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

SEAT ANGLE
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.
| 265 male students | P5 | % | P95 | % |
| Total leg length | 101.2 | 100 | 118,4 | 100 |
| Upper leg length | 58.6 | 57,91 | 67,8 | 57,26 |
| 89 female students | P5 | % | P95 | % |
| Total leg length | 95.8 | 100 | 111 | 100 |
| Upper leg length | 56.1 | 58,6 | 65 | 58,6 |
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.
| 974 male cyclists | 39 < 800 | 38 > 960 |
| Average seat angle own bicycle | 74.9 | 73.8 |
| Average optimal seat angle | 73.6 | 73.5 |
| 65 female cyclists | 4 < 700 | 6 > 860 |
| Average seat angle own bicycle | 74.6 | 73.5 |
| Average optimal seat angle | 73.5 | 73.5 |
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.
| FrameSize | Colnago | Merckx | Pinarello | Cannondale | Average |
| 48 | 75.5 | 76.3 | 74.5 | 75.0 | 75.3 |
| 50 | 75.2 | 76.6 | 74.5 | 74.5 | 75.2 |
| 52 | 74.4 | 75.0 | 74.0 | 74.0 | 74.4 |
| 54 | 74.3 | 74.4 | 73.8 | 74.0 | 74.1 |
| 56 | 73.6 | 74.0 | 73.5 | 73.5 | 73.7 |
| 60 | 73.2 | 73.0 | 73.0 | 73.5 | 73.1 |
| 62 | 73.4 | 72.8 | 73.0 | 73.0 | 73.1 |