rattleCAD 4.0 - FAQ

How does one parameter influence others?

At first try to do a step backwards and maybee also make a step to the side from your current understanding of bicycles and bicycle geometry.


the bicycle geometry is the abstraction of bike. It just defines the driving behaviour of the bike and the position of the saddle. that is all e.g: this work is mainly done in bikefit or an early concept phase of the bike and does not belong to any tubing. the frame geometry supports the concretisation of a bicycle. e.g: this work is mainly done by a framebuilder and is an answer to the requirement of the bikefitting

Based on one bicycle geometry you can build a lot of different frames with the same driving behaviour, but different headtube lengths and different angles of the top tubes. The frames will look different but supports the same geometry.


The most abstraction of a bicycle geometry looks like this:

In common, thats all you need to define the geometry of a bicycle. These parameters does not influence them selves.

The geometry of a bicycle can be seperated in 3 Areas:

  • Front
    • Handlebar position (in reference to the Bottombracket)
    • Stem angle
    • Stem length
    • Fork rake
    • Steerer/Headtube angle
    • Frontwheel radius
  • Center
    • Saddle position (in reference to the Bottombracket)
    • Seattube angle
    • Seatpost setback (perpendicular to Seattube)
    • Seatpost pivot offst (vertical)
    • Saddle height (height of the saddle itself)
  • Rear
    • Chainstay length (aligned)
    • Bottombracket depth (vertical)
    • Rearwheel radius

Why is there no Fork, Top- and Headtube in Bike Geometry?

Top- and Headtube are not part of the abstracted geometry of the bicycle. Defining the Fork and the Headtube starts concretise the bike frame base on the abstract geometry. The position of the Toptube depends on the length of the Headtube, while the angle of the Toptube depends on the length of the Seattube.
All this is done in the Bike Frame & Tubes Section.


... you may wonder how to configure the bicycles geometry in an other way you are more familiar with. Lets have a look to the following views ...

... the above view gives you the opportunity to handle stack and reach or the seat tube or the horizontal length of the top tube of a given frame.

... this view gives you the control on an other set of parameters.

... the last view gives you information about values that can not be changed directly in any way.

Step by step:

to demonstrate this, you can do the following, eg:

  • define a bicycle geometry you like and important for the show case with different headtube and seattube angles, maybee 74° for the seattube and 72° for the headtube. Stack & Reach in bicycle Geometry, as you like
  • switch to the frame details:
  • now change headtube length and toptube angle. What you can see is that Stack & Reach changes, but the position of the handlebar is kept, only the spacing between headset and stem is changed.
  • if you change the toptube angle to 0° and you change the headtube length, you will see that the toptube length changes depending on your headtube-length. The cause for this are the different angles of seattube and headtube ...