As we approach the 2030's, we are nearing the hundred-year anniversary of the advent and popularization of the DIY teardrop camping trailer. Since the very beginning, it seems these trailers were often thought of as a streamlined shape. The September 1947 article in Mechanix Illustrated titled "Trailer for Two" featured the byline A streamlined home on wheels that's light and easily towed; has a double-berth and a complete kitchenette. The problem - while the teardrop trailer featured in the article clearly bears that all-familiar vintage visual streamline teardrop shape, it is only a visual feature - from the perspective of the air in which the trailer must travel, the design is anything but "streamlined." Strictly speaking, this is only a problem if you want to solve it; we often build things where function follows form, aesthetics quite frequently supersede other engineering considerations. After all, the small size of the teardrop camper already makes it more efficient to tow than most other trailers, even in its traditional, beautifully un-optimized shape.
I first joined the T&TTT forum in 2007, and even then aerodynamics was of some interest to me, having a very fuel efficient TDI tow vehicle I had considered making a trailer for, I had wanted to maintain that precious fuel-sipping MPG. Accurate information was hard to come by, dispersed in esoteric corners of the internet and brick and mortar libraries, if it could be located at all - considered by some to be beyond the practical realm of the DIY camper designer. In the end I became discouraged and ended up building a Transit van instead! Much has changed since those days. Unknown to me at the time, that same year, another internet forum was just being born - called "EcoModder" - with the express intention of discussing vehicle modifications for increasing efficiency. In the years that followed, it blossomed into the highest quality repository of synthesized information on the topic of vehicle efficiency from the do-it-yourself perspective that has ever been created. The results people have achieved tinkering in their home workshops and driveways in that time has been nothing short of incredible, in some cases more than doubling the factory fuel efficiency of their vehicles. They also benefited from a surge in commercial interest in aerodynamics, from vehicle manufacturer's looking to meet stricter global fuel efficiency standards, and from advances in sectors like commercial over-the-road trucking - where no official fuel economy standards exist, but fuel is a significant operating expense that can be minimized.
Examples of The Worst Vehicle Designs
Most of the chart toppers in the worst vehicle category, as far as drag coefficients, come at little surprise. They are usually brick-shaped or boxy - The Jeep Wrangler at 0.454 Cd, Jeep Cherokee, and pickup trucks like the Dodge RAM 1500 or the Ford Ranger. The one that usually surprises people is the original VW Beetle, with a catastrophically bad 0.48 Cd, worse than even the most boxy, utilitarian designs. (Lower drag coefficients are better; some of the best production cars like the Toyota Prius come in around 0.25 Cd) The car's flat upright windscreen does it few favors, but the lions share of the problem is the rear of the car, with its fast-dropping roofline. The example is particularly pertinent, as you'll see the same problem on many classic teardrop trailer profiles. The smooth, yet steeply curved roofline keeps airflow attached, sometimes a good thing - but unfortunately it carries a large air mass along the skin in a direction of flow that creates an enormous drag component on the vehicle body, especially relative to its size. The rear end is effectively a well-designed air brake, not a feature you'd likely want on a vehicle intended to move at highway speeds. This is the same reason that the Forest River R-Pod travel trailers suffered owner complaints from worse gas mileage than the same sized travel trailers in the traditional boxy shape. The boxy rear end shape was, perhaps counter-intuitively, aerodynamically more efficient. This is also why a Jeep Wrangler is, while terrible, still more aerodynamic than a Volkswagen Beetle.
A Unique Opportunity
T&TTT users are an interesting group, both in avocations as well as opportunities. There is little doubt in my mind that there are probably more trailer designers passing through here in this one place than there are anywhere else on Earth. The sheer volume of unique one-off productions cannot be matched by any company or corporation on the planet. Every build is a new opportunity, a blank canvas - with far fewer design restrictions than any mass production vehicle designer has to contend with. Teardrops have long been limited by practical build considerations available to the do-it-yourselfer, but there too, things have changed. We have better tools and materials available than ever before. DIYers invented new techniques or applied old ones to new applications - the foamie is perhaps the most prominent and popular example. We even have new boat building inspired inroads into the DIY teardrop space with things like the kit-built CLC stitch and glue teardrop. If a builder was interested in applying aerodynamic principles to their design, the tools and methods to do so are better than they've ever been before - and it would be almost impossible to come up with a worse result than the traditional designs drag coefficient, affording a great deal of freedom to experiment with little downside. Leading by example, the EcoModder's have shown us that understanding and successfully applying good aerodynamic design is well within the realm of casual, yet thoughtful, at-home builders.
Benefits & Drawbacks
Every design decision is nearly always a compromise, you can't optimize for every variable all at once.
