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MTB geometries explained

14.12.2020 |Know-How

Stack, reach, top tube length, steering angle… Simplon explains the most important terminology regarding MTB geometries. Learn more!

Everything you need to know about stack, reach and more

Reach – what? Stack – how? Top tube length – why? If you’re thinking about buying a new bike, you’ll quickly be bombarded with terminology that can be a little overwhelming. But on the way to your dream bike, there’s one important step: finding the bike geometry that suits you. Because that’s the deciding factor in how your bike feels when you’re riding it.

Questions like “Does this MTB suit my riding style? Which frame size is best? What role do stack and reach play?” should be addressed beforehand.

We’ll show you the most important MTB geometries and explain why they are relevant and what to keep in mind when buying a new bike.

(A) Seat tube length

Seat tube length is measured from the centre of the bottom bracket to the top of the seat tube. It used to be an important reference measure for the size of the frame. For MTBs, it’s now less important because of the dropper Vario seatposts that are frequently integrated as a standard feature. Saddle height is primarily adjusted via the seatpost. For a particularly low sitting position on challenging technical descents, the seat tube is often as short as possible on an MTB. By the way: Because of that, riders can opt for a bigger frame to get an even longer reach.

Rule of thumb for saddle height: The inseam (measured from crotch to ground) is multiplied by 0.885.

Good to know: If you subtract the seat tube length from the saddle height, you’ll know how long the Vario seatpost can or must be at maximum insertion and extension. The height of the saddle is at approximately 40 mm.

(B) Horizontal top tube length

Whether your riding position on the bike is “extended” or “compact” is determined by the top tube length. It’s an important measure for estimating the approximate sitting position and thus riding comfort. It is measured in the imagined horizontal from the top tube / head tube junction to the centre axis of the seat tube.

(C) Steering angle

The head angle, also called steering (axis) angle, is paramount for the manoeuvrability and rolling smoothness of the bike in rough terrain. Rule of thumb: The slacker the angle, the safer and the smoother the bike acts. If you prefer agile and direct handling, you might want to opt for a bike with a steeper angle.

Both options have their pitfalls: While extremely steep angles make the riding experience slightly “bumpier” due to the low caster, they also make the bike respond to steering impulses much quicker. Very slack angles – below 64.5 degrees – might feel “strange” and give you an unusual riding behaviour at low speeds. They also require you to lean forward even more to go over the front wheel.

The steering angle strongly depends on which bike you’re riding – enduro, all mountain, cross-country or trekking. Just get an expert to help you and test it!

(D) Seat angle

Steep mountain passages are your forte? Then you might need a steeper seat angle on your mountain bike. Because: The steeper the seat angle, the more your centre of gravity shifts forward and the more ergonomic the power transfer. This makes riding uphill a piece of cake.

A slack seat angle means there’s more weight on the rear wheel. The crux: The front wheel might start to wander or lift off the ground on steep passages. This makes riding uphill harder. On flat terrain, however, you’ll have a much more relaxed riding experience, and there will be less pressure on your hands.

(E) Head tube length

Head tube length is measured from the bottom to the top of the head tube and tells you more about the height of the front – the so-called stack. Generally speaking: The longer the head tube, the more upright your position on the bike. The shorter it is, the more “extended” you are.

Tip: Mountain bikers who prefer to sit more upright should get an MTB with a longer head tube. However, if you’d like to get more pressure onto the front, you should opt for a short head tube.

(F) Chainstay length

Chainstay length is defined as the length from the centre of the bottom bracket to the centre of the rear dropout. Similar to the head angle, the bike’s smooth riding behaviour (long chainstays) is in juxtaposition with its agility (short chainstays).

Longer chainstays are ideal for those who appreciate a good climbing performance and a smooth rolling behaviour – for example, trekking riders or enduro racers. More playful bike enthusiasts usually prefer shorter chainstays. Doing wheelies or riding over obstacles becomes easier that way.

There are subtle differences between MTBs and E-MTBs, especially when it comes to chainstay length: With e-mountain bikes you can tackle steeper and technically more challenging climbs. To prevent the front wheel from lifting off the ground, longer chainstays are better.

(G) Wheelbase length

Wheelbase length is the distance between the axles of the front and rear wheel. How does that affect handling? A short wheelbase length equals more agility, a longer wheelbase length more running smoothness.

(H) Bottom bracket drop

The advantage of a greater bottom bracket drop is a lower centre of gravity and thus more agile riding behaviour in curves. The drawback: It only allows for less intense cornering because the pedal hits the ground much faster on technical descents with roots and rocks.

(M) Standover height

Another aspect of the bike geometry that needs explaining: standover height. It’s measured vertically from the centre of the bottom bracket to 150 mm from the top tube / seat tube junction. The measurement tells you how much freedom of movement there is between rider and bike. That’s especially relevant when getting off the bike.

Important: A bike with too much standover height can lead to painful bottom-outs and falls. The reason for that is that you can’t get your feet on the ground fast enough when dismounting.

Good to know: There’s no universal standard for measuring standover height. This makes it hard to compare the values of bikes produced by different manufacturers.

(R) Reach and (S) Stack

“Stack” and “reach” are both important measures to determine the optimal sitting position on the mountain bike.

The reach tells you how long the main frame is. It denotes the horizontal distance between the centre of the bottom bracket and the top centre of the head tube. Stack tells you how tall the frame is – in other words – how high the front is in relation to the bottom bracket. It’s measured vertically from the centre of the bottom bracket to the top of the head tube.

New school MTB geometry – these are the latest trends!

Simplon has its finger on the pulse of time when it comes to the latest MTB models. These are the new-school trends:

The reach becomes longer

A longer reach also increases wheelbase length which, in turn, leads to a smoother rolling behaviour. To make up for the longer reach, the stems become shorter and shorter.
The rider’s position on a bike with longer steering angle is much more centered, which improves balance in the direction of travel.
Another benefit: The rider can lean onto the handlebars and has more freedom of movement.

However, there are limits to how long the reach should be. The balance between the front centre and chainstay length has a significant influence on the riding behaviour of the bike.

The steering angle gets slacker

A slack steering angle improves rolling smoothness: For one, because the wheelbase becomes longer, but also because the steering responsivity is slower which gives the bike more riding stability.
You’re less likely to get sent over the handlebars. The reason: The front wheel is farther in front of the rider’s centre of gravity.

The seat angle gets steeper

A steeper seat angle allows for the distance to the handlebars to remain the same. A longer reach alone would make sitting on the bike very uncomfortable, and the rider would have to be in a very stretched-out position. A steeper seat angle also lets you put more pressure onto the pedals, which increases seated pedalling efficiency.

Good to know: On steeper terrain, the seat angle becomes slacker anyway, because the rear suspension sags deep into its travel during climbs. That’s why it makes sense to have a steeper seat angle on mountain bikes compared to road bikes or trekking bikes.

After having learned about the most important geometries, you’re perfectly equipped to find the mountainbike that’s ideal for you.

By the way: At the experience center in Hard, you’ll get all geometries optimally adjusted to your proportions. It’s worth stopping by! Or you can arrange an online consultation appointment directly via video chat.

Svenja B.

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