Wind Drag

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Wind Drag

Postby Jackw292 » Tue Jun 06, 2017 2:08 pm

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
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Re: Wind Drag

Postby working on it » Tue Jun 06, 2017 7:28 pm

Jackw292 wrote: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.
  • working on it (from another thread) wrote: 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.
  • 2013 HHRv,"squareback/simple" TTT, semi-offroad? 4x8, 2000+ lbs travel weight
  • featuring: 3500 lb Dexter axle w/brakes & HD leaf spring system > riding on General Grabber 27x8.5-14LT tires, LED lighting inside, A/C & heat, AGM battery 12vdc, 110vac from extended run generator onboard or park power, Coleman dual-fuel stove & Northstar lantern
  • 147697148333
  • 148599125895148106
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Re: Wind Drag

Postby Noreast » Thu Jun 08, 2017 2:13 pm

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.
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Re: Wind Drag

Postby working on it » Thu Jun 08, 2017 3:54 pm

Noreast wrote: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.
  • 2013 HHRv,"squareback/simple" TTT, semi-offroad? 4x8, 2000+ lbs travel weight
  • featuring: 3500 lb Dexter axle w/brakes & HD leaf spring system > riding on General Grabber 27x8.5-14LT tires, LED lighting inside, A/C & heat, AGM battery 12vdc, 110vac from extended run generator onboard or park power, Coleman dual-fuel stove & Northstar lantern
  • 147697148333
  • 148599125895148106
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Re: Wind Drag

Postby Socal Tom » Thu Jun 08, 2017 6:01 pm

The flat tail also provides a lot of drag, a tear drop shape helps with that significantly.
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Re: Wind Drag

Postby Aguyfromohio » Thu Jun 08, 2017 6:22 pm

Socal Tom wrote: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.

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