Some usual benefits:
- Increased fuel economy and range
- Decreased power requirement for a trailer of any given size to tow at any given speed
- Decreased wind and crosswind sensitivity
- Decreased drive train wear; lower oil, piston and/or gear temperatures in engines, transmissions, differentials. Decreased battery wear&tear by lowering average current draw in EVs and hybrids.
- Decreased oil loading, shear, and additive package depletion - shifting further from the severe service back towards the normal service maintenance intervals (most useful for vehicles with active oil monitoring systems or for people doing oil analysis) - decreased maintenance expenses
- Gains are permanent improvements, like insulation in your home they work continuously in a passive manner and reduce energy requirements perpetually for the lifetime of the vehicle. If you are building for posterity, it pays dividends
- Experimental design may not pan out, little or no gains are a real possibility on new untested designs
- Loss of some of the classic visual cues and traditional aesthetic
- Potential for decreased galley/kitchenette/rear hatch width and/or height: more packaging difficulty at the rear
- Interior intrusion of the wheel wells, possible loss of space or floor width at the axle
- More time consuming, potentially more costly (up front) construction
- Slight inconvenience of skirted/faired tire and wheel maintenance and repair
- Potentially reduced trailer maximum articulation left/right - decreased turning radius in tight backing or jack knife situations - could result in more unhitching and moving by hand, or sliding tongue extension requirements
- Requirement for better data tracking, record keeping, and analysis - keeping detailed fuel (charge) & maintenance log books if you wish to empirically prove out any new design features
- More time spent in the research & design phase of your build
- Increased reliance on friction, engine, or regenerative braking systems to slow down, or maintain downhill speed
- The "payback period" or the point at which the effort pays for itself in perceived effort, or real money terms, depends on 1. the efficiency of your design, 2. the miles per year you travel, and 3. the cost of fuel/energy. The less you use your teardrop, the longer any potential payback will take. A poorly constructed or maintained teardrop might wear out, break down or be scuttled before the design modifications would have a chance to break even if that is your goal.
I've been interested in the topic for many years, and have had quite some time to absorb some information, across many different sources of material and examples over that period. I collected up these third party resources in downloaded files, bookmarks, drawings and sketches. Keep in mind most of this information is based on things that have been understood for centuries, it just took time to find its niche and filter down more thoroughly into the public domain. None of this is recent technology or information, most from the early part of the last century or before. Really, only the applications have seen renewed commercial interest, primarily due to globally increasing real energy costs that started at the beginning of the 1970's and that will continue unabated into the future. Much of the validation, understanding and original application of these ideas predates computers and even wind tunnels.
The Basics
Let's get the boring stuff out of the way quick! As a vehicle designer, the primary reason you'd want to understand and apply aerodynamics principles is to counteract drag - the resistance of the air against your trailer's forward momentum. Since this is the topic you'd be describing the most frequently, a few common terms will come in handy to explain and discuss design features with other builders.
The first measurement we'll often talk about is called frontal area, A - this is the two-dimensional size of the "hole" that gets punched in the air when you are moving forward. We'll usually express this size in square feet, or square meters in metric. teardrops usually excel in this measurement, because they are small when viewed from the front.
The area of the grid that is filled in by the outline of the car is the frontal area.
The next measurement we'll often use is called the drag coefficient, Cd - this is a dimensionless number (there are no units like feet or inches) that describes how the shape of your vehicle affects how easily it passes through the air, compared to other shapes. The smaller this coefficient, the easier that shape moves through the air.
Our ideal for a ground vehicle would be to get as close to the streamlined half-body figure as practical.
To compare two designs, we take these two measurements and multiply them together, CdA - and we refer to this as the drag area. It's useful because we express this in square feet (or meters) and we can then easily see that a trailer with a better drag coefficient can be physically larger, but pass through the air more easily than a smaller trailer with a poor drag coefficient. As an example, we might have two trailers that have an identical frontal area of 22 square feet. One has a drag coefficient of 0.45, and the other a drag coefficient of 0.25. The drag area of the first trailer would be 9.9 square feet, while the second trailer is only 5.5 square feet of drag area, a vast practical improvement! Another illustration of this is to compare the drag of a bicyclist and a Toyota Prius. The Prius is much larger in frontal area, but has a much lower drag coefficient than an upright bicyclist, roughly 0.25 vs. 0.75. The result of this is that the two vehicles require about the same power to push through the air, the Prius just moves its larger size that much more efficiently through the air. Each and every square foot of the bicycle's frontal area appear 3 times "larger" than each square foot of the Prius, when it comes to pushing through the air.
