Teardrops: Does shape affect MPG?

[rruff]

My Teardrop had the Benroy shape, and the top of the TD pretty much matches the height of my JK. When I drive in the rain without the TD, the back window gets wet, when I drive with the TD in the rain the back window stays dry. The Benroy front curve, is a 19inch radius. The sloping back of a teardrop reduces the vacuum created as you drive through the air, so yes, I believe the tail end shape does matter.
Tom

The top-front curve is good (could be a bit smaller), plus side radius as well, and then you'd want a consistent taper to the rear rather than the typical curved top. If the rearward slope changes too rapidly, the airflow separates into turbulence. You want to keep it attached as long as possible, effectively decreasing the size of the hole you punch through the air. It is certainly possible to get better MPG with an optimized trailer (assuming it isn't too heavy) than without.

 

[SoCal Tom]

The top-front curve is good (could be a bit smaller), plus side radius as well, and then you'd want a consistent taper to the rear rather than the typical curved top. If the rearward slope changes too rapidly, the airflow separates into turbulence. You want to keep it attached as long as possible, effectively decreasing the size of the hole you punch through the air. It is certainly possible to get better MPG with an optimized trailer (assuming it isn't too heavy) than without.

The PO of my trailer originally painted it with pool deck paint ( contained sand for traction). I often wondered if it didn’t help hold the air to the trailer, like the dimples in a golf ball.


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[workingonit71]

Rounded front corners (sides and top) and kammtail would be best I think.

• 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. The kammback-ish spoiler on top keeps the rear clean,
  but I don't know if it helps reduce much drag.
• 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 being capable of off-roading in the least, so weight be damned, I was able to keep adding more features (+weight...now at 2200 lbs) and a stronger undercarriage (3500lb axle, heavy-duty springs) to make it more off-road capable.
• I've only towed it with my Chevy 2500HD, with open bed, so wind hits my tailgate first, then rises over the vertical lower front of the trailer, striking the 45 degree slope, as I had planned if using the smaller vehicles. I had visual proof, after towing over a dusty road, followed by a quick light shower of rain. The dust/rain left a flow pattern on the slope, but I saw nothing on the lower part(shielded by the preceding tailgate, 6 feet in front). Later on, I installed an aluminum tongue box, which made the 6 ft. gap smaller, and is v-shaped, which helps direct the wind around the sides, so it must be helping fight drag.
• In any case, my truck gets poor mileage anyhow, and I never could quantify the difference towing the trailer made, so unless I get a more aerodynamic tow vehicle, it really doesn't matter much what shape my trailer is. Most trailers on this forum are lifted, with higher ground clearance, taller/wider tires, sometimes with canvas tents/awnings/equipment bags on the top, and are towed by vehicles following the same pattern (taller, wider, equipment laden), so I don't see much point in worrying about maxing-out the trailer's aero-efficiency, IMHO, so I added more drag to my trailer. If I get a more aerodynamic vehicle, then I may remove it.
• 

 

[calicamper]

We just took a trip from Oroville, CA to Summerville, SC. We rented a Hiker Trailer 5x8 in Las Vegas, Nevada and completed our journey towing with a Subaru Outback 2.5. Without trailer and on the highway we would get 33 MPG with the trailer the best we achieved was 18 and as low as 15 with strong side winds. Fuel may make a difference as well as CA fuel is formulated specifically for CA but the trailer made a noticeable difference in fuel economy.

Several yrs towing with two 2.5 Subarus.
I have found that the biggest impact to the subaru mileage is wind drag. With our 4x6 trailer which sits at the bottom of the rear window (height wise) then gets a couple yellow costco bins set on top of the trailer. 1300lbs our long trips we average 20-21mpg in the 65-70mph every trip. If I add two mountain bikes to the roof we get knocked to 18mpg instantly. My 21ft sailboat at 8.5 ft wide and all up 26ft long hitch to stern behind the OB 1800lbs 16mpg was the typical average. The boat was sleek enclosed deck and sat dead even with the OB roof line with a 34foot mast sitting up over the car.

The taller boxy trailers Ive pulled all had us pegged at 16-14mpg averages.


Roof gear no trailer these arr my numbers

70-75mph
Narrow sleek roof box 1-3mpg hit. No head wind 1mpg , big head wind 3mpg.

Two bike in fork mounts are a solid 5mpg hit at 70+.
Same for the roof basket I borrowed a couple of times solid 5mpg hit at 70+ mpg

As for long trips towing I found 1800lbs to be about max limit for the cooling capacity especially in hot summer temps. Thats a big reason my camping setup I targeted 1200lbs or lighter for the trailer. We ended up with a 900lb empty and about 1300 packed set up. Works great.

But with 100,000 on the OB the original rear struts / springs are done. Im talking to RalliTEK they buld a kyb strut with their own springs that run about 18% stiffer which adds about 180lbs better support on the rear end. Thinking about getting stock height rears given our OB is constantly loaded and hauling people and stuff.

 

[fike]

If you keep your speed below 60mph, wind resistance isn't as important as weight. As you get above 60mph, the mileage impact of wind resistance increases rapidly.

You may recall the much-maligned 55mph national speed limit. 55mph wasn't chosen for safety. It was chosen during the oil crises of the late 1970s to reduce consumption.

If you want to drive 70mph with your trailer, then you need to worry about BOTH aerodynamics and weight. If you are content to go below 60mph, focus your efforts on weight.

I towed a tall-ish T@B camper with my Forester (sticks up at least 24" above the subie). At 65mph with wind, I was getting 10-12 mpg. At 55-60 mph with less wind, I was getting 15mpg. The same principle is true with my rooftop tent where the weight difference is negligible. Air resistance has a big impact on economy--from 17mpg-25mpg depending on headwind or tailwind.

 

[JJBiggs]

The best case scenario consists of weight - shape - height and rolling resistance.

Weight - less than 1000# is ideal. Shape - a slope or rounded overall shape is best for air flow. Height - staying below the maximum height of the vehicle towing will greatly help MPG's as the trailer will remain in the towing vehicles draft or slip stream. And lastly - tires. MT's will most certainly cause a reduction in MPG's. Always choose the best tire for your situation that meets the needs. The smoother the tread, the smoother it rolls.

C

 

[Ozarker]

A better shape would probably be a Bolger or Layden Sharpie, these are small micro cruising sail boats, the shape of the hull is about as efficient as you could get while keeping a utilitarian function, just turn the hull upside down.

MPG would be fine, but better would be the ability to ride out high wind storms while parked, IMO, point into the wind and ride out just about any head on gust.....