Low Profile and Drag Pop Top Wedge Foamy+PMF on a 5x10

[kipa]

This is my idea for a more EV friendly trailer.

Design goals:

1. Minimize aerodynamic drag to maximize EV range. The primary idea is to build something that will not cut my EV range by half! If I can keep it under 30%, I'll be happy. Under 20%, ecstatic. There's really no compromising on that issue for me and anything taller significantly impacts that.
   
   
2. Fit a queen mattress. I like big beds and I cannot lie. Planning to orient it lengthwise since I am taller than the box is wide.
   
   
3. Fully loaded weight of under 1500 lbs. Weight is an arbitrary limit for keeping it legal without registration here in Oregon. (I could get by with higher, but I doubt it will be even close to that maximum full loaded.)
   
   
4. Woodless construction for maximum moisture control. I am planning to go with 100% foam and PMF for the entire lower body and front cargo hatch. I also have a full size long bed pickup hard shell tonneau cover for the topper. Similar to the model, it will overhang the lower structure for excellent water shedding. It will also provide for extra cargo capability on top if I really need it.

I have not decided fully on whether this will be a tent or have rigid fold out walls, but I am leaning toward the latter if the rest of the build goes to plan.

Sucking at using CAD programs that never seem to do what I want, this is the model I built 1:12 scale to test my ideas. The outside dimensions without the tongue box are planned at 6x10'.

Now that I look at this photo, it's looking a bit like a coffin. Won't paint it black or brown I guess.

The wedge cover is currently set to taper about 6 degrees down toward the back from 24" to 18" at the rear of the enclosed area. I may try to do more than that, but I don't know the drag improvement would really be worth giving up the space, as the open rear cargo area probably needs to contain items that high anyway. Raising the front would just be increasing my frontal drag area, so I think this is a good happy middle ground that will give me over 7' of standing room under the peak.

The tongue box will be closer to 30" deep since the A-frame of the trailer imparts a drop from the platform. 3' long and 6' wide, tapering to 2' wide at the front..

I do not have a plan to build in a galley at this point, but it may come later, likely on a pull-out if I do.

Also, pay no attention to the wheel well positioning. It doesn't match the trailer base and I don't have the exact dimensions for where the axle goes yet since I am waiting on delivery. Obviously, there is a lot of flexibility to move as needed.

I am thinking that I will also incorporate some storage within the frame, as I intend to sheet the underside anyway to better clean up the aerodynamics. Likewise, the rear end cargo walls won't be solid on the very ack, but rather likely rails with plenty of tie-down options.

I may be able to go a little wider to better match the tonneau cover, but you get the idea.

So, without violating the design goals, what do you think?

 

[kipa]

 

[kipa]

As this is not an overlander build, another option I am considering, which would give me more room to increase the body height is to lower the trailer. Unfortunately, the HF 10x5 I will be using has the axle above the leaf springs, so that's probably going to require an axle change. The Timbren 4" drop would probably work well, but those cost more than the trailer, so it will likely be a torsion axle.

 

[kipa]

After seeing others doing full popup vs. a wedge, I think I am going to aim toward getting that into the design. In addition to the extra headroom, I am concerned that a wedge tilt is just asking for leaks against the rear high wall since the tonneau will leave the entire surface exposed when open.

 

[DrewsBrews]

IMO keeping the trailer frontal cross section within the "air shadow" of the tow vehicle is the biggest factor for air resistance. The width plays a big roll in that.

I'm just finishing up on a trailer that uses a fold open car rooftop tent over a storage box built onto the trailer frame. Mine ended up 840lb unloaded with a window A/C unit, not exactly light construction and a whole 5x6ft deck on the rear half. If the tent was mounted sideways (opening to the side) that would allow for a 4ft wide frame (or narrower). And if condensed down to just a sotrage box with tent mounted on top that would probably cut at least 200lb from the weight. That would be a super simple and quick build. Not exactly the least expensive option though as those tents are pricy. Though some $ would be made back up in being able to get a less expensive smaller frame.

 

[GTS225-TNT]

I'll give my 2-pence, here. (This has to do with your first design goal.)
In the drag racing community, there is a "rule-of-thumb", that for each 100 pounds you can lighten the car, you cut your elapsed time by .1 seconds.
My point is that weight requires energy to move it, and the more weight you attach to your EV, the faster you'll drain the charge.
That means that I suggest aerodynamics will have a lesser effect on your distance-to-charge ratio than the total weight of the camper. Yes, keep the camper body in the "shadow" of the towing vehicle, as DrewsBrews said, but if you can find a way to reduce your weight further, then I think I'd explore that.

Roger

 

[Tony Latham]

Why 6' feet wide for a queen mattress (that is 5' wide)?

I noticed a significant amount of drag when I went from a 4' wide teardrop to a 5' wide.

Tony

 

[S. Heisley]

First, Welcome!

