Seat tube angle
Top tube length and reach always have to be considered in combination with seat post length and seat post angle in the development of bike geometry. With the introduction of seat posts that bend backwards, it is now possible to consider these two dimensions independently from one another and coordinate them to the optimum degree. The frame geometry has what is known as a virtual saddle height. This is defined in length by the mean saddle height in relation to the height range of the rider for a specific frame size and an effective seat post angle. The real seat post angle can be used to influence the horizontal top tube length while maintaining a constant reach.
A: Seat tube angle
Example: Due to the individual adjustment of saddle height, the real saddle height generally deviates from the mean saddle height.
B: Effective seat tube angle
Depending on whether the saddle height is adjusted for a tall or a short rider, the horizontal top tube length increases or decreases in length, without affecting the reach in any way. During descents, therefore, the rider is always sitting in the optimum position on the bike and above the bottom bracket. In the case of tall riders, the backward-leaning seat post may result in the rider’s centre of gravity moving too close to the rear wheel axle, which will reduce the climbing ability of the bike. To better describe this effect, we use the term effecitve seat tube angle. This measurement is taken between the bottom bracket / motor axle and the middle of the saddle.