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Small Bed Pickup Rack
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Posted by: dalecyr ®

01/24/2011, 17:36:36

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This is another opportunity to keep the resident Engineering-
Technology Sponge from killing, or causing serious bodily harm
to, himself and / or others.

I want to build a rack to haul "stuff" in/on/above my small
bed pickup.
(I'm starting to regret selling my '65 Chevy stepside,
with it's 49" x 97" ID bed)

The rack will be your standard 4 column, with rails between,
forming a rectangle, with a cantilevered area over the cab.

"Stuff" will consist of plywood, lumber, ladders,
pipes, metal bar/tube stock; you know: stuff.

My rigger mentality makes me want to deal in numbers.
Ok, no problem. lessee here... I'm gonna need a max weight
to deal with, some guess as to distribution, and a safety factor.

Easy enough.
Conduit, pipe, metal stock (not too much at a time), ladders...
that stuff isn't very heavy.
Now, plywood... or sheetrock, now we're getting somewhere.

So, what if I build this rack to hold, oh, I don't know...
a hundred pounds?

Time for a little research...
(look up some stuff on the errornet (oops. internet))

If I use 3/4" plywood, or 5/8" sheetrock, they both weight
around 75 pounds per sheet.
YIKES! that's only one sheet!

Ok, time to re-adjust.
Let's say I build the rack to hold 500 pounds, with a safety
factor of 1.5:1.

That's still only about 6 sheets of plywood/rock.

Well... let's run with that for a minute...
It's easy enough to run the calculations for deflection
based upon material size/thickness, and using a
Uniformly Distributed Load (UDL).
(Yikes! that's gotta be bigger than I was hoping)

And it's not much tougher to accommodate the dynamic load
that will happen on any particular member during loading
and unloading.

Ok, got it.
Time to post a drawing and a WHOA THERE COWBOY!
Let's give this a little more thought.

What kind of questions are the *real* engineers gonna ask me?

Well... I'm pretty sure the calcs are right...
and I think the design is solid...
I know the weight I wanna hold...

ah HA! yea, I know the weight I wanna hold,
but this thing is attached to a *moving vehicle*!

And things that move have inertia.

So the load will want to slide off the back end of the rack,
then off the front end of the rack, then off the side of the rack.

Ok, I can strap down the load, or provide side rails, or both.
but now I've got a 500 pound mass sitting above the cab of the
pickup, wanting to go in the direction that we *used* to be
going in, at the approximate rate that we used to be going.

Ok, no problem, I'll just calculate the... uummm...

The stress on a...

What do I have here? Torque? A lever? A buckling force on
A column? A cantilevered load exhibiting a twisting force
on a column consisting of four widely spaced members,
while simultaneously applying a buckling force on each member
that varies with time?
(shudder)

Ok, now, hang on a sec.
I see home made, and commercially made,
racks on small pickups all the time.
Maybe they don't hold 500 pounds tho.

Maybe I need to reset my expectations.
Maybe I need to cut down on my caffeine intake late in the day.
Maybe I just need to sit quietly and let one (or several)
of the listers on this forum impart some wisdom.

Ok.
I'm quiet and listening now...







Modified by dalecyr at Mon, Jan 24, 2011, 17:39:34


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: Small Bed Pickup Rack
: Small Bed Pickup Rack -- dalecyr Post Reply Top of thread Engineering Forum
Posted by: dalecyr ®

01/24/2011, 17:58:01

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conceptual drawing attached.

 

Pickup_Rack.jpg (13.8 KB)  






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: : Small Bed Pickup Rack -- dalecyr Post Reply Top of thread Engineering Forum
Posted by: Kelly Bramble ®

01/24/2011, 18:51:21

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Well, I'll think about what you have proposed..

However, first thought I have is why don't you just buy one of those $369 utility trailers?

https://www.tractorsupply.com/trailers-towing/trailers/utility-trailers/carry-on-trailer-reg-no-floor-trailer-1000215

Humm, maybe I missing the fun part...








