Questions about design for ultralight bed-on-wheels (codename: Siesta)

[Betonhaus]

Hi I wanted to get a sanity check and part sourcing recommendations for a trailer I'm planning to build to tow behind my Fiesta. All I want is something that has hard walls so I can jump in and get comfortable even when it's wet or chilly outside instead of tenting. So far the concept I've come up with is shown below, based on my math it should weigh about 375-400lbs and cost about $3,000(CAD) for parts (before shipping, and I think i can shop around for some parts or get them on sale). It's big enough to fit a full/double sized mattress (to ensure it is narrower then the car and reduce wind resistance and lets me see behind me without using mirror extensions) with room in the front for luggage or whatev, shaped to improve aerodynamics.
The basic design relies on torsion half-axles (weight, independent so more stable) bolted to a welded steel frame consisting of two 2x1x.12" tubes (tubes to ensure the axles are secure but I think i can just weld a plate to bolt the axles onto so I can coat the entire frame with protective paint easily) with 80x2x1x.125" channel for the main support resting on top with some bracing, all screwed onto 1/2" mdf plywood (heard you the first three times) flooring for additional structural stability, the walls are made of 1.5" ridged foam insulation coated in a poor-man's fibreglass (cotton canvas and truck bed liner) for added rigidity and protection (possibly improved aerodynamics if the rough surface causes the golf ball effect in any meaningful way), then with two 26x36" door kits (found here= Doors (teardroptrailerparts.com) ) they also act as ventilation but I'll likely stick a crank style dome vent on top as a third ventilation source.
Based on all the time I've been working on this, this should be the lightest I can make it without going into extremely expensive materials. If I only load camping gear into it it shouldn't have more then maybe 600lbs resting on the frame, and if I always use the jacks when parked the weight of anyone inside it will be distributed on six points, should be more then enough for the frame to handle.
I guess my main questions are:
1. Is my logic sound? will the trailer be able to handle it's load with these parts? is it okay welding the frame together like this?
2. Is there an alternative source for the doors I could use? After searching and searching I've concluded that a prebuilt door is the best and this site is the only place that offers these doors, but they account for basically half of the entire cost of the trailer.

 

[NatersXJ6]

I would stay away from MDF, especially in the flooring. It is heavy, holds screws poorly, and any twisting will tear it apart, assuming it doesn’t get wet and just melt.

Your drawings aren’t really enough to get the whole picture in my head, but I’m guessing that you could go lighter on the frame and more rigid in the floor and come out ahead.

 

[old_CWO]

I am not sure about availability in the great white north, but there are aluminum bolt together trailer kits available from places like Northern Tool. They offer a 4x8 and a 5x8. Some home teardrop builders use these with good results. The trailers are obviously imported from China so I assume they are available somewhere in Canada that's similar to Northern Tool or Harbor Freight here in the states. That might get you a quick start to a nice lightweight trailer - according to the web site, the 5x8 ship weight is 186 lbs with a capacity of 1715 lbs. Since you are towing with a car, the bolt together trailer should be more than durable enough.

 

[Buddha.]

The unusual shape reminds me of this other trailer aimed at aerodynamics.

https://www.canamrv.ca/rv-brands/safari-condo-alto/2476/a21-series/travel-trailer/10056/

 

[Betonhaus]

I am not sure about availability in the great white north, but there are aluminum bolt together trailer kits available from places like Northern Tool. They offer a 4x8 and a 5x8. Some home teardrop builders use these with good results. The trailers are obviously imported from China so I assume they are available somewhere in Canada that's similar to Northern Tool or Harbor Freight here in the states. That might get you a quick start to a nice lightweight trailer - according to the web site, the 5x8 ship weight is 186 lbs with a capacity of 1715 lbs. Since you are towing with a car, the bolt together trailer should be more than durable enough.

Thanks for the heads up. I did consider a trailer kit but the ones that I could find were either too long or too short. Making it out of scratch costs no more then $800USD for the frame, axle, and tires and weighs 150lbs or less. I can find steel trailers that are cheaper but considerably more heavy, and even the aluminum trailers are a bit heavier, and a 5x8 is a bit too long, the front of the trailer is tapered so the walls are in line with the hitch, with the idea that if I'm close to jackknifing I'll see the trailer get close to my car.
My current cost and weight estimate is as follows, and I'm figuring out ways to reduce both and can likely source parts locally for cheaper

part	cost (CAD$)	weight (lbs)
frame (using online metal store, can get a local quote for less)	$400	80lbs
plywood floor	$62	75lbs
Axle/suspension (did find it elsewhere for half but didn't include wheel hubs so will investigate)	$450	29lbs
fenders (using black plastic ones but might change)	$48	2.5lbs
tires (12")	$170	42lbs
jacks	$86	20lbs
doors (the most expensive bit and what I want to source elsewhere for)	$1300+S&H	unknown, guessing 50lbs
mattress (tri-fold 6" foam)	$240	50lbs
3/4" Rigid foam for walls	$55	8lbs
Canvas drop sheet (substitute for fiberglass fibers)	$45	6lbs
Truck bed liner (substitute for fiberglass resin, tinted to match color of car)	$215	10lbs
total (will be revised downwards.	$3,050	375lbs

All in all that largely meets my target for an attempt at an ultralight trailer that I can afford to make while still being pretty decent quality, I intend to be able to doll it up enough to be able to make more to sell. I've reached my weight target and cost is okay but I think I can bring it down a bit.

