Aerodynamics and Improved Fuel Efficiency

[peterh]

As the cost of gas has gone up we've been grumbling about its impact on our wallets, and more than once people like me or Brian have mentioned how large a role wind resistance plays in our fuel consumption. Well, yesterday one of the online science magazines I browse through had this article about how researchers in the Netherlands were able to improve fuel efficiency by getting truckers to put skirts on.

Now, before all you women start suggesting kilts are the new men's fashion accessory, it was actually the truck trailers that wore the skirts, and using the skirts increased fuel efficiency by 15%. The article made me think about how some careful thought and experimentation with our trailers might yield some really useful suggestions on how we might change our aerodynamic profile and reduce our fuel consumption, making our favorite hobby both more environmentally and economically friendly.

Any thoughts?

--Peter

[Per Walthinsen]

For most of us with eggs it is probably a case of small improvements stemming from significant effort, i.e. maybe not worth it. As fuel prices go up maybe the cumulative effects of small changes can amount to something, I don't know.

If we had a way of actually measuring the effects of our efforts maybe we could channel our mods to those that really make a difference, but I have no way of doing that.

A sizable part of the motivation for changing from a very high-frontal-area vent cover to a low area and presumably low-drag MaxxFan was just that: drag and its effect on fuel consumption. But, the things I can do from here on in are minute, expensive, and of questionable value. For example, there is a way i could eliminate one of the two battery boxes on the tongue (no batteries in them, actually). The problem is that the tow vehicle actually moves the wind aside in front of it. (Fairing the whole front in like an Oliver might help). But, probably no positive effect at all.

Most of our eggs are likely quite slippery already, with the possible execption of Bigfoot, Trillium and other more squared-off models. Putting smooth underbellies on our eggs would be impractical and too heavy I think.

To get better fuel mileage I seem to be left with few real options: 1) Decrease weight 2) Drive somewhat smoother and slower 3) Pump up the tires slightly more 4) Wax the thing until it is slicker than snot 5) Only drive downhill, 6) Plan my route only in the direction of the prevailing wind, and 7) Slipstream trucks as if my life depended on it.

I plan to do all of the above.

[james kent]

That wax job is going to look real good pinned to the back of that semi.

[CD Smith]

This thread brings to mind the episode of Mythbusters about pickups.

Their results was;
1) Least efficient - tailgate down.
2) better efficiency - tailgate up.
3) best efficiency - truck box covered.

Efficiency increases as the square area directly behind the cab decreases.

Also, I infer from this that an SUV/VAN would be more efficient than a pickup as a tow vehicle for our eggs because the aero dynamic flow of air is less disturbed over the short distance between the rear of the SUV and the egg when compared to the longer distance between the truck cab and the egg.

[Rick B]

I think that the frontal area of trailers has an effect on drag. and I do believe that weight can be an issue as it takes more horsepower to move heavier weights.

Our Bigfoot is a bit more than 9 feet tall, and at it is molded, has rounded corners. 3000 series of Bigfoots have additional height of about 1 foot, as well as weight considerably more. I would assume that I would get poorer gas mileage with the 3000 series 24 foot model than my 25 foot lower profile, lighter unit. Head winds play a greater role with frontal area. In our recent trip into Oregon, we had head winds for more than one day and it reduced our fuel mileage from 10.5 to 11 down to about 8.8 us. I keep my tires aired up to near the max. and we have a tonneau cover over the Tundra's box. I also had the MaxFan vent installed over the front bed fan to reduce drag.

Rick B our unit weights about 6532 lb on the axles, with tanks empty but otherwise in camping mode (food in cupboard, fridge, clothing etc.)

[Bruce H]

The commercial trucking industry is driven by economics. Manufacturers in the US realized that aerodynamics have a significant impact on fuel economy. Federal regulations that restricted the length of tractors were dropped. This allowed manufacturers to use additional length to incorporate more aerodynamic designs and build safer longer wheel base vehicles. The "ant eater" Kenworth was introduced in the 80s. The designs of most competing models have also changed to maximize aerodynamic efficiency since then. Commercial truck manufacturers have employed designers with expertise in fluid dynamics. That is the science of how objects move through fluid or gas. Such expertise is also found in the design and manufacture of aircraft. Improvements in engine design and aerodynamics have resulted in a dramatic increase in the fuel economy of commercial trucks. During the past 35 years the average miles per gallon for a loaded 80,000 Lb truck has increased from around five mpg to around 6.5 mpg. That is running 65 to 70 MPH. At the same time the horsepower of the engines in these vehicles has increased dramaticly from around 300 horse power to around 500 horse power. If one of our 5,000 Lb tow vehicles pulling a 3,000 Lb fiberglass trailer could get the same relative fuel efficiency it would get 65 miles per gallon.

