At 3:39 AM +0200 6/12/04, Listar wrote:
>Date: Thu, 10 Jun 2004 19:52:11 -0700
>From: Mike <watershed@xxxxxxxxxxxxxxx>
>Subject: [OM] Re: An SUV Question (roof rack capacity)
> >
>>....and attach a piece of 3/4-inch plywood to the rack
>>> using through-bolt U-straps with metal backing plates on the other
>>> side.....
>>
>3/4" plywood will not offer nearly enough stiffness for a platform to
>support a tripod. It would probably work well enough for you and a buddy
>to sit back and sip a couple of cold ones and watch the independence day
>parade over the heads of the [hoipolloi]. I know because it's been
>done :>) For photography you would have to build a torsion box of some
>design and fill it with foam. Attach as per Joe's suggestion. I've built
>a custom car top carrier and mounted it on Yakima racks with U bolts.
>Works fine. Of course add the stabilizer jacks.
Ahh. A report from the field. Stiffness varies as the cube of
thickness (of the box, or of the sheet, as appropriate), so it
doesn't take much added thickness to add much stiffness. One can get
plywood up to one inch thick fairly easily, and face-glue multiple
sheets together. After that, a box is best, as you suggest.
I've built torsion box beams before, so I know what you mean, but I
think the term needs to be explained.
The basic issue is that the upper and lower sheets must be firmly
glued to the edge sections between the two sheets, so the sheets
cannot move even slightly with respect to the edge sections. In
short, screws and nails aren't enough; hard-setting glue is required.
For example, the box could be two sheets of 5/8 plywood with edge
sections consisting of pieces of 2x4 cut to form a rectangle joining
the sheets at their perimeter. The sheets would be glued to the
edges of the 2x4s using a hard-setting glue. Later, the space within
the box would be filled with high-density urethane foam. Given the
load, it may be best to put a some 2x4 spacer blocks in the center of
the sheet, to the maximum span is one foot, to reduce the load on the
foam when someone stands in the middle with feet together.
Where-ever a bolthole appears, there should be a piece of 2x4 glued
between the sheets, centered on the bolthole, so tightening the bolt
won't crush the box.
By the way, if the box/sheet will be out in the weather much, even if
it will be painted, it should be made of marine plywood, and in any
case the glue should be totally waterproof (like epoxy).
Joe Gwinn
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