Geometry of the bicycle: Height and length of the frame. Stack and reach

Classical bicycle geometry

The shape of the bike has evolved remarkably over time. Although the most common geometry in recent decades has been formed by two adjacent triangles joined by the seat tube.

For many years a characteristic common to all the frames was the horizontality of the tube that joins the seat and the handlebars. The length of that tube largely determined the size of the bike. At present, due to the incorporation of new materials and manufacturing procedures, the tendency is for this tube, which for years remained horizontal, now appears inclined ( sloping ). With this modification, the seat tube is shortened, which increases the rigidity (less flexibility) of the frame by making the rear triangle smaller and somewhat reduces the materials used and, therefore, the overall weight. In addition, it allows the number of sizes offered by the manufacturer to be reduced since the seat post will allow a greater range of adjustment. On the other hand, the more compact and resistant frames are more reactive and facilitate the handling of the bicycle, although stability on descents and in high-speed corners is somewhat penalized. The great drawback of this modification is that it makes it very difficult to choose the size of the bicycle and the comparison between the dimensions of different models and between different manufacturers.

Stack y reach

To solve this problem, we can resort to two measurements that will perfectly define the geometry of a frame in terms of its size, these are the height ( stack ) and the length or reach ( reach ) and will have as a point of reference the bottom bracket axis and the top of the steerer tube (measured along its axis).

The height or stack of a frame will be defined as the vertical distance between the bottom bracket axis and the top of the head tube.

The length, scope or reachwill be the horizontal distance between the bottom bracket axis and the top of the head tube.



Since we are not going to find any exactly horizontal or vertical element in the box, both will be imaginary lines, so we will need to use a tool, such as a spirit level, to be able to determine them.

In future articles we will develop the subject of bicycle sizes and the importance of choosing them correctly.