/ic/ - Artwork/Critique

I've been working on building forms from imagination, and am gonna start doing more realistic vehicles next week after I've practiced this stuff a bit more.
Can anyone advise on how to make these forms show a bit more depth? I put a lot of work into constructing everything accurately in perspective, but then I only show the contour of the objects and they look pretty plain. These are some of the better ones where I put some contour lines on to help.
I haven't practiced values yet, but I'd like to get some critiques early on while I'm still just doing basic lines and shapes.