Echoing what Marcus (horizenjob) said, the hard mill scale on hot rolled must be removed before welding. It is often a real bitch to get it off. I used some hot rolled plate in some areas and it was a lot of extra work. Use cold rolled. You'll save in time all that you spend on the material itself.
I went 1x1-1/2 on the main rails, which is over-built. However, I didn't have any good analysis tools (like FEA) at the time, I'm an amateur welder, and I have a high-torque V6, equal to a Chevy 305 or Rover V8, as a power plant, which I plan to take to the 300 HP area. The extra size and weight weight of the tubes seemed like an good insurance policy to me. If you did an analysis, 1-1/4 square would likely be fine too. There's a couple of V8 powered Locosts that have used 1-1/4 square tubing for the main rails and the owners seem fine with the result.
Cheers,
_________________ Damn! That front slip angle is way too large and the Ackerman is just a muddle.Build Log: viewtopic.php?f=35&t=5886
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