Index:
◦ Choosing your first bike
◦ Frame geometry: Top Tube, Reach and Stack
◦ Other indicators
◦ Which discipline are you heading towards?
◦ Wrong size bike? How to tell!
You have decided what kind of bike you want - mtb, road or gravel - but are you ready to buy it?
If you have evaluated several brands and feel bewildered in front of the proposed size guides, don't worry, you are not the only one.
Some brands use numerical wording while others use letters, these indicate the frame size, but what do they mean and how do you choose the first time if you don't have a frame to compare with? Don't panic, in this article we explain how to choose the right size bike!
Before buying a bike, especially if it is your first time or if you do not know the measurements of the frame geometry, we recommend that you consult a specialist shop. You can try out different bicycles to find the one that is most comfortable and suited to your needs. The professionals will guide you in choosing the right model, taking into account your build, your riding style, your needs and your budget.
Many shops also offer a Bike Sizing service and the possibility of a biomechanical examination, a test on a simulator that allows you to get the right posture in the saddle based on your measurements and preferences. This will help you find the most comfortable and suitable position for your physical conformation and prevent and avoid problems caused by an incorrect posture. More on this in this article.
The second tip for best orientation is to measure your height and your inseam, as many brands use these measurements as a starting point for suggesting the correct size. However, height is only an indicative value: physiological build and structure also influence the ideal size.
If, following the brands' indications, you are in between two sizes, consider that a smaller frame is suitable for a dynamic and fast riding style, while a larger frame offers more comfort when pedalling.
Frame geometries and measurements can vary between manufacturers and brands. There is no standardisation, because each brand uses its own geometries and measurement methods.
As Fabio explains, the three main measurements to take into account are:
The Top Tube, the Reach and the Stack.
The Top Tube or Horizontal Tube
The horizontal distance between the centre of the head tube and the centre of the seat tube or seat tube. It is important to note that not all brands measure the virtual length of the horizontal tube in the same way. For example, Colnago records the horizontal distance from the head tube to a vertical projected from the top of the seat tube, without taking into account additional backward projections of the seatpost.
The top tube measurement allows you to understand how the body is positioned in relation to the handlebars and the centre of the bike. If the top tube is too long, you risk leaning too far towards the handlebars and your riding position will probably be more aggressive, which could be uncomfortable on longer rides.
The Reach
It is the virtual line that is calculated from the projection of the bottom bracket to the centre of the head tube.
A low Reach value identifies a shorter, more reactive and nervous bike. A high Reach will tend to unbalance you forward, loading the front wheel more. If the Reach is too long, you run the risk of leaning too far towards the handlebars, which may cause discomfort or negatively affect handling.
The Stack
This is the virtual line that stretches between the height of the head tube and the virtual line from the bottom bracket. It affects the saddle - handlebar height difference, so a high value of this will mean a higher handlebar and therefore a lower height difference, if you are looking for more or less aerodynamics this metric will make a difference!
A bike with lower Reach and Stack measurements tends to offer a more collected position, favouring responsiveness especially in technical sections and acceleration. Conversely, a bike with a higher Reach and Stack promotes a more upright position, increasing stability but slightly compromising responsiveness.
There are, however, other indicators that allow us to better understand the dimensions, angles and thus the geometry in the frame, which we must take into account when choosing a new bike and which are mentioned in the brands' size guide tables.
Let's look at some of them:
• Wheelbase: is the distance between the front and rear axles. The greater the wheelbase, the more stable the bike.
• Standover height: This is measured from the centre point of the top tube to the ground. This measurement is crucial when assessing the ideal bike size. Indeed, it is important to be able to touch the ground with both feet when out of the saddle.
• Head tube angle: Fundamental to assessing the stability of a bicycle, this represents the angle of the head tube in relation to the ground. The more the angle is closed, the more responsive the bike will be, on the contrary with an open angle the handling increases.
Please note: open angle conventionally refers to a smaller head angle, i.e. an angle that positions the fork more horizontally, typical of gravity MTBs > MTBs generally have head angle values between 63° for downhill-enduros and 68° for XCs.
