Seat Tube Angle
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The seat tube angle is the included angle between the seat tube and the top tube (assuming that the top tube is level, or the angle between the rear of the seat tube and the ground plane. Seat tube angles are most often in the range of 72 - 74 degrees on modern bicycles.

The seat tube angle effectively fixes the saddle in the horizontal plane relative to the bottom bracket and crank axis. This in turn fixes the fore and aft placement of the knee and influences the hip closure angle and maximum spin rate. While I believe this to be secondary to the effect on spin rate, the seat tube angle also has an influence on the ridersí center of gravity.

A shallow seat tube angle displaces the hips rearward and like longer cranks, tends to restrict the crank RPMís due to the larger angular change of the femur and results in the need for a shorter top tube length due to a more upright pelvis and lower back.

A steeper seat tube angle moves the hips forward and allows higher crank speeds due to the reduction in angular change of the femur. A longer top tube length can be used because the pelvis and lower back is tilted further forward.

Slight increases in seat tube angle can be used to compensate for excessive crank length to allow a faster rate of spin. The actual range of motion will still be the limiting factor as far as spin goes and in no case should the forward displacement of the saddle be greater than the difference between the ideal crank length and the actual length being used.

      Things to watch out for!

In an effort to produce smaller bikes with shorter top tubes, some manufacturers have increased the seat tube angle. While on paper, this does provide a shorter top tube, in reality, the saddle will need to be moved rearward to place the rider in the correct position, thus nullifying any top tube reduction. 

Top Tube Length



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Last modified: July 09, 2014