rattleCAD 4.0 - Design Principle - Geometry

What is it for?

This page describes the Design Principle of rattleCAD 4.0, especially the definition of the geometry of your bike.

Content of this page


From the predecessing Bike Fitting you get the position of Saddle (Nose) and HandleBar in reference to the BottomBracket.
As a next step you have to define a Base Geometry that defines the main axis and centerlines and at least the driving behaviour of your bike.
The frame of your bike, especially the configuration of all of the tubes will happen in the next Tab: Bike Frame & Tubes. Please consider: the position, angle and length of the TopTube and DownTube is not part of Bike Geometry.

bikeGeometry! bikeGeometry! bikeGeometry!

Position in the rattleCAD Design Process

Geometry - GUI

To refine the Geometry of your bicycle rattleCAD provides the Bike Geometry tab:

bikeGeometry! bikeGeometry!

Geometry Design Principle: Outside-In Method

rattleCAD 4.0 follows the Outside-In Method:
The position of Saddle and HandleBar gives the Outer Boundary.
Every parameter changes the results inside this boundary.

This method represents the Main Design Principle of rattleCAD 4.0

Primary Parameters of the Geometry Model

This minimal set of parameters allows you to define your Bicycle Geometry Skeleton base on the position of Saddle and HandleBar.

bikeGeometry! bikeGeometry!

Summary Dimensions

This additional resulting dimensions lets you check your resulting geometry.

bikeGeometry! bikeGeometry!

Resulting or Inverse Geometry Parameters

Some of this resulting dimensions can be used as an additional Input Parameter.

bikeGeometry! bikeGeometry!

Parameters and Dimensions together

You can show all these paramters and dimensions at once, of course

bikeGeometry! bikeGeometry!

Geometry Design Exception: Inside-Out Method

    After the chapter above, you may ask:

  • Where can I set the Frame Height or SeatTube Length and the Length of the TopTube, because I want to copy an existing Frame.

    You can do this in rattleCAD, of course:

  • But this will not follow the main Outsie-In Design Principle.
  • you have to handle some specific behaviour yourself.

Secondary Parameters

Together with the Primary and Resulting Parameters from above you can define your Bicycle Frame in the classic way with the Secondary Parameters:

bikeGeometry! bikeGeometry!

Limitations of the Inside-Out Approach

While working in this Inside-Out Design Approach you may expect that all other parameters also work from inside the geometry towards the position of Saddle and HandleBar.
But, as you can see, some parameters, like the Stem are not part of the Secondary Parameter Set, what includes that they do not work from inside to outside but still follow the Outside-In Design Principle.

bikeGeometry! bikeGeometry!

Secondary Parameters follow the Inside-Out          ... and Primary Parameters follow the Outside-In Design Principle


You wants to change TopTube Length and Stem Length.

Not OK:

  1. change TopTube Length
  2. change Stem Length


  1. change Stem Length
  2. change TopTube Length

Why is this:

  • changing the TopTube Length: (Inside-Out Principle)
    • keeps the position of the SeatTube
    • moves the position of the steering axle of HeadTube
    • moves the position of the HandleBar
  • changing the Stem Length:(Outside-In Principle)
    • keeps the position of the HandleBar,
    • moves the position of the steering axle of HeadTube
    • and change the Length of the TopTube

All Options together in one View

bikeGeometry! bikeGeometry!