Understanding the geometry of the bicycle

Over the past 100 years the design of the bicycle has evolved in a number of ways to meet the needs of a wide range of users and to improve the performance of the bicycle in different disciplines.

Today there are dozens of specialized bicycle types for different cycling modalities and their differences are related to their performance and qualities. Many of these differences are directly related to the geometry of their frame.

When it comes to buying a new bike, most riders understand the importance of finding the right size frame. However, many of us do not know the meaning of those tables shown to us by the seller or found on the Internet, with the measurements that define its geometry.

So let’s take it one step at a time and unravel the aspects about the geometry of the bike frame and how it influences our riding, performance and safety as cyclists.

How the size of a frame is defined

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

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

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

Although a good start to determine the right size of your bike is to know the size that fits your physiognomy, it is also important to know that due to the existing differences between geometries and even between brands, these may vary the fit between each user. That is why knowing what the different sizes in the geometry tables mean for each cycling style will be very helpful to get the bike that fits you best.

What is the Stack and Reach of a bicycle

The two fundamental elements in the geometry of the painting are what is known in English as: Stack and Reach. The Stack could be defined as the height of the painting and the Reach as its length or range.

Stack: is the vertical distance between the center of the bottom bracket and the center of the top end of the head tube.

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

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

For example, a road bike designed for distance, usually has a bigger Stack, which allows a more upright and relaxed posture. On the other hand, a road bike oriented to speed or track competition, will allow a more aggressive posture due to a lower Stack.

When we compare the Reach of a road bike for endurance cycling and another for sprints, the Reach of those designed for speed will generally have a longer reach than the road bikes for distance. This “stretches” the rider’s posture, creating a lower front profile, which improves the aerodynamics of the rider, achieving greater speed with the position acquired while riding the bike.

The term Reach is also sometimes used to describe a cyclist’s range. Note that it is different from frame range. Rider’s range refers to the rider specifically and is measured from the tip of the saddle, to the end of the steering stem or stem. It is independent of the frame range. This range can be manipulated by changing the position of the saddle -forwards or backwards- on the seatpost, and the length of the steering stem.

How the Steering Tube Angle and Length Affect

 

Steerer tube angle and length are two key measurements that have a direct influence on how you ride your bike.

Steerer tube angle refers to the perspective or inclination of the steerer tube to the ground and will commonly be described as a low angle (slack) and/or a high angle (steep). A low steering tube angle requires more effort to change 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 wider head tube angle than a road bike, which provides greater stability on difficult and technical trails.

The length of the steering tube is measured from the bottom of the tube to the top.

Bicycles 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 steeper position designed to reduce the rider’s aerodynamic profile.

What is Offset, Trail and Head Angle on the Fork

 

Fork Offset, Trail and Head Angle are intrinsically linked.

Offset: is the distance between the front wheel axle and the head tube axle.

Trail: is the distance between the point of contact of the tyre with the ground and where the “Head Angle” line touches the ground.

Head Angle: is the angle between the projection of the head tube axle and the vertical projection of the front wheel axle.

The smaller the Trail value of the fork, the faster the bike will respond. This is related to the angle of the head tube and the travel of the fork. The smaller the gap, the more agile the steering will be. On a larger trail, the turning radius will be larger and therefore less agile, but the bike will be more stable.

How the seat tube angle and length affect

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

The angle of the seat tube does not change as much as the angle of the head tube, generally positioning it between 71° and 75° 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 to adjust the rider. So, whether you are a long-arm or a short-arm rider, it is worth considering this.

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 of between 74° and 74.5°. As the bike gets bigger, the seat tube angle tends to decrease, even to 72.5° or 72° on larger bikes.

Changing the forward or backward position of the saddle can also influence the seat tube angle by making it lower or more inclined. Saddle back is measured horizontally from the tip of the saddle to the centre of the saddle post.

That often means that on a larger bike you cannot bring the saddle forward enough because the seat tube angle has moved back too far. Even with a seat post in line. On smaller bikes we tend to find it higher, at 74°, usually allowing the saddle to be centred better on the seat post.

The seat tube angle is rarely considered an important factor when it comes to handling or riding style, although a lower seat tube angle can increase the space, leaving more length for the rider’s bending when riding.

Wheelbase and bike performance

The wheelbase is another indicator that helps determine the difference between fast handling and stability.

It is measured simply by the wheelbase of both wheels.

It can be represented in two parts: rear centre: from the centre of the rear wheel axle to the centre 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 driving of the bicycle. If the distance is short, the turning radius will be smaller. This translates into the possibility of making tighter direction changes. On the other hand, if the distance is wide, the turning radius will be greater, giving the bike stability when pedaling in a straight line, but the direction changes are wider.

If you are considering a bike to pedal on uneven terrain, you will want that stability, but if you are going to race, you will want to be able to get through the corners more quickly.

What is the Bottom Bracket Drop

Another element that will influence the stability of the bicycle is the drop or angle of the centre line or Bottom Bracket. This measure 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 such as larger tires playing an important role.

The standard on a road bike used to be 70 mm. Partly to allow more clearance in the corners, although you can handle this with shorter cranks.

On road bikes, the fall of the Bottom Bracket will be lower to increase stability, because that way, its center of gravity is being lowered.

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