Finding & mounting to wall beams?

I read the tire rack mounting thread, and I'm just a little confused about which screws are best when trying to secure stuff to the wall beams in an enclosed trailer.
Thread-making screws seemed like the best idea, but do those secure the load, or do you then get machine screws? And if I read Mcmaster's site correctly, these screws need a pre-drilled hole or punch?
Also, once I figure out which screw to use, is there an easy way to figure out exactly where the beam is? They're offset just a bit from where you see them at the roof corner right? Can I trust that the plywood sides are fastened to the beams?
Thanks for the help.
Michael

With mine, the verical wall beams were hit by every screw in the plywood. You are correct that the viewable beams in the ceiling don't neccessarily correspond to the wall beams. You can use a stud finder to find the vertical beams, but since you have visible screws on the walls why not pull a screw and verify that it is in a stud and then drop a plumb bob from it?

Make sure your screws are not too long, it would suck to pierce through the trailer outer skin...no I didn't...but almost...I wanted the longest screw I could get in there and also leave a little wiggle room for the head of the screw sinking into the wood a little bit.

Sounds like a good application for nutserts...

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]