How do you pick the material style and thickness for a project?

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I am undertaking the building of a car hauler for my Jeep, 16' deck, last 2' being a slight dovetail so I can easily drive the jeep on without ramps. Jeep is 3500lbs, but using two 3500lb axles.  I am just stuck between 5x9 or 4x7.25 C channel. The 1.75lbs per foot adds up, both cost and weight. The 5" channel seems to only have a slight weight carrying advantage. So how do you pick one over the other? And at that you could add in some 2x4 box steel, probably 0.250 wall and make it a little more rigid and end up in the middle of the 5x9 and 4x7.25 weights, probably a little more expensive though, and for a trailer might have rust issues inside then. But how do you pick? The plans I am using reference using either channel.
Reply:I read the blue prints. That information is always on the prints. There are books, and charts to help with weights, and spans. Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Originally Posted by CEPI read the blue prints. That information is always on the prints. There are books, and charts to help with weights, and spans.
Reply:I have a small trailer (8k) that uses 2x5 square tube, 1/8 wall,   the 5 inch channel seems like it would indeed add up to a heavy trailer.   Often when picking supplies, I'll look at the available materials pile. You could also build it landscape trailer style with a tube frame upper and an angle iron lower frame.    Sounds like you have plans already,   The biggest loads are when towing and moving down the road, surging, bucking, bouncing, I'd go with the 4" channel  and find the spot where the front wheels sit,  go back about 2' from there and double up the 4" channel from there to the tongue, that will help the load carrying between the axles, anytime I've seen a trailer bend its on the exposed tongue section.   Post lots of pics!CEP is right too, an engineering book is someone else doing the hard work and breaking stuff in the lab.    The farm book is overloading stuff and breaking it in the field
Reply:I was a project superintendent for a very large marine construction company for years. We were always required to have an engineer on site. Most of our work was cut and dried. But if something didn’t look quite right our engineer would send off an RFI, so the design engineer could explain him self. If they came to the conclusion the design engineer’s way would not work, then we got a change order to fix it. That’s where the money just rolls in! Don’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Originally Posted by fast*stI have a small trailer (8k) that uses 2x5 square tube, 1/8 wall,   the 5 inch channel seems like it would indeed add up to a heavy trailer.   Often when picking supplies, I'll look at the available materials pile. You could also build it landscape trailer style with a tube frame upper and an angle iron lower frame.    Sounds like you have plans already,   The biggest loads are when towing and moving down the road, surging, bucking, bouncing, I'd go with the 4" channel  and find the spot where the front wheels sit,  go back about 2' from there and double up the 4" channel from there to the tongue, that will help the load carrying between the axles, anytime I've seen a trailer bend its on the exposed tongue section.   Post lots of pics!CEP is right too, an engineering book is someone else doing the hard work and breaking stuff in the lab.    The farm book is overloading stuff and breaking it in the field
Reply:Originally Posted by CEPI was a project superintendent for a very large marine construction company for years. We were always required to have an engineer on site. Most of our work was cut and dried. But if something didn’t look quite right our engineer would send off an RFI, so the design engineer could explain him self. If they came to the conclusion the design engineer’s way would not work, then we got a change order to fix it. That’s where the money just rolls in!
Reply:Only problem I had with engineers, was our own. But generally only after they had been out of school a few years. On one job I had the project engineer tell me I had a derrick anchored out wrong! Really? How many engineers do you know, know how to run anchors for a derrick? I never really had much dealing with the design engineers, our engineers dealt with them. I just ran the work in the field. Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Originally Posted by Jeff000I am just stuck between 5x9 or 4x7.25 C channel. The 1.75lbs per foot adds up, both cost and weight. The 5" channel seems to only have a slight weight carrying advantage.
Reply:Originally Posted by DSWSimplest way to do this stuff is to simply buy preengineered plans. Plenty of places sell them.
Reply:Originally Posted by Jeff000I'll have a ton of pictures for sure. If I doubled up the channel, would you suggest offsetting it a few inches? Or almost just stitch them together? I was actually thinking of using a composite tongue instead of just A frame and taking the tongue tube right back to the axles. With a 3/4 ton truck it was hard for me to just ignore weights and go 5k axles. But I have to keep in mind I might not always have a 3/4 ton.
Reply:Originally Posted by CEPI’ve seen some of these plans from Northern Tool. Any 4th grader could build a 20,000-pound gooseneck trailer using them! http://www.northerntool.com/shop/too...=Trailer+plans
Reply:Originally Posted by CEPI read the blue prints. That information is always on the prints. There are books, and charts to help with weights, and spans.