Hi all. I need help with something, but I need to "set it up" in our heads first...
Envision if you will four rectangles, each made from the same piece of plywood (3/4").
Actually they're more 4 sided near rectangular structures (quadrilaterals) in my case that are almost rectangles, but for now, and for purposes of simplifying things please just think of them as rectangles, roughly the same size.
Imagine I want to put these rectangles together at some small height to make a larger near rectangular structure. By corollary, we can all agree then that each rectangle shares 2 of its adjacent sides with 2 of the other rectangles.
Why didn't I just make 1 big rectangle?....Answer: I had to work around some obstacles the 4 cut pieces of plywood have to be cut around...more on this below. If I may make an analogy, imagine two square columns that fall smack dab in the middle of my project. Plunge cutting out holes in the project to accommodate the columns will not help, as these columns run--in my analogy--floor to ceiling uninterrupted: there's no way to get above or below them.
To keep this entire structure elevated I'm able to support it from below in addition to the following. First a diagram:
Using a 2 dimensional graph might help.
The piece of wood in Quadrant I is bolted down in the Northeast, Quadrant II in the Northwest, Quadrant III in the SouthWest, and Quadrant IV in the SouthEast. Each quadrants get at least one bolt.
(Ok, about now you're asking: "what's this guy smoking?" Please bare with me, it will all make sense it a moment.
I'm looking for "under the plywood" methods of each piece supporting its 2 adjacent neighbors so as to be firmly resistant to up or down movement.
….I didn't come here completely unprepared. I've been studying the mechanisms of Draw (not drop) leaf tables, which tend to have under the table supports only coming out of one of their 4 sides--to the extent that you talking in such terms is helpful. I'd need them coming out of 2 adjacent sides on each piece likely.
Now for the real world application. I think things will make more sense now.
Here is a purchasable, almost rectangular cargo cover for a Jeep Wrangler:
Notice how my two aforementioned columns are the vehicles "roll" (technically Sport) bars.
Notice distally in the picture a white vertical structure. That's part of the (interior of the ) hard top. Notice 3 black "dots" on the horizontal white lip near the bottom of the hard top. Those are 3 M8 size bolts (there are 3 more on the driver's side) that help keep the top on the vehicle.
Notice how this manufacturer's $699 cargo cover is made of metal (I think, or at least don't try putting weight on it then.) That allows him strength and thinness, as he gets around the problems I'm going to talk about by simply putting the cover on BELOW the hard top. That can make it hard to get off if you need to transport some object of height, like a plant, that it's not easy to store on its side but won't fit below or above the cover.
I won't have that option to go under the hard top, working with 3/4" plywood that is. It would look bad, especially from the exterior; rain would get in.
In the bottom left corner of the picture there appears to be some bolt in the cargo cover. Ignore it.
Please observe the window in the side of the hard top. It is slightly inset. That means that if I want my wood cover to abut the interior walls of the hard top, I've got to slide my project in below the point where the bottom of the window starts jutting ever so slightly into the interior of the vehicle. Lowering the wood down like it was some door on the back of truck, that when angled down becomes the ramp to load the truck with just won't work. It will hit the window insert. I originally designed my wood cover that way, and it was fine when I had the vehicle's soft top on, whose windows do not inset. The hard top's windows, as shown here do inset.
Finally, notice that the hard top makes a slight turn towards the center of the vehicle both in its front and back, so forget about sliding the wood in from the front to back, or back to front.
Okay, confession time: since the vehicle narrows slightly back to front, put my four pieces together and you actually get an isosceles trapezoid, not that it changes much. My 4 pieces are like rectangles that you started to make into an isosceles trapezoid, that you changed your mind about after cutting one side. There's no name for that other than a quadrilateral.
So, there's lots of obstacles that I think necessitate the four pieces if I want a cover to fully encapsulate the area. I want to put weight on this. I don't want it to make that much noise, or bounce up and down.
IMG_2520 shows the support structure below: 3 pieces as marked, that are screwed and glued. You do see a purchased fabric cargo cover that hides my unfinished (read: not made to look pretty) work, which without, the fabric cargo cover could not hold the weight of heavy objects.
IMG_2522 shows a wing I made to get around the Sports Bar, at least on the side closer to the front of the vehicle. It just folds up to the sky during installation, sliding it in from the back of the vehcile, and once the wing is beyond the Sports Bar and closer to the front seats, that wing angles down to be attached to one of the hard top bolts....only what's shown is the soft top, where the window inset doesn't get in the way.
That hinge in IMG-2523? I have to unscrew it each time to get the wing installed with the hard top inset side window issue. Remember that's a picture with the SOFT top where there is no window inset to be causing me installation/de-installation grief.
IMG-2527 shows the overall device. I'd like the area between the back of the vehicle and Sports bar to have wings but I never made them. The two rectangles at the top of the picture, in 1/3 and 2/3 sizes fold down towards the ground to angle behind the back seats and their standard 1 passenger/2 passenger configuration. This way nobody can see what's in the cargo area even if one or both portions of the back seat are lowered, and I can store long things sideways to the front seats as these pieces angle up to accommodate such things. Those two flanges screwed on to hold vertical pieces of pipe play no role in the discussion here. They were used to hold a pipe lattice structure to prevent heavy objects stored on the cargo cover top from becoming projectiles in a short vehicle stop.
IMG_2528 shows the device from the underside.
IMG_2529 is a better picture of the support structure where in the distance the wood angles down towrds the front of the vehicle, matching the angle of the back seats when lifted up to hold passengers. There's foam on the horizontal runs that run on the sides, from the back of the vehicle to behind the back seats, that cut down on noise.
So between the Sports bars and the window insets on the hard top side windows, I have obstacles to deal with that suggest that if I want a cover that fully encapsulates the footprint at the height where the hard top connects to the vehicle, that I think 4 pieces is the way to go. Two pieces, one closer to the back of the vehicle one closer to the front will hit the window inset as they lower.
I thought maybe two pieces cut at the center, passenger's to driver's side of the vehicle where wings fold down to fill the space between the Sports bar and the hard top edge, but that won't clear with window inset either.
The professional model is probably in two pieces, but in its going below the hard top, doesn't have to deal with the window inset issue.
This has been long and I thank you all for your expertise. I'm thinking that maybe 2 X 3's go under the plywood on a single bolt, capable of pivoting, that jut to support their neighbor piece. I'm thinking one side of the vehicle gets assembled, raised like a draw bridge, then the other, and then that "draw bridge is lowered" ...all while not somehow hitting the columns that are the Sports Bars; the underlying support structural twisted out of the way to facilitate closure.
Sorry, if it was trivial I wouldn't be here.
Happy holidays everyone. Thank you for your patience.