Steering angle to stack: Geometry on a mountain bike

You can't go far wrong with entry-level hardtails. The geometry of these mountain bikes does not have to compensate for any particular drive influences from the brakes or chain. Nevertheless, there are small but subtle differences that are decisive for the riding behaviour and riding position. With MTB fullys, however, the frame geometry is more decisive. Due to the suspension on the rear triangle, engineers are faced with major challenges in terms of pedalling efficiency and damping behaviour. So if you are flirting with downhill-orientated MTBs such as all-mountain or enduro bikes, mountain bike newcomers need to be careful: extremely slack steering angles, generous reach values and short chainstays are in line with the trend, but only make sense under the dictates of experienced riders. Beginners are often unable to utilise the advantages of this type of geometry and find it difficult to ride off-road.

Why it makes sense to look at the geometry tables before buying a bike

For experienced bikers, a glance at the geometry table is often enough to assess the riding position and character of a bike. Especially when buying mountain bikes online or from direct mail order companies, the manufacturers' geometry tables help you to find the right bike. This is because the lengths, heights and angles ultimately determine how a mountain bike rides, whether the riding position is sporty and stretched or more upright, or how well the bike climbs. In addition, certain geometry values such as the seat tube length and reach are crucial to finding the right frame size. Therefore, it doesn't hurt to know how to interpret these values.

The most important values of mountain bike geometry at a glance

If you understand the technical jargon in the manufacturers' geometry tables, you can guess how the bike will behave off-road before you buy it.

1. Top tube: The top tube is measured horizontally from the centre of the head tube to the seat post. A long top tube ensures a more stretched and sporty riding position. A short top tube allows the rider to sit more relaxed and upright.
2. Seat tube: The seat tube is measured from the centre of the bottom bracket to the end of the seat post. Some manufacturers specify this length in centimetres or inches as the frame size. The rule is: the greater the stride length, the longer the seat tube can be. In addition to the reach, the seat tube length is decisive for the frame height - and therefore the size of the bike.
3. Protrusion height: Standover height is the vertical distance between the centre of the top tube and the ground. In technical terrain, a low standover height offers more freedom of movement. This value is particularly interesting for riders with short legs and should be as small as possible.
4. Stem length: Long stems provide a sporty riding position, short models for direct steering behaviour. There is a lot of tuning potential here for little money to customise the riding position of a bike. Stem lengths usually vary between 40 and 100 millimetres.
5. Head tube length: A long head tube ensures a high front end. Downhill, this provides extra safety. Uphill, however, the front wheel rises faster. A long head tube therefore reduces the bike's climbing ability.
6. Chainstay length: The chainstays are measured from the centre of the bottom bracket to the centre of the rear wheel hub. Long chainstays improve the climbing characteristics of the mountain bike, but it loses some of its playfulness downhill. Short chainstays make it easier to lift the front wheel over waves and obstacles. In addition, bikes with short chainstays are easier to pull onto the rear wheel, for example for a wheelie.
7. Steering angle: One of the most important parameters in MTB geometry. It describes the inclination of the fork. A flat steering angle makes the bike track-fast on downhills. If the steering angle is steeper, the bike is more agile. However, a steering angle that is too slack quickly makes the bike sluggish. On the other hand, a steering angle that is too steep makes the bike nervous at high speeds. Overall, the steering angle depends heavily on the bike category: The more suspension travel a mountain bike has, the slacker the steering angle usually becomes. While this angle is often around 68 degrees on hardtails, 63 degrees is not uncommon on enduro bikes.
8. Seat angle: It describes the angle of the seat tube. A slack seat angle creates the feeling of pedalling from behind. A steep seat angle improves climbing ability. The saddle offset can be adjusted to find the optimum riding position for you and to equalise the seat angle. The trend in current mountain bikes is towards ever steeper seat angles (75 degrees or even steeper).
9. Bottom bracket height: BB height in English. This indicates the distance from the bottom bracket to the ground. A low bottom bracket ensures a safe downhill ride. The disadvantage: The pedals touch down more quickly off-road.
10. Wheelbase: The distance between the front and rear wheel hubs is measured. A long wheelbase gives a very smooth ride. If the wheelbase is short, the bike is more agile. On 29-inch mountain bikes, the wheelbase is usually significantly longer than on 27.5-inch bikes.
11. Stack: This value describes the vertical distance between the bottom bracket and the top of the head tube, i.e. the height of the main frame. With a high stack, you tend to stand upright on the bike when descending. If the stack is too high, you may not get enough pressure on the front wheel and your control will suffer.
12. Reach: The reach indicates the horizontal distance between the bottom bracket and the upper edge of the head tube. In other words, the length of the main frame. In recent years, the trend in mountain bikes has been towards a long reach paired with a short stem. This should ensure direct steering behaviour and a smooth ride. However, this combination requires a lot of physical effort and is not very intuitive for beginners to ride. Reach values can also be used as a guide when choosing the right frame size.