If you don't already have one, create a Shortcut to the rFactor.exe (e.g. on the Desktop).
Rightclick on the Shortcut and select "Properties".
In the now opening Window, in the Field "Target", add "trace=3" at the end of the Field (you have to let a space between the existing Entry and trace=3).
It should look as already described then:
...rFactor.exe trace=3
or, if the Entry is covered with Brackets:
...rFactor.exe" trace=3
Without that, the game don't generates trace Files.
If you are done, start the game by using the Shortcut.
A trace File will be generated now, everytime you start the game and can be found in your rFactor Installation Subfolder ..\UserData\LOG (like i've already described before).