Postural assessment can be divided into two closely related directions: bike sizing and bike fitting. For a long time and even now, these two notions become intertwined. The easiest thing, so that we talk about the same thing, is to explain to you how I use these two terms.

Starting with the summary::

  • bike sizing is the method used to buy a new bike
  • bike fitting is done with your current bike.

As you will understand, if my explanations are clear enough, a good bike sizing should precede the bike fitting.

In summary first for those who are in a hurry:  bike sizing is the art and science to determine your ideal position in order not to go wrong with the purchase of a bike and its components. It is the position that you can maintain for a long time without loss of performance related to discomfort or pain.



At the very beginning, bike sizing was simply measuring the inseam to determine the correct frame size. It soon became clear that this measure was not enough and could be used at most to evaluate the seat height. Other anthropometric measures were then added, which is logical, the length of the legs not being proportionally related to the height of the trunk or to the length of the arms. Two factors are however neglected here: the flexibility of the cyclist and, at the same time, his ability to maintain the position imposed by the results provided by the formulas.


In the meantime, bike sizing has evolved a lot; it became dynamic. The tool used is a fit bike (or size bike) allowing, while the cyclist pedals, to change in all directions the position of the saddle and the handlebar. A saddle, handlebar and crank length change is instant so you can easily compare the effect. I use the F.I.S.T (Fit Institute Slowtwitch) protocol to make you travel through different positions and arrive at YOUR position. It is the one that corresponds to your type of practice, your morphology, your flexibility,  taking into account your possible pathologies and your traumatic history.

Do not make a mistake when buying your new bike. I, unfortunately, welcomed many cyclists who chose the bike according it’s look or being a victim of marketing or a proportion. They could not be comfortable and had to resell it at much less than the value. You do not have to adapt to the bike. You must have the possibility to buy the bike that suits you,  right from the start. A powerful database of more than 100 brands and more than 10,000 models will allow us to find your dream bike. As a result of this session, you will be given a list of models and your fit coordinates so that you can immediately order your bike with the right length of the stem, the height of spacers needed and the length of the cranks. Your bike shop will be grateful.

The limits of bike sizing:

  • The fit coordinates provided may differ from your final bike as the components (saddle, handlebar, levers, and pedals, …) are probably different. The ideal is to optimize your position once you after several rides with your new bike.
  • Only brands whose stack & reach measurements are provided in addition to the angles of the head tube and seat tube may be considered. These values are necessary for applied trigonometry.
  • Sizing works very well for road, triathlon and time trial bikes; unfortunately, it is not applicable to mountain bikes. Why? Comparing the geometries, you can see the enormous variety of angles of the head tube and the seat tube. Added to this are the various forms of handlebars mounted on MTBs. Adding the types of practice and the lack of information provided by some manufacturers, we quickly realize that even after defining your position, it is impossible to draw up a list of bikes list that correspond to it. On the other hand, if you have a model in mind and the information provided is sufficient, it is possible to calculate the possible positions and to simulate them on the fit bike.

Please note: The components are taken into account for the calculations, but the choice of the colors and the precise model according to your budget is not part of the bike sizing!

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Theoretically, a 1 m 80 tall cyclist could ride a frame between 54 to 58 (S-XL), depending on the brand. How is it possible? For two reasons:

  • You read it above, the morphological proportions. Whether you have long legs and a short trunk or the opposite means passing from one frame size to another.
  • This numerical or alphabetical value designating the frame size is measured differently, depending on the manufacturer. If you compare the geometries of the different brands and the nominal size of the frame, you will find that not all manufacturers measure the heights and lengths of their frames in the same way, which explains why you can end up with a bike that is too small or too big if you change your brand but you want to keep you 56. In the past, bicycles had a top tube that was horizontal and 56 in one brand was also a 56 in another. With the arrival of sloping frames, whose top tube was no longer horizontal, things changed. Unfortunately, the belief remained that the frame size is always measured in the same way. 


