I build race car replicas for people all over the country. Mainly sprint cars and supermodifieds but some stock cars. I build all my cars out of styrene tube or rod. Of it is a space frame it will not sag with tube because of all the different pieces used in a frame like that. If it is a single tube frame then use styrene rod because it's solid. My cars end up weighing around 4 lbs when complete and the frames, and axles are all styrene. I even build slot cars, that really take a beating, out of styrene. When I first started this 5 years ago I did use brass tubing because that's what I was told that would hold up best. Well it does hold up but why go to all that trouble and just make a good frame, with good connections out of styrene. Another tip is to use a tooth pick and spread the CA glue around the connection like a weld. I sometimes use JB weld on parts with a lot of strain.