The RV industry has been much less influenced by economics. As far as I can tell most manufacturers give no consideration to aerodynamic efficiency, especially the aerodynamic efficiency of trailers. This is a complex subject because of the variations in the aerodynamics of different tow vehicle trailer combinations. The dramatic increase in fuel prices is now forcing consumers to evaluate fuel economy more than ever. This presents a significant opportunity and incentive for RV manufacturers to develope and market trailers with better aerodynamics.

[Steve L.]

This thread brings to mind the episode of Mythbusters about pickups.

Their results was;
1) Least efficient - tailgate down.
2) better efficiency - tailgate up.
3) best efficiency - truck box covered.

Efficiency increases as the square area directly behind the cab decreases.

Also, I infer from this that an SUV/VAN would be more efficient than a pickup as a tow vehicle for our eggs because the aero dynamic flow of air is less disturbed over the short distance between the rear of the SUV and the egg when compared to the longer distance between the truck cab and the egg.

Which, FWIW confirms what aerodynamics engineers told me in two seperate auto companies I work(ed) for. Those net-type tailgates sold for aerodynamic reasons are in fact worse for overall drag.

[Pete Dumbleton]

I recall reading an article of tests done in the late '80/early '90s that the tailgate question was in part dependent on the truck body and bed length. Some did better with gate open and some did better with gate closed.

There's also the bottom to consider; even the eggs with fiberglass bottoms have the frame and axle, etc., outside, creating turbulence.

I did find that raising my old Jayco (slanted top and flat sides) so its bottom was level with truck's bottom made it tow noticeably easier, despite being higher on top.

[steven jones]

since i'm doing complete restoration, i'm taking aerodynamics & weight reduction into account with my design... the trailer will probably be aluminum, & the underside will be smooth... the wheel-wells will have removable covers... i may remove the front window to reduce frontal-area drag, eliminate the roof vent for a lower-profile vent, removable awning stowed inside, etc...
the effects will be unmeasurable, since i don't have a baseline to compare to, but anything i can do to get further down the road for less $$ will be done...
--- steven

[Lisa H.]

One relatively easy fix to improve fuel economy is to put a fairing on top of one's vehicle to help the air move more efficiently over the trailer rather than hitting the blunt face of the trailer. I plan on making a plexiglass fairing to install on the Yakima rack I have on the roof of my SUV. Although Yakima sells fairings for their racks, the fairings are too short. I need to examine what attachment devices are available to affix the fairing to the rack. I've seen a fairing that's sold on either JC Whitney or Northern Tool and Dye but it was quite expensive. I think it's worth the experiment. Any thoughts. . . .?

[Pete]

Pulling my 17' Oliver with my 2wd 2008 Tacoma 4 door at 55 mph, I got 15.3 mpg this weekend on my outing to Inks Lake State Park. I usually travel at 65 and w/out the trailer I get 23.5 mpg.

I wonder how much of the reduction in mileage is due to the fact that I'm turning 2200 rpm at 55 w/out overdrive as opposed to 1850 at 65 mph using overdrive .

Sometime, if I'd like to toss some $$ out the window just for knowledge purposes, I may drive a distance on back roads, at 55 mph, w/out the trailer and w/out using overdrive (ouch!) just to measure the mpg to compare to the towing mpg.

If I do, I'll post the results here.

[Chris Z]

Lisa H
I've had the same thoughts. When we did our 5000 mile trip with 5 grandkids, we had a plastic turtle on top of my Suburban. I was wishing then that it had a better slope on the front of it to deflect the air. I also wondered how well it would work since it is so far from the top of the truck to the front of the trailer.
That turtle was a life-saver for room to store food and chairs and things but doesn't seem right to spend $800 or something for an aerodynamic one. Take a long time to re-coup that.
Let us know if you do that and report any results.
Chris