trailer design?

This summer I'm going to tear down a 2-horse trailer and rebuild it into a small heavy flatbed/stakeside (5½x9½'). My design goal is to be able to carry 3500# loads. What I need to do is to choose the lightest steel pieces which will take the load. Unfortunately, my intuition was developed building large oceangoing ships, and I tend to massively overdesign every steel structure I make. If I'm making a surface plate stand for example, there is little penalty, but on a hitch-mounted trailer, every pound you put into the trailer subtracts from the payload. This isn't a standard trailer design as far as I can tell, so I can't just buy some plans and order steel.

I plan to use steel channel for the frame box, angle for the stringers, and use

2x6" tongue-and-groove fir for the deck. I'm leaning towards 4" steel channel and 2x2x1/4" angle.

Ideas? Comments? Suggestions? All welcome ..

Grant Erwin Kirkland, Washington

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Grant Erwin
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By steel channel, do you mean the stuff that looks more or less like an I beam that only has flanges on one side? That sounds very heavy to me.

I would go down to you auto wrecking yard and see how cheap you can get steel from a truck frame. The frame will be a lot stiffer for torsion if you box the frame. I would not use any angle. See if you can find some small channel that was used for industrial shelving. You could also see if you can find some pieces for pallet racking. A couple of

10 foot beams might work real well for the long frame pieces. They would already be boxed. According to my Grainger catalog , the heavy " seismic " 9 foot beams are good for 7,000 lbs. I assume that is for a pair, but it does not say.


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The problem with trailers is that you have all sorts of momentum loads that completely screw up normal beam calculation formulas or tables. Technically you can still do the calcs but you need to take into account the mass of the load, mass of the trailer, spring rates, yada, yada.

I'd suggest stopping by a few of your local rental places as well as any utility trailer places and have a look. bring your caliper and take some notes. One of my local places has some heavy duty trailers similar to what you are thinking about.

If you plan on just 2 stringers front to back, 4" just "feels" too small for a 3500# load. You could go from a 4" to a 6" channel, double the strength, and add only 50 pounds. When you buy channel, get the lightest weight (thinnest web) channel in each size. Heavier channel in the same size just has a thicker web, the extra steel doesn't do you much good.

My local steel yard has some "Z" shapes d> This summer I'm going to tear down a 2-horse trailer and rebuild it into

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Google for trailer plans. I found loads of sites. This looks good.

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John Manders

Another thought. If you are stripping the trailer frame, consider having it galvanised. It lasts a whole lot longer.


Reply to
John Manders

If you have the math skills the book How to Design and Build Trailers - Vol. 2

by M.M. Smith is a great book. Northern tool carries it. It tells you how to design your own trailer . It is not a set of plans for a trailer. But has all the info for stresses moments, inertia etc etc.

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Hi, Grant. Several years ago I bought 2 books on trailer design and building from Northern Hydrolics, or what ever their name is. They send catalogs about every 3 months and usually they go into the recycle bin. Can't find them now to see if the books are still offered. I just glanced through the books and they look like what you want. I never did build the trailer. Too easy to go rent what I need.

Paul Redmond, OR

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I can't offer much wisdom with respect to the frame but I'd suggest not using tongue and groove for the deck. Moisture and dirt get trapped in the joints and you get a rotted out bed in no time flat. Better to use just plain 2x6 and a wood preservative for some degree of longevity. If you intend on hauling dirt, sand or gravel, stuff that might fall through the cracks (I'd also suggest spacing the deck boards about 1/4" apart for drainage), invest in some sacrificial OSB to cover the deck whilst hauling the loose stuff.

Reply to
Dennis Shinn

Thin steel over 3/4" or 1" marine plywood works pretty well.

Thin steel over pressure-treated 2"x6" works better, especially for heavy loads.

The wood supports the load while the steel provides an easy-sliding surface for ease in unloading/cleaning.

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