rattleCAD 4.0 - Basic Concepts

Impact of Seatpost Setback on the Saddle Position

Seatpost and saddle are positioned seperately in different ways:

  • SeatPost is positioned by SeatTube-Angle
  • Saddle is positioned by Saddle-Height and horizontal Offset from Bottom-Bracket


The geometry of the seatpost is more complex as some may expect or as manufacturers specifiy it.
When you take a look on the different seatpost models, you will see that they provide several different solutions to clamp the saddle.
Further on the position of the saddle clamp changes depending on the seattube angle.
In the most cases the manufacturer provides a seatpost setback as the only parameter to geometricaly describe its products.

  • But how does the geometry of these clamping concepts impacts the position of the saddle on the bike?
  • And is the given seatpost setback parameter sufficient?

Real world examples

Lets have a look on real world examples (graphics and dimemsion are aproximetly derived from web-resources).
For this analysis the seattube angle is set to 73° and the saddle rails are horizontal.

Seatposts with different concepts and geometries ... open graphics in full size


Use provided graphics of different manufacturers and compare concepts and given specification
Try to find a more specific specification of seatposts that give as more information on how they impact on the position of the saddle on a bike.

If possible, find one single value, something like an effective seatpost setback.

This effective seatpost setback will be defined like following:


The effective seatpost setback is

  • the horizontal distance of
  • the mounting position of the saddle (center of the saddle clamp) to
  • the horizontal intersection with the seattube axis.

At least the effective seatpost setback is a result of the given geometry of the seatpost and the angle of the seattube.

Results of this comparison

The analysis of the geomtrical concept of the given seatposts brougt us to the following results. The read an blue lines and arcs shows the geometrical interdependencies with their specific expressions like shown in the images below.
The red lines shows the lengths in the analysed situations, whild the light blue arcs show the pivoting behaviour of the saddle clamp and the darkblue lines shows the clamp length of the specific models. The dimensions give us information about the specific expressions of these lengths.

Seatposts with different concepts and geometries ... open graphics in full size

To get a clearer view on these results the following view shows only the geometrical concepts of the analysed seatposts.

Seatposts with different concepts and geometries ... open graphics in full size

Next to the understanding of the geometrical concepts of these different models lets take a look on the values delivered by this analysis. The accuracy of the values is of course to high for this type of analysis but should make it easier to understand the results and to reproduce this analysis.

Ritchey, 2-Bolt 3T, zero25 DEDA, superzero
specified Setback 25.00 mm 25.00 mm 21.00 mm
effective Setback 16.09 mm 21.50 mm 21.96 mm

* SeatTube-Angle: 73°, Saddle-Rail: horizontal

Conclusion for rattleCADs seatpost representation

All the seatposts available on the market provides a mechanism to move the saddle in a horizontal direction and to pivot the saddle in a given range. For an exact computation of the affect of the different seatpost geometries and clamping concepts the provided seatpost-setback is not sufficient. To compute the pivoting effect the distance between the saddle clamp and the pivoting position of the saddle clamp is also required.

The relvant parameters for the Bike Geometry in rattleCAD

The image below shows the relevant parameters to define the vertical and horizontal position of the saddle.

horizontal and vertical positioning of the saddle in rattleCAD

While the orange dimension (205) is responsible for the horizontal position, the vertical positioning is much more complex and requires the following variables:

  • Seatpost
    • Seatpost Setback
      • perpendicular to the seatpost axis (23.9)
    • Seatpost Pivot Offset
      • vertical dimension (29.4)
      • is a radius and defines the center of a curve (circle) the saddle clamp is pivoting about
  • Saddle
    • Saddle Height
      • vertical dimension (40)
      • the heigth from the rail to top surface of the saddle

Representation ot the analysed seatposts in rattleCAD

The analysied Seatposts represented in the rattleCAD Geometry sketcher:

Seatposts represented in the rattleCADGeometry sketcher ... open graphics in full size


... so what is is the conclusion beside the concept rattleCAD works with different seatposts?

While rattleCAD requires Seatpost Setback and Seatpost Pivot Offset to represent the geometry of the seattube, all the discussed values seems to be unimportant in real world.

While the manufactuers layout of the seatposts, especially specified by a single seatpost setback value, that differs so much to the effective seatpost setback, the position of most front edge of the saddle clamp is still close to the horizontal intersection with the seattube axis.

This brings us to a further result of this analysis, which is the front clamp Offset. It defines the most front position of the saddle clamp. Together with the length of the saddle clamp, this values limits moving the saddle back- or forwards.

Offset of the front edge and length of the saddle clamp ... open graphics in full size

Ritchey, 2-Bolt 3T, zero25 DEDA, superzero
front clamp Offset -3.92 mm -3.74 mm -2.79 mm
saddle clamp lenght 38.70 mm 38.29 mm 48.80 mm

From the perspective the selection of a specific seatpost does not have a big impact to the geometry of bike. In other words the exchange of seatpost on an existing bike has only less impact on the positioning of the saddle. So different seatpost setbacks have less impact to the positioning of a saddle as manufacturers want us to believe.

Although the impact of seatposts setback specification seems very less, and the values like the pivot offset are not given, the implementation of seatposts in rattleCAD gives you the opportunity to deal with this parameters and to do your own analysis.