[Evodyne]

Skippy-I hadn't even thought about the reaction forces to plunging. You are absolutely right! Thanks for setting me straight.

Ger, can you clarify? I was envisioning a single piece of 80-20 with 3"x 6" cross section. Span would be over 80". I can understand going to two, an upper and a lower with each carrying one linear rail. The torque box would bridge the gap between them? If that's the case then wouldn't a pair of plates (front and back) that sandwiched the two 80-20 beams do the same, or am I missing your point?

Give the span I'm crossing I just don't want something that weighs too much.

Thanks for your input-I appreciate it!