twimTurbo,
you need to persevere with those rectangles. Since a rectangle is a surface which in Sketchup can be made in to a 3D object by use of the PushPull tool.
I gather you have drawn the chassis in 2D with a collection of lines, so as you have a representation of fig 4.2 in your Sketchup model. Is that correct?
Think more about planes (surfaces) rather than lines.
Strangely enough lines on their own aren't much good in Sketchup for modelling 3D objects. Having used the more 'engineering' type CAD packages, I was drawn in (no pun intended) to using Sketchup with this approach of creating drawings with lines. But really you want to be drawing rectangles (to draw a rectangle and before releasing the mouse button type the size (e.g. 1000mm,200mm) and then release, this will give you a rectangle to say representing the underside of a bottom rail) to create the components of the chassis. Create all the reactangles required to create the frame. Arrange them togther, the angled one are a bit fun to get lined up and then use the PushPull tool to 'lift' each reactangle by 25mm. Thus you will get a 3D model of the bottom chassis rails.
I know it's frustrating but getting the hang of this will get you a long long way on getting to grips with Sketchup and it being useful. As I said before, I had two or three attempts at getting Sketchup to do what I wanted, but a bit of determination and a study of the tutorials will help.
The best tip is to stop thinking like a draughtsman would, i.e. as one would when drawing with a pencil ruler and drawing board, and go along with the surfaces approach used by Sketchup.
Don't give up.