I would stay away from MDF, especially in the flooring. It is heavy, holds screws poorly, and any twisting will tear it apart, assuming it doesn’t get wet and just melt.

Your drawings aren’t really enough to get the whole picture in my head, but I’m guessing that you could go lighter on the frame and more rigid in the floor and come out ahead.

I think i did make the assumption that mdf is waterproof and as strong as plywood yeah, spruce plywood is lighter and not much more expensive. I intend to have the bottom of the trailer sealed with truck bed liner and the inside I might just use a sealant and paint, I'm not sure if i can find marine grade plywood locally but i'll try.
I'll try to throw together a more proper blueprint with measurements, but i can add these photos for now

 

[Betonhaus]

i do need to look at how other people are doing their doors. i want something sturdy that seals in case it rains or something (and at least keeps the moisture outside), and unless I construct a heavy door frame from scratch the only other solution i found was to buy a premade door that can be popped into place. It's something i do consider important, but the doors will be the most expensive single component so I'd like to shop around for alternatives.

 

[andytruck]

Have you looked at motorcycle trailers for inspiration?
Your doors are vry pricey. look at teardrop doors

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You can probably make the frame lighter. Only needs to be strong from the hitch to the axles. Behind the axles then the walls will give the floor strength, you can use less steel there. There is no stress from towing on the floor behind the axles.

You could make the stabilization legs work with 3. Two rear, one front at the tongue.

Torsion axles are expensive. Look at getting leaf spring axles, cheaper and they are repairable. 1200 lb axles should be super cheap.

edit** For the plywood, simply use Homedepot fiberglass epoxy and cloth to waterproof it. It will ad a lot of strength. I have a fiberglassed piece of plywood and it is like steel.
edit* if you are wanting to sell them, then just sell as you would a used trailer, dont claim to be a manufacturer, just sell as-is.

 

[NatersXJ6]

Truck bed liner has a bad tendency to peel off of plywood. You are probably better off with just a good quality plywood and paint. Think about the span inside and across your frame members. 1/2” MDF will most definitely collapse through if you kneel in the wrong place. Plywood will be much stronger. The you mentioned selling these, you are likely going to end up in a pit of regulations that will consume the vision for your project and leave you disappointed. Build for yourself and enjoy.

You probably need to buy some small quantities of these materials and experiment before dropping thousands on a “maybe”.

For instance, find your doors first and then match wall thickness to the doors.

Make your frame taller and use thinner gauge metals for more strength (rigidity) vs weight.

 

[Teardropper]

+4 or 5 whatever the count is on nixing the MDF. It won't last. It won't hold screws. Use 1/2" exterior plywood.

I don't understand all of the channel you've got running lengthwise. If that's to support the floor, it's not needed.

I think you should reconsider using the channel for the chassis. Square or rectangular tubing is much stronger by weight and you can butt weld it without fancy fitting.

Tony

 

[Betonhaus]

Ok yes I'll go with plywood instead of mdf. but looking at structural load tables it seems that it is far safer to only have 12" between supports instead of 24", by like a LOT.
I might switch to tubing, but I'm still a little concerned about rust inside the tubes. I realize I can just cap them with a little vent then spray linseed oil or some ********, but I'm unfamiliar with the process and not confident i can do it correctly. but realistically it's probably not an issue and all tubing would be fine.

I realized that 1) I converted the price of the doors to cad twice and they are only $700 2) I can live with a 26x36" Doors and skip the oversize fees of the 30x36" door 3)I only need ONE and can just use a Window for the other side so the cost comes down to $500USD/$630CAD which helps, and i can get the axel/bearing hubs for $265 instead of $450
That brings the cost down to $2,200ish, depending on S+H. ebay didn't seem to have a decent door.
yeah maybe I'll just paint the plywood, but i want to do the undercarriage to protect it from rocks and water splatter. I'm planning to reinforce the truck liner by putting down a natural fiber sheet live a canvas drop sheet then saturating it with truck liner to give it rigidity like fiberglass, and I can staple the canvas to the plywood to secure it.