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: : : Small Bed Pickup Rack -- Kelly Bramble Post Reply Top of thread Engineering Forum
Posted by: dalecyr ®

01/24/2011, 18:56:45

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my trailer backing skills are worse than my welding skills.







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: : : : Small Bed Pickup Rack -- dalecyr Post Reply Top of thread Engineering Forum
Posted by: Pinkerton ®

01/24/2011, 19:08:31

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If your welding skills are that bad you probably should not be building something that goes overhead anything or anybody, and that also carries dangerous projectiles.

Dave







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: : : : : Small Bed Pickup Rack -- Pinkerton Post Reply Top of thread Engineering Forum
Posted by: dalecyr ®

01/24/2011, 20:01:50

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That thought had crossed my mind too.

But, if I overlap the joints, I can always thru-bolt them
in addition to welding.

I'm not above making *sure* this thing can handle the load. ;)








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: : : : : Small Bed Pickup Rack -- Pinkerton Post Reply Top of thread Engineering Forum
Posted by: Pinkerton ®

01/24/2011, 19:23:30

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OK, I'll be serious for a bit.

The rack is an idea as is Kelly's suggestion for a trailer. Harbor freight have several trailers at good rates and they are sturdy. I have assembled many for friends and they live the dirt road life out here in the boonies just fine. Their folding trailer is especially a good deal.

OK, being old and lazy I do not like to contemplate lifting 75-lb anythings above my head. Does your truck have square holes in the top edge of the tray walls for dropping the rack legs into?

So, just to toss a cat in among the pigeons, what about...

Dave


 

side_rack_001.jpg (43.0 KB)  






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: : : : : : Small Bed Pickup Rack -- Pinkerton Post Reply Top of thread Engineering Forum
Posted by: dalecyr ®

01/24/2011, 20:21:23

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Dave:

If I understand your drawing correctly, the load would sit
just above the pickup sides.

That takes care of a lot of issues, but not the most pressing one.

The local steel supply company sells their stock in 20' lengths.
They will cut it in half for free, but that reduces the potential
usefulness of it, cuz I hafta plan my own cuts better.
And sometimes it just wastes stock.

So, part of the design parameters are to be able to haul an
un-cut piece or three from there to home.
And to haul my 24' extension ladder.
And other "long" things, like 50" x 16' panels of cattle fence.

I haven't gotten to the point of attachment yet;
I know that will carry it's own set of challanges.

The *current* issue is that I dont know which formula to use
to calcualte what that load wants to do to the rack
in a accelerating/braking/turning situation.

I *think* the whole rack acts as a "machine", if the columns
are strong enough. If they are not, they will buckle under
the lateral load.

I know I can use gussets to "strengthen" the columns,
but I'm not quite sure how to calcualte that either.
Even so, I need to know how much lateral force a given size
can resist.

And I don't know enough to know if this '4 widely spaced member
column' acts like an end supported beam.

as I'm writing this, I *think* I realize that this will act more
like a wire frame box, because the pickup side walls act as
the bottom members of the box.

So, now I think I'm dealing with (only) a racking component.
albeit in various directions.

right?

Now that I've deluded myself into thinking this is just a simple
problem of sizing a wire frame box to resist a specific
lateral load that would cause racking of the entire machine,
this seems like a much more managable problem.

now... where is that formula for calculating that...
there must be one somewhere around here...








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: : : : : : : Small Bed Pickup Rack -- dalecyr Post Reply Top of thread Engineering Forum
Posted by: Pinkerton ®

01/25/2011, 09:47:18

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Dale,

It is intended as TWO individual hanging racks off the side of the truck. The 4' x 8' panel would slide into the racks on it's edge.

The base of the rack that supports the panel is around level with the BOTTOM of the out-side of the truck side panel, maybe 12" off the ground at most. You would be lifting the panel only about 12" high and sliding it in. You could even have rollers on the bottom.

You have stopped thinking and started assuming again
!!!

Dave








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