If you have determined that you need shorter cranks this must be taken into account in the further steps for calculating sitting position geometry. Shorter cranks require a higher saddle. (Add 1 cm for 160mm cranks and 1.8 cm for 152mm cranks.) A spacer of the corresponding thickness can be placed on the lower pedal.

The efficient use of energy when cycling is not only dependent on the correct saddle height but also on the position of the saddle relative to the bottom bracket: the “fore-aft position”. If you sit too far back from the bottom bracket you will tend to push the pedal more forward instead of using the weight of your body to push the pedal downward (centre of gravity over the bottom bracket).

The correct saddle position can be determined using a plumb line from the knee. The knee plumb line is an (imaginary) vertical line running from the surface of the kneecap of the forward leg ideally down to the base joint of the big toe and accordingly onto the pedal axle when the cranks are horizontal (Fig. 3). If the knee plumb line is too far forward, the saddle must be shifted further back in its supports. If the knee plumb line is too far back the saddle must be shifted further forward. If the saddle cannot be moved far enough in its supports then the frame can be built with a different seat tube angle.

For a very athletic riding style and sitting position, where the handlebar height is 10-13 cm (4-5”) lower than the saddle height, the knee plumb line should fall 2-3 cm (3/4-1”) in front of the pedal axle because this allows for a wider angle between the thigh and upper torso. A wider upper torso angle in the athletic position improves breathing and leg circulation.

If the seat tube angle is too shallow the muscles of the upper leg cannot work in equilibrium, and more demand will be placed on the hamstrings and gluteal muscles. Without the appropriate support, this situation will cause the pelvis to tip backwards, which can lead to a rounded back and problems in the cervical spine. 

Once the drive position (saddle height and fore-aft position) is correct, the adjustment of the sitting position can begin.