Practical Design Features
Let's first take a look at an illustrated breakdown of where the largest drag features are located on a compound vehicle (tow vehicle+trailer). This will guide our efforts towards the highest reward areas we could focus on:
That diagram gives us the well understood fractions of what each part of the vehicle contributes to drag - 25% for the front, 30% for the wheels and undercarriage area, 20% for the break/space between the two vehicles, and 25% at the rear. Let's assume the front of the tow vehicle is a fixed or unchangeable variable and we design the trailer to fit perfectly behind it (not sticking out in any dimension, taller or wider.) The breakdown then for the focus of the trailer designer is 27% for the space between the trailer and the tow vehicle, 40% for the undercarriage and wheels, and 33% for the rear of the trailer. The overall size or frontal area is something also well within our control, and we can always work to fit more into a smaller package. A perfect example of this is the popup standie designs, you have nearly double the interior volume, but it moves down the road at roughly half its "camping size!"
Undercarriage and Wheels - 40%
This is our single largest target, and fortunately also the easiest from a design and build perspective to patch up. We want the air to flow under the body and around the tires and wheels as smoothly as possible to minimize these often neglected, but massively turbulent drag areas. A few quick line items to explore:
- Smooth bottom - A perfectly flat substrate material on the underside of the trailer/frame members
- Choose half axles over a full beam axle - eliminates the tube perpendicular to the airflow - spheres/perpendicular round tubes/cylinders are very high drag coefficient shapes!
- Trailer jack swings away, up and out of the airflow, perhaps located between the trailer tongue a-frame members, rather than outboard
- Eliminate all "warts" along the bottom, such as permanent mount stabilizer jacks or plumbing features that hang down in the airflow
- Wheels, tires, fenders, mudflaps - don't let anything stick out into the airflow or beyond the width of the tow vehicle. Fully skirting the wheels flush with the sidewalls substantially eliminates this incredibly large source of turbulence and drag
- Moon or Pizza Pan style hub cabs can reduce the wheel turbulence - often seen on aero semi truck drive axle wheels
Our second largest target, and also a bit of a misnomer. The drag is substantially located at the rear of the trailer, but our work to change and improve the rear shape follows on from the shape of the entire body profile. For this area, we have to understand how to get our build closer to the ideal of a "streamlined half-body" as shown in the generic shape profiles above. This is easily the area of focus where most of the creative design re-work could happen. Ecomodder Phil Knox, aka "aerohead" developed a few very simple tools to make starting this process easier for us. The first are what are called design templates. Very simply, they are just an outline/shape that is a very conservative, nearly ideal streamline. You can use these two templates when you are sketching your side view, and your top-down view (plan view.)
The first, side view:
To use this template, you just overlay your trailer design on top of this drawing. You must follow a few simple rules to line everything up correctly.
- The bottom of the trailer tire needs to be on the flat line at the bottom
- The tallest point of the roof needs to line up with the tallest part of the template, where the 0 degree mark is indicated. Slide the template left or right so that the 0 mark is lined up perfectly with the peak of your roof line - for this part, it doesn't matter if the front lines up with your trailer at all.
- Do not scale the drawing in such a way that it is stretched out or squished in any way - the aspect ratio must not change. You can resize the drawings so they fit up in height, but the ratio of height to length of the template needs to stay constant to work properly.
We are only concerned with the outline here, the car and tire illustration is just there to indicate that this is a "mirror image" that creates the right shape profiles on both the left and right sides. The line marked "Location of Max. Roof Camber" is lined up at the rearmost point of the widest point of your trailer body where you want to start a side taper, the width of your trailer will perfectly line up with the widest points on the teardrop shape. Remember, in this view, we are looking down on the trailer from above.
Normally, teardrop trailers do not have sides that taper inwards towards the rear. Tapering in this direction comes with sacrifices in ease of build, but also can account for nearly half of your potential gains in this section of the rear design optimization. You could compromise and only taper a portion of the build, leaving straight walls where it is more practical to do so for space and building considerations.
Some other notes I have picked up about these templates. They are very conservative - you might be able taper more steeply, but it is risky to do so. You can accidentally, and very easily, create much more drag by going "inside the lines" (steeper taper) versus going more boxy - outside the lines. Going more aggressive should be undertaken very cautiously, ideally with experimentally verifying the resulting drag is OK. Going too aggressive is how the classic teardrop, VW Beetle, R-Pod, etc. end up being such poor designs, strictly from an aerodynamic point of view.
Once you reach the rear of your trailer design, you probably aren't going to want a trailer that is so long that it comes down to a perfect point in the back. Wherever your design stops, the safest and usually most practical aerodynamic thing to do is chop the back off with a perfectly straight vertical, flat rear hatch. This is called a kammback - it is basically just the rear shape of the ideal streamline but chopped off cleanly at some arbitrary point that matches the length you want your design to be, in relation to its height. The more you optimize this shape, the smaller the flat spot on the back will become, and the more creative you'll need to get with the galley or rear hatch access.