 

[billiebob]

Hi I wanted to get a sanity check and part sourcing recommendations for a trailer I'm planning to build to tow behind my Fiesta. All I want is something that has hard walls so I can jump in and get comfortable even when it's wet or chilly outside instead of tenting. So far the concept I've come up with is shown below, based on my math it should weigh about 375-400lbs and cost about $3,000(CAD) for parts (before shipping, and I think i can shop around for some parts or get them on sale). It's big enough to fit a full/double sized mattress (to ensure it is narrower then the car and reduce wind resistance and lets me see behind me without using mirror extensions) with room in the front for luggage or whatev, shaped to improve aerodynamics.
The basic design relies on torsion half-axles (weight, independent so more stable) bolted to a welded steel frame consisting of two 2x1x.12" tubes (tubes to ensure the axles are secure but I think i can just weld a plate to bolt the axles onto so I can coat the entire frame with protective paint easily) with 80x2x1x.125" channel for the main support resting on top with some bracing, all screwed onto 1/2" mdf plywood (heard you the first three times) flooring for additional structural stability, the walls are made of 1.5" ridged foam insulation coated in a poor-man's fibreglass (cotton canvas and truck bed liner) for added rigidity and protection (possibly improved aerodynamics if the rough surface causes the golf ball effect in any meaningful way), then with two 26x36" door kits (found here= Doors (teardroptrailerparts.com) ) they also act as ventilation but I'll likely stick a crank style dome vent on top as a third ventilation source.
Based on all the time I've been working on this, this should be the lightest I can make it without going into extremely expensive materials. If I only load camping gear into it it shouldn't have more then maybe 600lbs resting on the frame, and if I always use the jacks when parked the weight of anyone inside it will be distributed on six points, should be more then enough for the frame to handle.
I guess my main questions are:
1. Is my logic sound? will the trailer be able to handle it's load with these parts? is it okay welding the frame together like this?
2. Is there an alternative source for the doors I could use? After searching and searching I've concluded that a prebuilt door is the best and this site is the only place that offers these doors, but they account for basically half of the entire cost of the trailer.
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You need to check out commercial manufactured trailers. And check out how steel channel bends, use it in its position of strength.

Primary loads are at the axle mount and they will try to twist the outer rail, hence the channel should be on edge, not flat.

Also pick materials which are lightweight, MDF is very heavy.
Pick materials which will survive cold, hot temperature swings and wet, dry transitions, again MDF will dissolve.
Look at the loading, orient every product to add strength to the whole.

There are some great youtube videos on building the walls with EPS foam and canvas, definitely the cheapest, lightest, simplest way to build an ultra light tear drop. I think the is a thread on it here too.

 

[Teardropper]

I'm planning to reinforce the truck liner by putting down a natural fiber sheet live a canvas drop sheet then saturating it with truck liner to give it rigidity like fiberglass, and I can staple the canvas to the plywood to secure it.

Is this for the bottom of the floor? Just give it a couple of coats of thinned oil-based polyurethane. Bed liner is like a sheet of rubber. It won't add rigidity to canvass.

Tony

 

[billiebob]

I can staple the canvas to the plywood to secure it

you will trap moisture and create rot, all products need to drain naturally, just prime and paint the underside.
On the 12" on center thing, yes 12" oc will be stronger but 5/8 T&G plywood with supports 24" oc will be more than you need in an EXPO trailer. If not, why not go 8" oc..... Weight, weight is the issue, and cost. I agree with open channel over tube. Quite right, channel will drain and dry naturally, tube will capture water, and silt, and salt.

On framing seriously, just copy the manufactured trailers, they run a steel perimeter and run steel cross members left to right for good reason, weight and strength. You need longer pieces length wise with longer spans, shorter pieces laid out across the width. Don't reinvent the wheel. that trailer builder who builds 100 a day has all the engineering and cost calculated to sell a strong, light product. Wether you build in wood or steel or plastic, always orient the product to carry the load, put the members on edge, never on the flat.

 

[Betonhaus]

a bolted together aluminum frame using 2x.125" square tubes for the main frame and 2x1x.125" tubes for lateral supports would work, right? pricing it out it doesn't seem so bad compared to the quote for steel channel i got.


Sorry I'm still trying to figure out the best design so i keep changing things up, this was a long term project when i didn't have a cost estimate or a job, but now that I'm within the desired range for cost and weight and I have a job finally it's getting close to the time to ******** or get off the pot.

 

[Teardropper]

2x.125" square tubes

Do you mean .125" 2" x 2" tubing? That would be much better than your channel.

As far as cross members, you need one at the axle. You should have a second one a couple of feet from the front of your frame for the tongue to tie to.

You don't need cross members to support a 1/2" floor. You're not walking on it, you're distributing your weight while laying flat.

Here's a well-designed trailer frame. For your little trailer, you don't need the A-frame but you do need the box at the front to tie the tongue to.

Tony