things to consider

Depending on what you’d like to use your gravel bike for, there are a few more things you should look out for when choosing your preferred geometry.

If you plan on riding through the winter and using mudguards, make sure there’s really wide tyre clearance to enable you not only to run mudguards, but also a wider, more aggressive tread to tackle the mud.

If bikepacking will be a priority for you, consider how you’ll fit bikepacking bags. Higher top tubes can give bigger main triangles, which provide more space for frame bags. On the other hand, a sloping top tube will provide more exposed seatpost, which means you might be able to attach a larger seatpost bag.

A more upright position can mean there’s more space between the handlebar and the top of the front tyre for handlebar bags, which is especially important for shorter riders.

Another factor that smaller riders will want to pay attention to is toe overlap. This is where you hit your foot on the front tyre when turning sharply. Thankfully, due to a longer wheelbase and slacker head tube angles (especially in smaller sizes), this tends to be less common on gravel bikes than road bikes.

Toe overlap won’t render your bike unrideable, but if you have it you’ll need to be mindful when you’re steering sharply, typically during lower-speed manoeuvres.

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