by KCStudly » Sun Jul 12, 2020 11:55 pm
I don't understand why everyone keeps saying that FG is brittle. They clearly are just propagating the experience of other people under different circumstances.
While trying to decide between PMF and epoxy/glass weave for myself, I tested. I actually tried it for myself. Now maybe a thick slab of laminated FG cure will be so strong that when you eventually get it to fail it might snap, but that's just not how a couple of thin layers of 6oz cloth works when laminated lean over extruded foam.
After years of curing, to this day I can still fold my test coupon into a U-shape (glass to the outside of the bend)... that's 2 plys of 6oz weave over 3/4 inch thk foam board (though with the weave not filled, primed or painted, and only laminated on one side)... and it comes right back. No cracking, no delamination, nothing.
Whack the face of the panel against the pointy corner of the bench fairly hard and, what do you know, it didn't shatter, splinter or crack. In fact I couldn't find a mark.
Not only that, but 8oz canvas glued with TB2 and painted with 1 coat of latex on the exact same size and shape of foam weighed exactly the same as 2 plys of 6oz glass and epoxy.
gregkn 73 is right, the strength of that FG topper tub comes from the perimeter walls of the tub acting as vertical stiffening, and the ribs formed by the raised channels where the metal channels recess. Look how tall those tub walls are; very deep resulting in a strong moment of inertia.
One last thought, if the RTT OEMs could get away with making their products lighter and easier to install, don't you think they would? All engineering is a compromise. If you want it lighter you either have to sacrifice durability, longevity, or cost of materials and manufacturing. If you go too light it could fail altogether. Just saying.
Using vertical ribs underneath the RTT floor longitudinally in the direction of travel would not increase the wind resistance so dramatically, but when properly filleted and glassed into the underside of the floor would add a bunch of stiffness. Could even be inverted like the ribs in the glass RTT, then subtract these bumps from the foam layer. Think of it like the ribs in a pickup truck bed or the roof in an old large American station wagon.
KC
My Build:
The Poet Creek Express Hybrid Foamie
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"
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