Understanding the geometry of the bicycle

In the last 200 years, bicycle design has evolved in various ways in order to meet the needs of a wide range of users and improve bicycle performance in different disciplines.

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Currently, there are dozens of types of specialized bicycles for different types of cycling and their differences are related to their performance and qualities. Many of these differences are directly related to the geometry of your frame.

Bike geometry

When it comes to buying a new bike, most riders understand the importance of finding the right frame size. However, many of us do not know the meaning of those tables that the seller shows us or that we find on the internet, with the measurements that define their geometry.

So let’s go into parts and unravel the aspects of the bike frame geometry and how it influences our ride, performance, and safety as riders. You may like to read the best hardtail mountain bike under 1000.

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Bicycles come in different sizes, which generally refers to the size of your frame. Size is defined by the length of the seat tube.

Choosing a bike with the proper fit makes riding more comfortable and gives you better control over it. However, it must be borne in mind that the size of the frame is not a standard in itself, as it varies between brands, countries, and disciplines within cycling.

So the size of the frame of a road bike is not the same as the size of a frame of a mountain bike. Also, not all brands use the same system. Some show the height in centimeters, others in inches, and others by sizes (S, M, L, referring to the small, medium, and large respectively).

Although a good start to determine the appropriate size for your bicycle is to know the size that fits your physiognomy, it is also important to know that due to the differences between geometries and, even between brands, these can vary the fit between each user. This is why knowing what the different measurements that we find in the geometry tables mean for each cycling style will be very helpful to get the bike that best suits you.

What is the Stack and Reach of a bicycle?

The two fundamental elements in the geometry of the frame are Stack and Reach. The Stack could be defined as the height of the frame and the Reach as its length or reach.

Stack: It is the vertical distance between the center of the bottom bracket with respect to the center of the upper end of the head tube.

Reach: It is the horizontal distance from the bottom bracket to the top of the head tube.

Keeping these measurements in mind will allow us to compare between different bicycle manufacturers or brands. Remember that the different brands and styles of bicycles will have different geometry despite being the same size.

For example, a road bike designed for distance tends to have a higher stack, which allows a more upright and relaxed posture. On the other hand, a road bike geared towards speed or track competition will allow for a more aggressive stance due to a lower stack.

When comparing the Reach of a road bike cycling endurance and one for sprints, The Reach of those designed for speed generally has a range (Reach) longer than road bikes to distance. This achieves “stretch” the cyclist’s posture, creating a lower frontal profile, which improves the aerodynamics of the rider, achieving greater speed with the position acquired when riding the bicycle.

The term Reach is also sometimes used to describe the reach of a cyclist. Note that the scope of the chart is different. The rider’s reach refers to the rider specifically and is measured from the tip of the saddle to the end of the steering stem or stem, regardless of the scope of the box. This reach can be manipulated by changing the position of the saddle – forward or backward – on the seat post and the length of the steering stem.

How it affects the angle and length of the head tube?

The angle and length of the steerer tube are two key measurements that have a direct influence on the handling of the bike.

The head tube angle refers to the perspective or inclination of the head tube with the ground and will commonly be described as a low angle (slack) and/or a high angle (steep). A low head tube angle requires more effort when changing direction but gives the bike more stability.

A high head tube angle, commonly found on road bikes, requires less effort to ride and allows for a more agile bike. Mountain bikes have a looser head tube angle than road bikes, which provides greater stability on difficult and technical trails. The length of the head tube is measured from the bottom of the tube to its top.

Bikes with a long head tube raise the front end of the bike, placing the rider in a more upright position. Bicycles with a short head tube lower the front end of the bike, placing the rider in a more inclined position, designed to reduce the aerodynamic profile of the rider.

Offset, Trail and Head Angle bike geometry

The offset of the fork or Fork Offset, the delay or Trail, and the steering angle or Head Angle are intrinsically intertwined.

Offset: It is the distance between the axis of the front wheel and the axis of the steering tube.

Trail: It is the distance between the tire’s contact point with the ground and where the “Head Angle” line touches the ground.

Head Angle: It is the angle between the projection of the steering tube axis and the vertical projection of the front wheel axis.

The smaller the Trail value of the fork, the faster the bike will respond. Being that it is related to the angle of the steering tube and the offset of the fork. The smaller that space, the more agile the change of direction will be. On a larger Trail, the turning radius will be larger and therefore less agile, but the bike will be more stable.

How it affects the angle and length of the seat tube?

Seat tube angle refers to the angle of the seat in relation to the ground and will have implications for your bike setup. An easy way to identify it is by measuring its angle against an imaginary straight reference line between the toe, front, and rear.

The angle of the seat tube does not change as much as the angle of the head tube, generally positioning it between 71 ° to 75 ° of angle, regardless of the discipline of your choice.

If the angle is too low (slack), on larger bikes, the seat tube will extend too far back, presenting the need for adjustment to the rider. So if you are long-armed or short-armed, this is worth keeping in mind.

The angle of the seat tube will affect where the saddle will be placed. It’s one of those standards that has never really changed. If you look at a small bike, it will have a raised seat tube angle, between 74 ° to 74.5 °. As the bike gets bigger, the angle of the seat tube tends to decrease, even down to 72.5 ° or 72 ° on larger bikes.

Changing the forward or backward position of the saddle can also influence the angle of the seat tube, making it lower or steeper. The saddle setback is measured horizontally from the tip of the saddle to the center of the mount.

That many times means that on a bigger bike, you can’t push the saddle far enough because the angle of the seat tube has gone too far back. Even with an in-line Seatpost. On smaller bikes, we tend to find it higher, usually, 74 °, which allows for better centering of the saddle on the seat post.

The seat tube angle is rarely considered a major factor when it comes to handling or riding style, although a lower seat tube angle can increase space, leaving more length for the rider to flex while riding.

Wheelbase and bike performance

The wheelbase is another indicator that helps determine the difference between fast handling and stability. Its measurement comes simply from a distance between the axles of both wheels.

It can be represented in two parts: rear center: from the center of the rear wheel axle to the center axle or Bottom Bracket. The front center, considered from the center of the Bottom Bracket to the center of the front wheel axle.

The wheelbase will have a direct effect on the handling of the bike. If the distance is short, the turning radius will be smaller. This translates into the possibility of making tighter changes of direction. On the other hand, if the distance is wide, the turning radius will be greater, giving stability to the bike when pedaling in a straight line, but the direction changes are wider.

If you are considering a bike for pedaling on uneven terrain, you will want that stability, but if you are going to race, you will want to be able to go through the curves more quickly.

Another element that will influence the stability of the bicycle is the drop or angle of the center axis or Bottom Bracket. This measurement refers to the difference in height between the bottom bracket and the horizontal line of the wheel axle. This standard has changed over time, with factors like larger tires playing a role.

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