Wind Drag

[Jackw292]

Hello All,
My first build, 5x8 trailer, hauling with my Sonoma. My truck height and trailer height will be virtually the same. How much wind drag will I really have to worry with? Should I incorporate a radius on the top of the trailer just because?
I guess I should include that my build will be more or less square. Oops

[working on it]

Hello All,
My first build, 5x8 trailer, hauling with my Sonoma. My truck height and trailer height will be virtually the same. How much wind drag will I really have to worry with? Should I incorporate a radius on the top of the trailer just because?
I guess I should include that my build will be more or less square. Oops

• I'm an adherent of the squareback school of design, mainly because I like simple design, strength of structure, and the fact that I had no way to bend 3/4" thick plywood (my choice of material, prior to ever designing it). But, I tried to get some aero benefits if I could, by trying to hide the TTT in the wind shadow of my Tow Vehicles, which I intended to use, at the time: by making the trailer 4 feet wide (less than the width of the cars), sloping the square front end, and putting a Kamm-type rear spoiler, to disrupt suction/drag, on the rear.

• My original concept was to make it simple, strong,and with a 45 degree front slope for aerodynamics (since I couldn't bend 3/4" ply). The Kammback was also in the original plan, though I used a semi-rigid piece of conveyor belting to create the Kamm effect. Aerodynamics was always a factor of the design, because I had intended it to be towed in the air shadow of either my wife's Cobalt (@55.5 inches roof height, and roughly 40 inches at the trunk), or my HHR Panel (@63 inches roof height), while the trailer's front began sloping back @ 40 inches, rising to the roof @70). But it ended up way too heavy to be towed by either (1600+ lbs, way over their tow limits of 1000 lbs each). Ended up towing with my HD truck, so the aero is not a factor, nor the weight. The trailer doesn't affect mpg at all; truck doesn't even notice it there. That's ok, though, since I started out wanting it to be semi-off-road capable, despite the Cobalt or HHR not beng capable of off-roading in the least, so weight be damned, I was able to keep adding more features (+weight) and a stronger undercarriage (3500lb axle, heavy-duty springs) to make it more off-road capable.

• I keep on modifying the TTT to get closer to my "off-road capable" and "instantly ready-to-roll" goals, and have started to add non-aerodynamic racks and fender mods to achieve those goals (more mods to come, I'm sure), and with the trailer now over 2000 lbs already, you can see that I have abandoned aerodynamics and mpg gains, entirely. If I were you, starting a build to be towed with an open-bed pickup truck (Sonoma), choose lightweight materials and radiused curves for mpg and aerodynamics, or go to the darkside, and choose to build with heavy materials, uncaring about mpg and drag, like I did. Or, maybe you can combine the best of both build types, and get it all.

[Noreast]

I started out my designing with a similar conundrum... I was just going to build a 5x10 box....

But my car is also about 55" tall and I didn't want the square edge sticking up and addi a bunch of aero drag

I re drew it with a 12"/45deg slice out of that forward top edge but I did not like how it looked. Too angular and looked half assed.

I settled on a 24" front radius and gently sloping rear hatch. It's much more pleasing on the eyes. Cutting a radius that size isn't that hard either. Sloping the hatch gave me more headroom when it's open. I'm sooo much more happy with the revised and radiused shape vs my original plans with straight cuts.


If I sloped it more (like a weekender) it would be more aero but I'd loose a lot of my galley storage.

[working on it]

I started out my designing with a similar conundrum... I was just going to build a 5x10 box....

But my car is also about 55" tall and I didn't want the square edge sticking up and addi a bunch of aero drag

I re drew it with a 12"/45deg slice out of that forward top edge but I did not like how it looked. Too angular and looked half assed.

I settled on a 24" front radius and gently sloping rear hatch. It's much more pleasing on the eyes. Cutting a radius that size isn't that hard either. Sloping the hatch gave me more headroom when it's open. I'm sooo much more happy with the revised and radiused shape vs my original plans with straight cuts.


If I sloped it more (like a weekender) it would be more aero but I'd loose a lot of my galley storage.

• I thought about starting my 45 degree sloped roofline at about 12" down front the top (of a 48" sheet), to get a larger door out of it. My fender placement didn't allow it to be further back, so taller would've been better. But, I already had plans to put two shelves along the front slope, one folding, and the upper one solidly mounted (for stereo, speakers, lighting controls, and a 10" diameter 110vac fan, hanging under it). It wouldn't fit in the space, so I dropped the slope start point down to where my wife's Cobalt trunk edge ended, 40".
• I also had plans for the rear storage area, and the other side of the bulkhead, which entailed a fully vertical hatch. I think the raised hatch-gap shield (made from stiff conveyor belt material) was sufficient to break up the suction-causing airflow over the roof and behind the hatch, at least as good as the little spoilers of some SUV's. I never considered a radius cut, since one only, would've served no purpose in an otherwise straight/angular design.
• How many boxcars have you seen with radii and spoilers? When Aero isn't needed (my HD truck doesn't need the help), then why add any just for looks? A front radius wouldn't be needed without a rear to match, and a rear radiused hatch door would've complicated my build (the hatch build is the real challenge for many, I've read).
• Just build what pleases you, as most do, none are identical. If they were, why bother to build your own. It makes camping more interesting, examining all the different approaches others have made to fit their needs and desires.

