John E Davies
Adventurer
You need flat (horizontal) mounting pads to rest the bars on and to attach the u bolts through. You don't need to make them out of channel - it just makes it simpler to use the same material as the uprights. You could use angle just as easily.
Use your judgement on the channel sizing - you get the rigidity from the U shape, not so much from the width of the "ears". I wouldn't go any narrower than 2 inches in the main dimension, since you need the room to drill holes for the u-bolts.
You will need a hacksaw anyway to cut the pieces to length - you can easily trim any excess material off the ears to allow the upright to drop down into its slot. It would even be better to use the wider size and trim to fit, since that will make a stronger attachment. The downward load will be carried straight from the upright to the frame and not through the bolts. Does that make sense?
As far as using the square tubing - it will be a little stronger, but also heavier and more expensive per foot. Aluminum is usually priced by weight. It will also require longer bolts to go through both sides, and you run the risk of squashing it if you torque down the hardware too much. My trailer is made with 2x2 square tubing and I like it a lot, but channel would have worked just as well. If you don't want to install round cross bars, the square tubing would work well as a replacement, and you could just bolt it straight to the pads without u-bolts. But then you can't easily mount factory boxes and racks......
IMHO, if you are going to go to the trouble of building a rack system, it should be able to accept standard Yakima or Thule mounting parts, depending on your preference. I am a big Yakima fan.
It's a lot harder to explain in words than through a picture......
Good luck.
John Davies
Spokane WA USA
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