Trailer Aerodynamics

What makes a vehicle slice through the wind easier?  How does a trailer fit in the mix?  The science is complex, but there are some good concepts of trailer aerodynamics that are important to know.  Is there anything I can do to improve my rig?

Trailer aerodynamics is not just the trailer, because aero involves the entire rig.  The tow vehicle is the leading edge, and the trailer is the tail.  Both are involved, and both are important, since the wind must pass by it all.  A ton of science has gone into the study — for Semi-Trucks.  Less with RV’s and smaller trailers, but there is a lot to learn anyway.

Fuel Economy is the big perk we think about for aerodynamics, but trailer stability and drivability are also benefits.  If you travel much with a trailer, aerodynamics are important.  And, the larger the trailer, the more important it is.

Yes, the best place to implement aerodynamics is at the factory.  But, if you already have the rig, that’s not an option.  Yet, there are things to do.  On the other hand, in DIY, we are the factory!  So, think about trailer aerodynamics early for best advantage.

Image above from ElectricTowCars.com  Pretty cool information there.

Aerodynamic Theory

Before we look at details, let’s first talk global theories.  What is “Aerodynamic Drag”?  In simple terms, we can think about aerodynamic drag as “Air Disruption“.  The more a vehicle disturbs the air, the more drag it creates.  So, improving aerodynamics is about minimizing disruption of the air as we pass through it.

The most important 3 things for air disruption (aka aerodynamic drag) are these:

1. Frontal Area.  (Front view size.)
2. Smoothing (aka drag coefficient).
3. Speed.

These are also the 3 main parts of the aerodynamic drag equation.

This equation says the Drag Force (Fd) is equal to a Coefficient of Drag (cd), (multiplied by) Density of the air (ρ (greek letter Rho)), (multiplied by) Velocity through the air, squared ( V*V ), (multiplied by) the Frontal Area of the object, (divided by 2).

Thinking about disturbing the air sets the stage for concepts of aerodynamic drag appropriately.  And, it’s easy to think about.  A larger object moving through the air will disturb it more.  Smoothing the flow around an object disrupts it less.  And, as you go faster, it increases air disruption.  Improving Aerodynamics is all of these.

We can talk about the theory of best shapes, like airfoils, but that’s kind of moot because cars and RV’s with those TRUE shapes don’t exist commercially.  Yes, there are some really cool prototypes, and there are some moving toward those styling cues.  Anyway, I love theory, but in this article we will focus on practicality.

That’s the physics.  To improve trailer aerodynamics, we have to work on those 3 things.

Application Of The Theory

Many people like to focus on the coefficient of drag (smoothing).  (Like one of my favorite forums, Ecomodder.com.)  However, the biggest contributor is speed (V^2) by far!!  If you double your speed, you quadruple the drag.  Also, the drag coefficient is on par with the frontal area.  With an existing rig, it’s easier to mess with changing the coefficient of drag than to reduce frontal area, but there are many things that do both.  We’ll talk about some below.

Now, let’s think about each of the 3 items above with respect to disturbing the air.  Think about it in terms of your whole rig, not just the trailer aerodynamics.

1. Frontal Area

   

   This one is easy.  With a bigger object (front view), more air must move out of the way.  So, the bigger it is, the more drag (more fuel it requires).  Keep in mind that we are talking about the front view.  Long and thin, like an arrow doesn’t have much frontal area.  So, smaller (front) is better for trailer aerodynamics.

   Here is an analysis by Computationalfluiddynamics.com.au.  This shows drag differences with a reduction in the frontal area.  Please note size compared to the tow vehicle, but we’ll talk about that below.  I’m not sure I believe the 48% number, but from a strictly theoretical standpoint without other drag, maybe that’s true.  The most interesting part for me is the graphical view of the pressures.

   

   Please note that the image shows both a lower trailer top, and a trailer closer to the tow vehicle.

2. Smoothing (aka Drag Coefficient)

   The smoother the skin of the object (including the rounding of the edges and tapering of the tail end), the less turbulence the object will create as it passes through the air.  We call it Cd.

   Unfortunately, “Drag Coefficient” is not well defined, like you can’t calculate it to see how much of a difference each idea will give.  That’s what engineering does sometimes to make the equations work — they make a Coefficient.

   We have to experimentally measure Cd in a wind tunnel – which is not practical for DIY’ers.  Fortunately, we do know things that make it greater or smaller.  This is one we can play with, and we’ll talk through some ideas below.

3. Speed

   This is the most bothersome.  Who wants to go slow?  I like to go fast, but as we see from the equation, this is by far the most significant factor.  And, it’s the easiest one to control.  So, to reduce air disturbance, slow down.

Small, Smooth, Slow.  Doesn’t sound very exciting.  And, it’s kind of the opposite of the way we like to do things.  Well, except for smooth.  Anyway, that’s the application of the theory.  Now let’s talk practical.