On the other hand, a closed angle is defined as a wider head angle that positions the fork more vertically, as in the case of racing bikes > with values ranging from 70° to 75°, also depending on frame size.
• Angle of the seat tube: i.e. the angle of the seat tube to the ground; when this has a high value, the bike tends to be more snappy and nervous, and vice versa.
• Bottom bracket offset: An essential parameter for assessing the stability of a bicycle. The bottom bracket offset is the distance between the horizontal line of the bottom bracket and the horizontal line joining the wheel hubs.
A higher offset indicates that the bike has a lower centre of gravity.
• Rear stay: This is measured from the centre of the bottom bracket to the centre of the rear wheel dropout. A short rear stay makes the bike more agile and lively, but also less stable and comfortable. Conversely, a long rear stay offers more comfort and stability, but sacrifices responsiveness.
The geometry of the frame varies according to the chosen discipline. Here are the main characteristics of the frame for each discipline:
Do you want to be fast and aerodynamic?
Choose an ‘aero’ racing bike with aggressive geometry. Aero bikes are designed to be more aerodynamic. The head tube of aero bikes is very short, usually less than 140 mm, to create a low front profile that reduces air resistance.
In addition, these bikes have a larger frame reach than endurance bikes, allowing the rider to assume a more elongated position to minimize air friction. The top tube is generally straight, unlike other models, which reduces the distance between the seat tube and the saddle, increasing the bike's rigidity and further reducing the frontal profile.
Do you plan to ride long distances and prefer to be more comfortable in the saddle?
Look for endurance bikes. These have different geometries to emphasise comfort rather than aerodynamics, as they are designed to tackle various types of terrain and for prolonged periods of time. On these bikes, the rider assumes a more upright position thanks to a longer head tube and wheelbase, a reduced reach, and an increased stack.
This configuration allows a less aggressive and more comfortable ride. In addition, a wider handlebar, usually 44 cm or 46 cm, helps to improve the bike's stability.
Are you thinking of a gravel bike?
The geometry of gravel bikes is still different. The steering is raised to allow a more relaxed riding position, and the top tube is generally more inclined than on road bikes. The steering angle is more ‘open’, meaning that the wheel hub is more forward, favouring more generous wheels and providing a smoother and less nervous ride, as well as improved handling and precision on uneven terrain. A longer wheelbase ensures greater stability, which is achieved thanks to the longer seatstays.
However, this configuration results in slower acceleration and restart response.
When you get on the saddle, many variables come into play: the rider's constitution and physical proportions, your riding style, the distances you want to cover and your level of competitiveness; so once you have chosen your size and tried the bike out, it is advisable to consult a biomechanic, as mentioned, to find the best position.
We talked about the importance of the Retül Bike Fit in this article.
Below you will find a few things to look out for to see if you have chosen the right size bike or not. 👇🏻
Your posture while pedalling is a first indicator, if your torso is bent too far forward and your arms are extended, your bike may be too big. The correct leg position is when the angle formed by the thigh and the extension of the shinbone is between 35° and 40° and the position of the foot should be parallel to the ground. If to achieve this position you have to maximize the length of the seatpost you may need a larger size, the same applies if you pedal while touching the front wheel.
In conclusion, remember that choosing the right bike is not just a question of size, but of finding the right balance between comfort and performance.
If you already have a bike but are thinking of changing it, start with the size of the Top Tube and then assess whether the other sizes and geometries are suitable for you.
However, the advice is always to contact a professional bike fitting shop after the purchase to adjust the individual areas of the bike.
If you are looking for such shop, please contact Mooveo , among the many services you can try the Retül Bike Fit.
If you don't have a bike to compare, follow these tips, keep in mind the basic measurements such as Top Tube, Reach, and Stack.
With the right information and a little bit of attention, you will be able to find the perfect bike for your needs, which will offer you not only optimal performance, but also an enjoyable and safe riding experience.
Happy cycling!
