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What is a Gravel Bike Part 3. Geometry and terms.


The heart of any bike is its frame.

The geometry is the most important. Get the wrong geometry and set up for your unique size and it doesn’t matter how good the components are, what its made of or how ‘light’ the bike, it will never deliver the optimal SPH rating for you or your riding preference.

It’s even more crucial on a Gravel Bike.

Why? I’m glad you asked.

One of the single biggest fun factors of gravel is the variety and diversity of where you ride.

This means you can encounter all sorts of environmental factors, even on a local suburban ride. And so you’re seldom in one position for long. You are riding in a very dynamic manner and you’ll be moving around a lot more on a gravel ride as well as using more upper body. The funny thing is, although a lightweight bike is nice, it may not be the best for you and your riding. That’s where geometry comes into it.

Every year, new bike models are launched laden with the latest tech and adorned in splashy new colors.

While new paint jobs are the easiest to notice, more often than not, they are backed up with the promise of being faster, stiffer, lighter, usually from slight deviations in measurements, angles and dimensions. It’s these details that are far more consequential. And they might not work for you!

Even the slightest change to bike frame geometry can significantly impact how bikes feel, and ride.

Bike geometry involves a lot of measurements, math, charts, and diagrams that can look overwhelming. But geometry shouldn’t be a scary monster; you just have to know where to look.

Ready to take the plunge and dive in?

Key Bike Geometry Words

Understanding what these words mean, will make the difference between a dream ride or a recalcitrant hack. Or in GVR terms, SPH … Smiles Per Hour!

  • Stack
  • Reach
  • Wheelbase
  • Head Tube Angle
  • Rake
  • Trail
  • Bottom Bracket Drop. Or BB Drop.

Once you understand these you can gauge a good idea of how a bike will feel and handle, and whether it will give you the SPH your seeking! And remember, there is no right or wrong with your SPH, it is as unique as your fingerprint! All of these measurements apply to every bike, from BMX to Tour de France race bikes.

Manufacturers invest significant research into geometry, so don’t worry, about making the measurements yourself, the websites will be accurate.




Stack and Reach

Frame geometry diagram showing stack.
This bike geometry diagram shows the stack of a Santa Cruz Tallboy mountain bike; (image/Santa Cruz Bicycles)

Stack is the term for the vertical measurement from the center of the bike’s bottom bracket to the midpoint of the top of the head tube. Reach is the horizontal measurement between the same two points.

These two measurements are the fundamentals of setting the positioning of the rider on the bike.

These two measurements will provide a clue to the bikes character. It can be aggressive, aero and racy, or more comfortable and suited for endurance.

Bikes with a long reach and short stack will put riders in a low, aerodynamic position. Exaggerated, imagine a “Superman Flying” position. This is your race bike geometry for riders who want to go fast or who race. To really get the most from this style of bike, you need to be prepared to develop flexibility and good strength conditioning.

Conversely, bikes with a shorter reach and higher stack offer riders a more upright position, taking pressure off the upper body and back. However, don’t think this means it is a slow bike. Far from it. Ultimately it is your fitness that will determine how fast you can ride. But is your SPH (Smiles Per Hour) is all about long rides, more adventure and exploring and rides to far away bakeries, this is probably the style of bike you might love. Descriptions like “endurance” are found for these bikes. Gravel bikes though are the ultimate expression of this style of bike. And the right gravel bike is far from dull!

If you don’t know where your own fitness lies, check out these other blog posts HERE that our professional physio partners have curated. If you are relatively new to cycling, no matter what form, Road, Mountain Bike, Gravel or even Hybrid, we strongly recommend a professional fitness assessment. Have a look HERE for what Barefoot Physio offers.


Frame geometry diagram showing reach.
This frame diagram shows the reach of a Santa Cruz Tallboy mountain bike; (image/Santa Cruz Bicycles)


Wheelbase and Chainstay Length:

A bike’s wheelbase is the distance between the front and rear axles – centres of the wheels.

Bikes with a shorter wheelbase will turn a lot quicker and are sometimes described as agile or direct, while longer wheelbases tend to feel more stable but will lack some of the immediate turn. Now this is the interesting thing, on gravel you want a bit of slack in the steering because heading down hill, over a rough surface, you want the bike to be stable, you don’t want to spend your time ‘over correcting’ the bike which at best is tiresome, at worst and accident waiting to happen. The trick is to find the sweet spot.