[Socal Tom]

The flat tail also provides a lot of drag, a tear drop shape helps with that significantly.
Tom

[Aguyfromohio]

The flat tail also provides a lot of drag, a tear drop shape helps with that significantly.
Tom

Yes, in most situations the biggest improvements come from the tail of the vehicle.
Any bit of taper or rounding helps, and there are noticeable benefits up to pretty extreme tail tapering.

[aggie79]

The flat tail also provides a lot of drag, a tear drop shape helps with that significantly.
Tom

Yes, in most situations the biggest improvements come from the tail of the vehicle.
Any bit of taper or rounding helps, and there are noticeable benefits up to pretty extreme tail tapering.

How do these thoughts/theories apply for a trailer being towed in the "disturbed air" behind the tow vehicle?

[Socal Tom]

The flat tail also provides a lot of drag, a tear drop shape helps with that significantly.
Tom

Yes, in most situations the biggest improvements come from the tail of the vehicle.
Any bit of taper or rounding helps, and there are noticeable benefits up to pretty extreme tail tapering.

How do these thoughts/theories apply for a trailer being towed in the "disturbed air" behind the tow vehicle?

if the TD is close enough, it acts like an extension of the vehicle. for example, I have a jeep wrangler 4door. Without the TD behind, the rear window ( which is a essentially straight up and down) gets covered with rain when driving without the TD. With the TD is stays dry when we are moving at speed. The TD fills the "vacuum" area that would otherwise be created behind the jeep and keeps the water from being pulled behind the jeep.
Tom

[Aguyfromohio]

if the TD is close enough, it acts like an extension of the vehicle. for example, I have a jeep wrangler 4door. Without the TD behind, the rear window ( which is a essentially straight up and down) gets covered with rain when driving without the TD. With the TD is stays dry when we are moving at speed. The TD fills the "vacuum" area that would otherwise be created behind the jeep and keeps the water from being pulled behind the jeep.
Tom

That's my crude understanding of it as well. A tapered trailer, close behind a square-backed tow vehicle can reduce flow separation and turbulence, reducing drag. It all gets complicated fast if we want precise answers, and we end up at three dimensional computational fluid dynamics. But people who race bicycles and cars know about drafting and try to exploit it, and every bird and fish in nature have a more blunt head and a more tapered tail end.

My only point in all this is that the shape of the back end matters much more than the shape of the front end for reducing drag.
If you are spending the time and money to make one end of your trailer pointy, putting the taper in the back is better for reducing drag.

[Aguyfromohio]

...How do these thoughts/theories apply for a trailer being towed in the "disturbed air" behind the tow vehicle?

I think I can be a bit more helpful than the answer above, where I just punted to a rule of thumb.
Here's a great image of a tractor trailer rig showing the contributors to drag


It shows what's important for a tow vehicle with a trailer.
The front drag depends entirely on the tow vehicle , the trailer plays no role in that 19%.
The 14% drag contribution by the gap is interesting. It's probably worse for a typical automobile with a teardrop, because the gap is bigger, and the trailer roof is relatively taller.
But the closer the trailer gets to the tow vehicle, the more we drop that component of drag.
The under body and side drag components are probably more important for big semi trucks than for a jeep with a teardrop, because the semi truck is just so much bigger. Far less skin drag and under body drag on camper rigs.

That 33% base drag component is the part we are really talking about here. If we fill that region with a tapered teardrop trailer, we get some benefit.

Now here's the only image I could quickly find for a modern looking SUV type tow vehicle. It's from an SAE paper by engineers at Jaguar looking at the effect of bleeding air from the high pressure in front of the vehicle to the low pressure region at the back. I honestly can't tell if this image is with or without that bleed air.




That red region is where a perfect trailer shape should fit to really reduce drag.
We see right away it's a pretty bad shape for a practical trailer. Too small and short and low to be comfortable.
But if you are truly interested in reducing drag and increasing fuel economy with the trailer:

- get as close as you can to that shape
- keep the trailer tongue as short as you dare so it's close to the tow vehicle
- keep the roof of the trailer low so it's not higher than the roof line of the tow vehicle

All the above discussion ignores vortex shedding and the associated drag from that phenomena.

My take away is that a smallish teardrop ain't so bad for aerodynamic drag and fuel economy, and a square back trailer costs some additional fuel economy to get that nice usable space in the back of the trailer.

That aerodynamic shape I posted above in this thread looks to me like the Schlörwagen, a 1939 German aerodynamic concept that is among the sleekest ever designed. Notice that it's not prismatic, with straight slab sides. It's also rounded port-to-starboard, like half a jelly bean.
Here's a video of it in a wind tunnel

https://www.youtube.com/watch?list=UU2iaRtEaE3Xbj3wPBot1-ZQ&v=JRfb2-tyRyg