bike buyers advice, bike tech talk

Understanding bicycle geometry

Reach, stack, steering angle …? I beg your pardon, what? Anyone who deals with bicycles is quickly overwhelmed by the technical jargon of the industry. We explain the most important geometry data and measurements of modern bicycles in understandable language. We also reveal which geometry dimension influences the riding behaviour and how. This way you can interpret the geometry table we provide for each bike yourself. However, because this is a complex undertaking in which you have to compare several values, we have already taken a look at the geometry of each bike and tell you how the bike rides on the trail and whether you will sit sportily or comfortably on your dream bike.

Table of Contents

Stack (1)

Like the reach, the stack is a virtual measurement between the bottom bracket and the top of the head tube, but measured vertically. With higher stack values you stand lower behind the handlebars. When sitting in the saddle, high stack values have a more comfortable effect on the riding position. Attention! The stack is extremely dependent on the suspension travel of the fork and the wheel size. The more travel a bike has and the larger the wheels used, the higher the stack value. Bikes with a particularly low stack value tend to have a sporty riding position. Because other geometry dimensions also play a role in these statements, we have developed a formula that automatically tells you whether you will be sitting on your bike in a sporty or comfortable position. With our bike-test.com seat position assessment we tell you exactly how you will sit on your dream bike.

Reach (2)

The reach is a virtual measurement. It measures the horizontal distance between the bottom bracket and the top of the head tube. This value defines the length of the main frame. This value varies between the different bike categories. In recent years, however, the reach value has increased considerably on all mountain bikes. In return, shorter and shorter stems have been installed. This has made the handling of mountain bikes more and more direct and comfortable while maintaining the same seating position.

Top tube (3)

The top tube is measured horizontally from the centre of the head tube to the seat post. A long top tube tends to make the rider sit on the bike in a sporty, stretched position. Cross country bikes in particular have a relatively long top tube. 

Seat tube (4)

The frame size given in centimetres or inches usually refers to the length of the seat tube. The longer the seat tube, the larger the frame. The seat tube is measured from the centre of the bottom bracket to the end of the seat tube. Extremely long or short seat tubes can make it difficult to adjust the seat height, especially in combination with a telescopic seat post.

Seattube angle (5)

The seat angle describes the orientation in which the seat tube is welded to the bottom bracket. A seat angle that is too flat leads to a stern-heavy riding position and inefficient movement of the leg muscles. Many people then have the feeling that they are pedalling “forward”. Good values for the seat angle of mountain bikes are between 73-75 degrees. If the seat angle is too steep, knee problems can occur. The seat angle of a bike can also be compensated to a certain degree by the saddle clamp, depending on whether you mount the saddle far forward or far back.

Head tube (6)

The head tube contains the headset and the fork. The longer the head tube, the higher the handlebars and the more comfortable the riding position of a bike. Cross-country or marathon racers prefer the shortest possible head tube in order to get a lot of pressure on the front wheel when descending and also to sit nicely streamlined in the saddle. Enduro riders stand better behind the handlebars with a longer head tube and thus have more control over the bike on steep descents.

Headtube angle (7)

As a rule, the steering angle becomes flatter the more suspension travel a bike has. Flat steering angles suggest a downhill-oriented design of the bike. Viewed in isolation, however, the steering angle is only one parameter for the optimal geometry. In general, a flat angle ensures a track-following ride, while a steep steering angle makes the bike agile. Downhill-heavy mountain bikes like an Enduro have a steering angle between 65 and 64 degrees. Normal hardtails with 100 millimetres of suspension travel have a steering angle between 69 and 68 degrees. In addition to the steering angle, the caster of the front wheel also affects the handling of a bike. This caster in turn depends strongly on the offset of the fork.

Bottom bracket drop (8)

The bottom bracket drob is the vertical distance between the bottom bracket and the wheel axles. It is often given in a negative number, sometimes also in a positiv. The lower the bottom brackets position under the wheel axels the better a bike corners and the more save it feels on the trails. Bigger 29er wheels offer the option to more bottom bracket drop then smaller wheels. This is one reason, why 29er wheels are rideing so well in technical terrain.  

Bottom bracket hight (9)

The bottom bracket hight also discribes the position of the bottom bracket. But unlike the bottom brackte drop it is not measured from another virtual point of the bike. It discribes the absolute hight from the ground to the middle of the bottom bracket. The lower it is the saver a bike rides. But bikes with to low bottom brackets have the problem that the pedal hits the ground very often when pedaling on the singel trail.   

Chainstays (10)

The chainstay length describes the length of the rear triangle. Short chainstays allow the front wheel to lift off easily and make the bike agile. Longer chainstays prevent the bike from rearing up on steep climbs. This measurement is extremely important for assessing the handling of bikes on the trail. We have also developed a formula for this to tell you with our bike-test.com Handling Score whether your bike is playful or runs smoothly.

Wheelbase (11)

The wheelbase is the distance between the rear wheel hub and the front wheel hub. A long wheelbase gives the bike a smooth ride, but may make it a little sluggish. In recent years, the trend on mountain bikes has been towards a longer wheelbase in combination with the shortest possible chainstays in order to combine agility and smooth running.

Stem length

With short stems, mountain bikes can be steered more directly through the curves. However, the riding position suffers. Depending on the bike category, the stem length varies from 40 mm (enduro) to 100 mm (hardtail). The stem also adjusts the bike to the individual size of its rider. Stems with a length over 100 mm are outdated and not comfortable to ride. 

Fork offset

Very few manufacturers give this measurement in their geometry tables. It describes how far the wheel axle is in front of the steerer tube. Suspension forks with little offset (44 mm) generate more caster when installed and thus make the handling of bikes even more true to track. Forks with a large offset (51 mm) generate less trail and make the handling of the bike a little more playful. However, in such a case you have to be very sensitive to actually feel the differences on the trail.

Trail

The Trail is the distance between the point where the front tyre touches the ground and the point where a virtually extended line, parallel to the steering angle, meets the ground through the front wheel hub.

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About Ludwig

Ludwig Döhl is known as fulltime bike nerd. He is riding, racing and working on bikes for over 20 years. He grew up screwing in bikeshops and racing the european cross country worldcups. With getting older he managed to live as enduro-mtb pro racer, before he was working as deputy editor in chief for BIKE - europe´s biggest bike magazine. Now he is trying to get even more people excited about a lifestyle on two wheels with this website. His knowledge about bikes depends on over 100.000 km in the saddles of over 800 different bikes he rode in the last decade. Having any question for Ludwig? Just drop it down in the comments and he will answer as soon as he is back from his ride.