Other obvious considerations - don't attach things to the trailer that stick out - nothing on the front, top, bottom, sides, or back. Roof vents, roof racks, antennas, bottles, tanks, spare tires. The more of these warts you can remove, smooth, or creatively relocate to enclosed interior compartments, the better your design will become. There is much, much more that can be discussed about this area of design and the overall shape, but this is a very practical starting point for your work.
EcoModder has an online tool that allows you to upload your side or top view drawing and it will help you quickly line up these templates with your drawing. https://ecomodder.com/forum/tool-aero-template.php
Tow vehicle to Trailer Gap - 27%
It would be nice if a trailer, that is the same size or smaller than the tow vehicle, would just slide into the air pocket behind the tow vehicle and experience no drag at its front end. Unfortunately, this is not the case. This space is something you should really focus on, especially if you have to give up optimizing on other parts of your design due to the shape, space, or features you want on your build. The ideal here is to have no space between the vehicles, with no gaps for air - this would be like driving around one of those articulating bendy busses that have a smooth accordion-type joint in the middle. That kind of solution will most likely be impractical for your build. The next best thing we can do is fill in that gap as much as possible, and make the front of our trailer as easy as possible for the air flow to reattach smoothly. I've included some illustrated examples of the effect this gap has on compound vehicles that were done by Phil Knox.
You can see that you might approach the gap filling in a variety of ways, but consideration to turning and the articulation angles must be taken with care. You can close the gap with the trailer shape, vehicle fairings, or a combination of the two. Semi truck trailers often use a combination approach to close the gap. The Camp-Inn 560 "raindrop" shape probably offers some advantages to note here, due to being able to have the leading edge of the trailer physically closer to the rear of the tow vehicle, as well as providing a fairly smooth path for the air to reattach along the sides of the camper body.
Take note of the final illustration "Deductive Pathway B." This demonstrates that you could in fact use a trailer to lower the total, combined drag coefficient below that of the tow vehicle alone with a well designed pairing, making the combination more efficient than the tow car alone.
Further Exploration
A few more illustrations:
The effect of radius corners and adding a boat tail on a bus profile.
The change in drag coefficient of various "kammback angles" - demonstrating you can make things better, or worse, with taper.
Fineness
Fineness, or fineness ratio, a term in fluid dynamics or aerospace engineering that describes the ratio of the maximum width of a fuselage relative to its length. For our sake, we want to make a kind of "smooth egg" that fits within our ideal streamline shape, and avoid things like sharp corners, edges and abrupt transition points. Radius corners are an example of improving the "fineness" of a body design. If we place something simple like a 3" radius along our entire top edge perimeter between the side walls and the roof, we can drastically decrease the formation of turbulence and drag vortexes that can spawn off that sharp edge. This is especially true in cross wind conditions, where the side of our camper effectively becomes the "frontal area" for some portion of the airflow over the body. Modifications like these can be very important and exhibit a large difference in total drag between two otherwise identical shape profiles.
Body Frontal Area Profile Matching
A common point of high air pressure in a towed, box-like trailer is the two upper front corners. If you make a trailer that matches the width of the tow vehicle at the belt line, just below the car's windows, you'll probably end up with a 5' - 5.5' wide teardrop. If you make the full height this same exact width, you often see from the front view, that the top corners end up considerably wider than the roof of the car. Cars often taper the cabin inward along the sides, above the beltline, for aerodynamic purposes. We usually don't need as much room at the headliner as we do at the shoulders and hips. If you were to copy this common tapering profile, you will significantly reduce your frontal area, matching the tow vehicle, but still maintain a large/wide area at the floor for your mattress and other features.
Verify Experimental Results
To verify your results, you need to measure them and keep records. A fuel log book is an excellent and easy way to compare results over time, or between two or more trailers. If you are not coming from a traditional trailer and this will be your first, you could rent a traditional style trailer, U-Haul, etc to develop some basis for comparison. Other common testing methods you can research are coast down testing - how much distance the vehicle takes to come to a complete stop from a set speed, and tuft testing, using pieces of yarn and tape all over the body to identify areas of turbulence, flutter, and poorly attached or disturbed air flow.
Final Notes
Identifying the right shape and design features is fairly easy - it's been well worked out for us by now, at least the starting point. Figuring out how to build to such specs is where the real innovation lies. How much can be applied and still end up with a good camper that is easy enough to build? It's really up to us, which kind of opportunities do we want to take, and how much we're willing to leave on the table. Here's to the next one hundred years of building!
