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Build Trailer from 1985 GMC S-15 Frame

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  • Build Trailer from 1985 GMC S-15 Frame

    I'm having trouble finding a forum anywhere dedicated to this kind of thing, but this seems like a good place to ask...

    I have a 1985 GMC S-15 (better known to many as a Chevy S-10 or GMC Sonoma) here which is no longer street-legal and not worth fixing. What I am planning to do is strip the truck down, leaving only the frame, rear bumper, rear axle/diff/springs/tires, and use the frame to build a specialized flatbed trailer.

    I am hoping someone has done something like this and can give me some tips, or at least warn me of some caveats. I primarily have concerns about the weight capacity of the single axle (albeit the truck originally weighed 3,000lb with a 1,700lb payload, so it should hold 2,500lb easily enough!), and the stability of what will be a relatively high deck. The frame is primarily 4" C-channel steel (3/16" or 1/4" thick), going up to 5" on the lower part. If I keep the solid rear bumper mounts and cut the frame just ahead of the driver's door, I will have nearly a 12' section of frame with the rear axle being 4' from the rear (creating a nice 60/40 weight split). (see hand-drawn with measurements and GM's schematic)

    First off, to get any decent width, I have to build the deck over the tires. This is more or less required anyway, as the top of the frame at the rear is level to the top of the tires. I presume I would need to leave at least 5" clearance over the tire, which puts the top of my deck at 32" off the ground, which seems a little high to me. The really unusual bit for a trailer, obviously, is the 1' drop and the narrow wheelbase (62" wide). I have three plans in mind, still weighing the pros and cons of each:

    - Option 1 is to build a solid flatdeck 32" off the ground, with sheet metal toolboxes underneath (built on the low section of the frame). The flatdeck would be built from 1 1/2" x 1/8" angle steel, and plain ol' 2x lumber.

    - Option 2 is to build a dual-level flatdeck. Upper section 6' long, lower section 5' long. The lower section would likely be built with drop-down doors. The front of the trailer (partially on the tongue) would have a steel toolbox built, with a ledge at a height equal to the rear deck, allowing a 6' panels made from 2x to be dropped over the low section (creating one long 12' deck). I like this option, as it creates a 20" deep lower section with sides where I can easily throw garbage and such when on the job, and not worry about it blowing away on the road. Also a place to put smaller things (tools, nails, etc) when hauling long lumber on top.

    - Option 3 is to build either Option 1 or Option 2, but make the rear portion of the deck hinged at the rear and latched down as a dumping deck. This would require building removable walls and a removable rear door (something I might have done anyway for use as a general utility trailer), and adding a mount underneath inside the frame for a ram of some kind (or a 4' jack-all until I can afford a ram!).

    Excluding the toolboxes (which I would probably add later to save money now, or temporarily build from plywood other than structural bits), option 1 is the cheapest at about $450 in material, including 1 1/2" angle, lumber, 4" tube steel for building the tongue, the hitch, and assorted other bits (safety chain, marker lights, etc). Option 2 comes in at about $600, and option 3 adds about $200 (excluding the ram). All of these are much cheaper than $2,000 for a single-axle, 12' x 7' flatbed with no walls or toolboxes!

    Other planned little bits:
    - I plan to leave the diff and U-bolt in place to keep the oil inside, rather than trying to strip the gears out. It's just easier, and I'm lazy. I am also considering leaving the rear brakes in place so that I can hook up a handle to the E-brake cable and engage it from the front. Reason for this is I will often be handling the trailer alone, and on even slight slopes it can be difficult to get the trailer in position then rush to get blocks under the wheels. I figure it should be safe to leave them in, so long as I remove the shoes to prevent anything strange happening. (Unless the brake shoes are also what engage the e-brake? can't remember...)

    - I will be re-using the S-15's taillights as well as some other detail bits, just to add a touch of cool. I would also like to mount the old headlights under the front corners of the trailer (facing backwards) to provide me with some surround-floodlight at night while reversing (as I often am working at night). However, the standard 4-pin trailer connector does not include wiring for reverse lights - any suggestions?

    - I might need to replace the aging springs. A 1,200lb load of 14' 2x10's in the back of the old pickup would drop the rear bumper about 8" (albeit about 80% of that weight was behind the rear axle). If I hit the Pick-N-Pull, how can I evaluate replacement springs? I know there are trucks there that have not been loaded and abused nearly as much as mine.

    - If the 32" high deck over the narrow 62" wheelbase creates a substantial stability risk, what are my options? Best I can think of is to mount the existing axle lower with smaller tires (current tires are 25" high), or get a proper trailer axle with really small tires. Either would let me drop the deck to 26" high. But by the time I hit that expense, I might as well not be using the truck frame, I won't save anything...

    Well, that's all. Does anyone have any suggestions for me?

  • #2
    I'm not a big fan because of the high loading height. There are plenty of plans on the internet to make a low low deck trailer including ramp to load things.


    The big negative on using a truck bed as a trailer is the high center of gravity. Careful how you load it. Since you have already set the axle, the only real question is how far out the tongue should go. Usually the longer the better.

    Most trailers don't have shocks on them. The shocks should actually help control some forms of sway. The first time you pull it, make sure you have a big truck pulling. If it does start to sway at high speeds you will be glad to have all the mass. Rule of thumb 5-10% of the weight needs to be on the tongue. With the high center of gravity I would be closer to 10%.

    As for the brakes. Check your local laws. Once a trailer gets up to a certain weight you will need brakes on the axle. I would highly recommend electric brakes - but by that time you might as well get a drop axle to go with it.

    If you don't need brakes (remember your truck will have to stop the entire weight of the trailer) you can just leave the old drum brakes on. Drum brakes by their nature don't drag. If you do want to use the brakes as a parking brake, you will need the drums on there.

    You will have to keep the center differential together. Inside the diff are C-clips which hold the axles in. You could remove the ring - or just grind down the pinion teeth to limit the drag.

    With electric brakes, if something goes wrong, hit the lever in the cab and the trailer can slow the entire truck down (well, as long as you have a load on the trailer - unloaded the trailer might just bounce a bit).

    As for load capability. The S-10 was probably rated somewhere around 1500lbs. I suspect you would be in the same ballpark. But just to compare, my little 4x8 trailer is also rated at 1500lbs and I think I got it from Home Depot for $450. Harbor Freight carries similar. The only thing I would wish is make the loading of sheet rock easier....
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    • #3
      Bear in mind the prices I quote are Canadian, and the cheapest (new) trailer I can get is a crappy little 4x6' with a 1,000lb capacity for about $800.00. A half-way decent 12x7 is around $2,000.00. The load capacity for my S-15 was around 1,700lb; but that was also with a 3,000lb truck attached. I'm guessing that 75% of that 3,000 bore on the front axle, with the engine and some of the cab weight, so that leaves the rear end with a potential capacity of 2,500lb, although I would likely never put that much weight on it.

      The laws here allow me to register a home-built trailer ("UBUILT" make) with a capacity up to 2,500lb with no inspection, so long as I promise that it is safe and has taillights (if necessary) and basic necessities (ie. safety chain). Over that and I need inspection for meeting codes, brakes, etc.

      Big truck. I have a 2010 Ford Ranger V6 SuperCab, I guess that doesn't count? :P I could borrow my neighbor's tow vehicle, a big old '82 quad-cab long-box Chevy with a big old V8 in it. He uses it to haul his 24' car trailer and his 28' utility trailer. (He's a racer)