The seat angle is the decisive factor for the position of the saddle, the so-called saddle fore and aft placement..

According to Gonzales and Hull this saddle set back position ranks third in importance when it comes to efficiency. Just as with the crank length, the seat angle is a factor that cannot be altered during the process of cycling. Because the seat angle directly influences the efficiency, it is of vital importance to determine the seat angle before purchasing the bicycle. The seat angle is the angle that is made by the seat tube and an imaginary horizontal line.

The most striking example of an adapted seat angle is probably the so-called American triathlon position. Here, the angle tends towards 90 degrees, whereas a standard frame angle measures 72 to 75 degrees. The triathletes claim that for them this position is more comfortable. They can ride their bicycles more efficiently, they can exercise more power, and the shift from cycling to running proceeds more naturally; however, solid evidence for these assertions has not yet been provided.

What is also striking is that cyclists assume different positions on their saddles when riding different tracks. When riding downhill, they tend to shift towards the back of the saddle, whereas during climbing they move somewhat forward on their saddle. This might lead to the conclusion that changing positions on the saddle has a mechanical or metabolical advantage.

How to determine the optimal seat angle? When the saddle is adjusted at the correct height and pedal and crank are positioned horizontally, the perpendicular line should go from the knee cap straight through the pedal axle. Research shows that there is increased stability of the saddle position when this perpendicular is appr. 2 cm behind the knee cap.<

Determining the seat angle by only measuring the upper leg-length does not suffice. It should be noted that measuring of the seat angle should be conducted while the cyclist is positioned on the bicycle. The sitting position on the saddle is strictly individual, as it is influenced by the width of the pelvis and the shape of the saddle. The position on the saddle determines the position of the knee during the cycling movement; thus, the position on the saddle influences the seat angle.

The standard-seat angle-geometry, at which large frames are equipped with a shallow seat angle (72 degrees) and small frames are equipped with a shallow seat angle (75 degrees), presupposes that people with longer legs automatically have relatively longer upper legs than persons with shorter legs. The following diagrams illustrate that this presupposition is incorrect. Diagram 1 illustrates the relation between total leg length and upper leg length with students of the Technical University of Delft (Molenhoek, 1994). These statistics clearly show that there is no difference in the relation total leg length / upper leg length between people with longer legs and people with shorter legs.

Diagram 2 shows the seat angle measuring results of 1028 persons according to the bikefitting.com measuring system. In this group there were 39 males with an inner leg length of less than 800 millimeters and 39 with an inner leg length exceeding 960 millimeters. Also included in this group were 4 females with an inner leg length of less than 700 millimeters and 6 with an inner leg length exceeding 860 millimeters. Generally speaking, this correlates with categories p5 and p95 of diagram 1.

Diagram 3 shows the standard seat angle for different frame sizes of four frame constructors who (still?) take the incorrect supposition as described above as their starting point.