[Chris P.]

Clay - seriously - buy some balsa sticks (1/8" square will suffice), some thin cardboard (like manilla folders), and some CA glue (you can get all this at any hobby shop. Try different ideas of frame design with this because even if you are very very good at visualizing in 3D, you will miss some very important design points drawing in 2D. Not to mention the fact that it's fun too! You'll be surprised how much you learn and how many other ideas you'll get. You can easily set up stiffness tests (torsion, bending) with it too and compare design ideas.

Judging by these drawings, it looks like you are going to have a wheelbase over 110". That's getting pretty long for a sports car. It also means that the cross bracing you are worried about will become even more important. The 3D balsa modelling will also clear up issues like those braces above the engine. Those may have to be removeable. Not that removeable braces are an absolute no-no - they are fine, but one should try to design without using them and only resort to them if proven to be needed.

One quick pointer on cross bracing. An ideal tube frame has tubes only in compression / tension. (This rarely ever works out perfectly in real life - there will be some bending here and there in final designs but don't design that in if you don't have to.) Look at the horizontal tube below the doors as Bob pointed out above. They should connect to the ends of other tubes, not the middle of other tubes. The "V" shaped bracing is not required - you can typically get away with one diagonal brace from corner to corner if the tube ends all meet up properly. You're goal is LOAD TRANSFER. Look at the load path through the tubes in your mind. This is where the balsa model will really come in handy because there are loads not purely on plane with the paper.

I'm at the same stage you are with my design - I get just as much fun out of designing as I do building, modifying, and driving. Cheers!