Chainstay length is the space between the center of the bottom bracket and the rear axle. Bikes with shorter chain stay tend to be more nimble which can be a benefit when negotiating single track and climbing.

Head Tube Angle

The head tube angle of a bike is the angle at which a line would travel from the center of the head tube down the steering axis to the floor (the dotted line in the image below).

Frame geometry diagram showing the head tube angle of a Santa Cruz Stigmata.
This bike geometry diagram shows the head tube angle of a Santa Cruz Stigmata; (image/Santa Cruz Bicycles)

The steeper the head tube angle is (larger number), the less input a rider needs to make in the cockpit for maneuvering. Steep head tube angles are a mainstay of aggressive race bikes that need to turn on a dime without requiring much from the rider. There is, however, a trade-off. While bikes with steep head tube angles have quick handling, they can feel a bit twitchy, especially at lower speeds or across rougher roads and terrain.

Bikes with more slack or less steep head tube angles (smaller number) feel much more stable but do not respond with the snappiness of their steep-angle counterparts while maneuvering.

Rake (Fork Offset) and Trail

A bike’s rake or fork offset is the distance between the front wheel’s axle and a line drawn through the center of the head tube to the ground. Bicycle forks place the front axle forward of this line. More rake typically means a bike will be a more comfortable and stable descender; less rake or offset means more attention will be required. However, the more rake a bike has, the less responsive steering becomes.

The trail is the distance between the contact patch of the front tire and the point at which the imaginary line through the center of the head tube hits the ground. Bikes with less trail tend to be quick handling and nimble. They can, however, also feel a bit twitchy. These bikes have steep head tube angles and less rake, so the front wheel is more firmly underneath that rider than further out in front.

Bottom Bracket Drop

The bottom bracket drop is the distance the center of the bottom bracket sits beneath the front and rear axles. To find this measurement, draw an imaginary line between the two wheel axles, then measure the distance from the bottom bracket’s center.

Frame geometry diagram showing bottom bracket drop.
This frame diagram shows the bottom bracket drop of a Santa Cruz Stigmata; (image/Santa Cruz Bicycles)

Bikes with lower bottom brackets feel much more planted while cornering, lowering the rider’s center of gravity. However, the lower the bottom bracket, the closer the pedals get to the ground.

The measurement is crucial for off-road and cyclocross bikes. Higher bottom brackets give more clearance to overcome obstacles and more space to roll over mud and debris. They also allow pedaling further into turns, providing more clearance for the inside pedal as the bike leans over.

Pedal strikes can be among the nastiest ways to go down, so it’s important to think about how to pair cranks appropriately with the height of a bottom bracket.

Seat Tube Length, Angle, and Standover

Seat tube length and angle are exactly what they sound like: the actual length of a seat tube and the angle at which it sends a seat post out of the frame. These measurements are essential because raising a seat post up and down also moves the seat forward and backward. This impacts how the stack and reach of a frame affect the rider.

“Effective” seat tube length is what the seat tube length would be if the top tube were parallel to the ground (most top tubes are not). This number is more useful when comparing frames, as modern bikes have top tubes that slope down from the head tube to the seat tube. And the amount of slope varies from bike to bike.

Standover height is the height of the bike’s top tube from the ground. For people who step off of bikes, it’s essential to know whether you can put a foot down comfortably.

This bike geometry measurement can be tricky. For example, most cyclocross bikes have higher top tubes than gravel bikes because their riders need more space in the center triangle to throw the bike over their shoulder.

There is more likelihood of hitting the top tube on bikes with higher standover measurements. They can also be a little more challenging to mount and dismount.

Adjustments to Bike Geometry

Of course, there are accessories riders can buy to help dial in fit further if a bike’s factory geometry is a bit off. Spacers, stems with increased reach or rise, shorter or longer crank arms, seat posts with a setback or no setback, and almost endless handlebar variations can all drastically alter the feel of a bike, as can bigger modifications like different forks.

But, as riders use aftermarket accessories to tinker with a bike’s fit, they risk getting rid of the features of a bike’s design that enable it to ride as intended.

Understanding bike geometry is an essential first step in selecting a bike, but there’s nothing quite like hopping on and going for a ride to get to know how a bike fits and handles.

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