One of my teachers, Dan Empfield, proposed in 2003 a brand new nomenclature, which is the only true way to measure a frame. Two measurements are used to determine the height and length of a frame. The “stack” is the height of the top of the head tube relative to the center of the bottom bracket; the “reach” is the horizontal distance of this same point of he frame compared to the of the bottom bracket. Today, all the serious manufacturers provide these two values in their geometry chart.

What is the utility of these two values?

  • For the customer: the stack/reach ratio gives an indication of the type of the bike; It will go from values below 1.4 for a “race” oriented bike to values above 1.5 for an “endurance” oriented frame.
  • For the bike fitter: these values are necessary to be able to prescribe to the cyclist the bike that suits him with the appropriate components.

What has also changed a lot is the variety of geometries. Almost all manufacturers now offer geometries in their catalog ranging from race geometry to endurance geometry. The variety is currently so huge that,  with the exception of very tall or very small cyclists, no one needs a custom frame anymore. By cons, for cyclists who are sensitive to the responsiveness of their bike and want a bike that matches the best possible their type of practice, their weight and of course to their morphology, there are very good frame builders knowing to exploit wonderfully the properties of the used materials.


You could order your bike without knowing your exact coordinates, but it is risky, the integration of cables in the cockpit being more and more common. Road bikes often have a monobloc handlebar, it is advisable to know one’s handlebar coordinates HX and HY  ( handlebar reach and stack) to the center of the bottom bracket, in addition, SX and SY coordinated to the saddle relative to the same point.


For triathlon and time trial bikes, the indicated frame size does not tell much about the position in which it will allow you to ride. Even now, some will advise you to take a size below your road bike’s. It’s rather risky as a generalization! Without preliminary bike sizing, you run the risk of buying a bike that will not allow you to ride in your ideal position.

Before buying such a bike, it is imperative to know the coordinates of the arm pads: Pad X (Arm pad reach) and Pad Y (Arm pad stack). The goal of bike sizing is to determine them and then use them to draw a list of bikes to get there: it’s “bike prescription”. You will understand that the cost of such an approach is tiny compared to the safety of not being mistaken.



In summary, first of all for those in a hurry: bike fitting is the art and science of adapting your bike and its components to the best possible position allowed by your equipment.

Compared to bike sizing: the philosophy remains the same: find harmony between you and your bike and find the best balance between performance and comfort. The difference: you come with your bike and it is on your bike that the fitting will be done. Depending on the needs, it is not excluded that we also use fit-bike, which has the advantage to make change faster than on your bike some parameters such as the crank length, saddle, handlebar, aerobars.

Video analysis is used to:

  • help the slow the human eye
  • better visualize what we are talking about (slow motion, freeze frame, angle measurement, …)
  • better compare initial position and final position
  • save the data for later use

Depending on the chosen formula, pressure sensors on the seat allow:

  • to precisely locate and quantify pelvic pressure spots on the saddle
  • to assess the mobility and stability of the pelvis on the saddle
  • to appreciate if your saddle suits your morphology and your practice.

The limits of fitting:

  • Your bike: If a good sizing has been done, your fitting will also have every chance to be good. It will be necessary here mainly to take into account the different components used to proceed to the fine-tuning of the new bike. On the other hand if a sizing has not been done and your bike does not have a geometry (size, type) that suits you, there is a way to adapt to the best, possibly by changing one or the other component, but this can, unfortunately, be done at the expense of the handling of the bike.
  • You: The bike can be adapted to the best on the spot, sometimes in several steps. You have chosen a fitting because of certain complaints? It will be considered and will be one of the drivers of your session. However, it would not be honest to promise you that a fitting will spontaneously eliminate your health issues. You may need to work on your flexibility, specifically strengthen different muscles, consult a doctor or therapist.

Some precisions:

  • The components are taken into account for the calculations, but the choice of colors and precise model according to your budget is not part of bike sizing!
  • If new calculations are desired more than six months after the bike sizing, I invite you to send a new contact form and to select “bike prescription”.

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