Now, looking at your responses so far, the others are right. Having the cabin of your trailer wider than the tow vehicle will cause a significant drag factor. I suspect you are trying to match your existing tonneau cover. If that fits your truck bed, you'd be better off using it there and making a pop-top foam cover for your build. Also, you'd get more for your non-drag and weight calculations if you build out from front to back instead of side to side. In other words, build a 5x10; and put your queen bed lengthwise instead of crosswise, if you really need that extra 2'. Another way to eliminate some of your drag is with curved sides and/or top, but if that is done, you should make the curve the equivalent of a part of a circle so the air will flow easily around it.

I also have a concern about using that tonneau cover on a totally foam base. I suspect that eventually you would have some problems with the constant weight of the cover on the foam base, possibly some bending of the sides of the foam base or even pulling apart at the corners. It might be better to add supporting 1x2" boards, at least at the corners and maybe near the lengthwise centers. If you have a hinge on the cover, you will also need a board there, to attach the hinge to. So, not totally foam but close to it.

Foamie people, please chime in with your thoughts.

 

[kipa]

Thanks for the answers so far!

As far as width, I am trying to keep the frontal area in the car shadow. I get up to 74" inside the mirrors on my EV6. The tonneau itself is only 72.75" wide, so with that as the limit, height is the only significant challenge with that as I try to keep the top deck of the trailer during travel even with or below the rear spoiler on the car. I may shift to putting it all on a the 4x8 trailer I already have. I might have to mock it up on the larger trailer to finalize that decision though.

Regarding weight, it's a common fallacy with EVs and trailers that it's a major factor. Most EVs recover over 80% during regeneration of the energy used to accelerate it. So additional load really only equates to an additional 20% or less energy for acceleration only. Aging Wheels does a great job of illustrating the relationship here. The short version is that rolling resistance adds up to a small fraction of the energy required to push turbulent air around. So while it's a factor, air flow remains a top priority.

I am shifting my materials plan for the base structure as suggested though. Leaning more toward ACM over foam for that with heavier aluminum trim at the corners. Running a continuous hinge for the popup sides on top of that should stabilize the structure further. So calling this a foamie/PMF build is probably no longer proper.

I am adamantly avoiding curves as a first(?) build in any case. I don't need that extra complication with the popup. Maybe on the cargo box up front and whatever I do at the rear . .

 

[kipa]

This is what I am thinking for the lower structure, with reinforcements embedded as needed. (Inside to the right.)

 

[kipa]

I am also experimenting with hybridizing the PMF by using TB2 embedded fiberglass mat. This provides better compression rigidity that canvas seems to lack. So that may replace the FRP panel on the lower as well as anywhere else I need greater rigidity.

 

[Tony Latham]

I am also experimenting with hybridizing the PMF by using TB2 embedded fiberglass mat.

Why not the real deal? Fiberglass cloth and epoxy?

Tony

 

[Pmullen503]

Why not the real deal? Fiberglass cloth and epoxy?

Tony

Having nearly completed a epoxy/glass trailer I can say it does cost a couple hundred $ more. HOWEVER, the performance and finish is well worth the cost.

 

[Tony Latham]

HOWEVER, the performance and finish is well worth the cost.

 

[kipa]

Honestly, I just hate messing with epoxy, and the extra cost is a factor I would like to avoid, but maybe the glue is a bad compromise. Cost-wise this about splits the difference between full on FRP and PMF. I'm not sure the extra strength and water resistance of epoxy is needed for this build.

 

[kipa]

I think I am going to downsize this to the 4x8 trailer and eliminate the rear cargo deck, as I can still get the space I need with that. I'm still going to keep the overall width, but that will allow me to lower the deck height by the ~2-3" an axle flip would provide without having to drop a bunch of cash on new axles. I don't believe the lowering to be a significant problem for ground clearance and departure angle. My EV6 is also lowered, so maintained gravel and dirt roads are the worst it will see aside from driveway transitions.

That will also allow me an extra 2-3" of height for the lower main structure, allowing me to keep a popup structure that should still provide full headroom inside without complicating the rigid wall system.

 

[Tony Latham]

I just hate messing with epoxy,

It's cake.

Just get the pumps when you buy the epoxy.

Tony

 

[kipa]

I think the cost difference with epoxy may be more than suggested. Going over foam, it's going to need multiple layers to provide the needed strength, so that's a lot more to soak into it. Not ruling it out, but the result I am getting with glue is noteworthy.

Here's my test piece with s-glass layered over the mat. It evens out the texture a lot, and would produce an acceptable surface for me.

 

[DrewsBrews]

Looking at the EV6 I bet the spoiler does well to seperate the air coming down from the roof. The sides, not so much. The air likely curls around the fender until finally seperating about where the hatch vertical lines are. I imagine the air shadow is narrower than it looks.

 

[Tony Latham]

I think the cost difference with epoxy may be more than suggested. Going over foam, it's going to need multiple layers to provide the needed strength,

The strength is in the glass, not the epoxy. You want the glass saturated but no more–it's just a waste of money and adds weight.

If you want to use less epoxy, use lighter-weight cloth. If you want more strength, use a heavier cloth. And this is true whether you use glass/epoxy, or an experimental adhesive such as TB2 or liquid-based varnish with fiberglass.

But I digress. I think you have your mind made up.

Tony