I clearly remember the day I went to buy my first bicycle, completely unaware of the cyclist terms, rules and world. I was so excited and so impatient. The seller, an old friend of mine, asked me: "Which size? A 55, maybe a 56?".
How is it "Which size?". Aren't bicycle frames all the same size? But of course, It wasn't this way.
In this article, I'll try to explain how to calculate the bicycle frame size for our needs.
Exactly like clothes, for bicycle frames, there are sizes too. And every size can satisfy a minor or major quantity of hights and cyclist's physical structures. The mid-tones can then satisfy every one of us, with every difference in height or build. So, before going to a shop to buy our first bicycle, we should know beforehand our frame size. We already gave some instructions on how to calculate the right bike frame size, but here we are focusing on the road bicycle frame.
It's not easy to understand the differences in the many bicycle frames, because the specifics may vary from company to company. It's generally based on one of the two main tubes which compose a bicycle (top or down) or on the saddle height (or, better, its crotch).
Let's take a better look:
- International sizes:
- It is indicated with the same letters as clothes: XS, S, M, L, XL, and it's related to the cyclist's height. The table here below can help you understand, considering that XS is up to 168 cm, S up to 173 cm, M up to 176 cm, L up to 179 cm, XL up to 183 cm and XXL over 185 cm.
- Horizontal length (top tube):
- In the picture below it is indicated with the letter A, measuring the top tube from the head tube to the projection of the seat tube on it. In traditional frames, like this one, the measure coincides with the top tube length.
- In “sloping” frames, since the top tube in inclined, this measure corresponds to the horizontal position, indicated with the letter L.
- Vertical tube (seat tube):
- In the figure, it is indicated with the letter B and it's usually measured from the center of the bottom bracket shell to the end of the seat tube, always indicated in centimeters.
As we can see, to consider the cyclist's height is certainly more immediate and easy to help us choosing the bicycle frame. However, since every bike frame company uses a different geometry, frame size can apply to different measures in a cyclist. More than a calculation, it is a mere suggestion, since these tables are pretty standard and don't consider a cyclist's build, or maybe their need of a particular seat, more comfortable for the back.
Mathematic calculation of the size
After considering the different methods that the companies consider building a frame, a question arises: which is the "perfect" bicycle frame for me? To have a more empiric answer, we can use a mathematic formula, which will give results to be compared with the company measures.
This calculation is a simple multiplication:
where C indicates the measure of the crotch (in the picture below you see how to calculate it), and 0.65 is a constant.
Let's try with a practical example: my personal size, after measuring the crotch is 85 cm. Applying the formula, the value will be 55. This value should be considered as "MY REAL SIZE", is referring to measure A and measure B, since usually, A and B are the same, in road bikes.
How to pick the right bike frame
Before buying a bicycle frame, you should take into account something else. As we already stated, every cyclist has a different build: with equal heights, the proportions between legs, arms and the rest of the body can sensibly vary. Once chosen the frame size, there are some changes that can be made on the handlebar, the saddle or the pedals, in order to guarantee the maximum comfort. So, the measurement seen above, being 100% reliable, becomes a mere basic indication for the research of the right bicycle frame size.
My advice is to try on the road your frame obtained with the indicated measurements, and see the impressions. The sellers usually help in this matter. If this isn't possible, there are some online size calculator that, despite don't take into account the different builds, can be useful in the hard search for the perfect bike frame size. In general, the choice of the frame size is not only given by mathematical factors but the complex total of factors making every cyclist different from one another. You obviously have to have a general idea - geometry and measures help us - but it's the field test that could be crucial and give us the definitive answer.
The eye of a good biomechanic could be more valid than a table. Moreover, the choice of the bike frame and its size could be dictated by the type of cycling you are going to practice. For example, a stiffer frame is more suitable for competitions even on shorter distances, while for more comfortable and longer rides, you'll have to consider other types of frames.
A special thanks for the collaboration to Fausto Filippucci, Federal collaborator and mechanic from 1988 to 2010 for the Olympic Italian team in Barcelona 1992 (Gold medal by Giovanni Lombardi in points road bike) and Athens 2004.
I'm from Piedmont and I'm 30 years old, I have been living and working in Lombardy for a few years. After a start without any competence in this field, I then approached the bicycle world more and more. Today I can call myself a bicycle traveller and videomaker who would never ever stop. I ride my bike trying to understand why it is so beautiful, rich, therapeutic. And every time I try, I do not understand it. So I must leave again...
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