Myths About Trailer Aerodynamics

We all seem to have a “look” that fits in our minds about what a good aerodynamic shape is.  When we apply that to our thinking about trailers we often come up with ideas that look like this:

A trailer with a nice rounded front, or maybe a trailer with a good “V” nose.  Looks good, but is it?

V Noose

How much does a “V” nose help trailer Aerodynamics?

Actually, it does help, but perhaps not in the obvious way.  Often we think of it as slicing the air, but a trailer following a tow vehicle is in turbulent air.  Yes, it makes an easier transition for air around the trailer, but it’s small.

Perhaps more important, the “V” usually consumes space between the tow vehicle and trailer – making less space for turbulence between them.  This comes, in part, because the “V” nose usually takes up some of the tongue.

Then there is the application.  A sharp “V” nose with sharp edges (like the white trailer image) is less effective than rounded nose with rounded edges.  While I don’t have good data to show, a good rounded trailer front is better than the “V”.

Front, Middle or Back?

Best bang for the buck in aerodynamics is at the back of the trailer, not the front.  It looks good in the front, but that’s the myth.  There is certainly value in smoothing and rounding the front, but the biggest drag happens at the back.  Front rounding is probably #4 on the list.

The middle (between the trailer and tow vehicle) is important for aerodynamic improvement too.  See above.  It’s part of why the “V” nose helps.  Turbulence there consumes significant energy.  More space (distance between car and trailer) creates more drag.  Of course, you can narrow the gap with a short tongue, but you’ll sacrifice other things that might outweigh aerodynamics.

There are confounding factors with a bigger tow vehicle – yes, it blocks more air, but it also creates more turbulence between.  The myth says a larger tow vehicle improves trailer aerodynamics – which has some merit (see the red image), but the net effect depends on many individual specifics.

Trailer Size vs Cool Shape in Aerodynamics

Frontal Area is super important with trailer aerodynamics.  While this is a point of interesting frustration, we want a big trailer, but size has a price.  The myth:  You can have a bigger trailer if it is more aerodynamic.  Smoother shape helps, for sure.  Yet, adding 10% to size is easier than taking 10% from whole rig Cd.

It’s more than the theoretical of the red image.  For example, the Aviator trailer in the photo above has a nice looking aerodynamics shape, but look closer.  How many things interrupt the shape?  The awning, the roof units, roof vents, side handles, and things under the trailer.  Yes, shape matters – everything.  Wind does not care how sleek it “looks”.

To truly reduce the Cd for your rig, it requires a lot of smoothing.

“Dirty” Air Under The Trailer

Another myth . . . Air under the trailer is “dirty” (turbulent) anyway, so things there don’t matter.  Well, everything matters.  Sometimes we can quiet the air with dams for a reduced effect, but it still matters.

Large commercial trucks use this approach, because it’s cheap to install, and it makes a difference.  Not only the long skirts along the sides, but also fairings around the wheels.

RV’s often have a smooth belly to avoid extra turbulence around pipes and crossmembers under the trailer.  It’s an easy way to improve aerodynamics without affecting other areas.

If you can put sheet material all along the bottom for a smooth belly, it makes a difference.  “Full Moon” wheels, and wheel fairings around, will reduces turbulence there.  (Just make sure you don’t block airflow for cooling the brakes.)

Wind Deflectors

Deflection shields that mount atop the tow vehicle are difficult to address as a myth.  If they are right, and close to the trailer, then they can help.  On the other hand, if done wrong, they will hurt more than help.

I will point out that they are more important if the front of your trailer is squared off like a brick.  If your trailer has nice rounding, the wind deflector is less effective.  Or, you can use a smaller one for some reasonable effect.

Just be careful when buying them.  The best application is something with appropriate size for your trailer, something rigid, and one that mounts as close to the trailer as practical.  A big screen atop a pickup cab for a bumper pull trailer will not help.  Just remember that you’re adding turbulence (air disturbance by adding a wind deflector) so it must offset and cause even less disturbance somewhere else, or it will hurt more than help.

Oh, and don’t leave it on your vehicle when driving without the trailer because that hurts both Cd and Frontal Area!

Aerodynamics are NOT the Whole Story

There is one other really big piece of the story with aerodynamics.  I’ll illustrate with a story.

On one trip across Wyoming, on the way out, the day was beautiful.  Our Subaru Outback ran 80 mph at 24 mpg.  On the way back, it was very windy, a hard head wind.  The Subaru struggled mightily to keep 70 mph and our fuel economy went to 11 mpg.  The change in fuel economy is not just due to the wind.

The other side of the story is my truck.  Even with a huge load, fuel economy doesn’t change so much.  It gets 14-15 mpg most of the time.  The worst I’ve seen is pushing it up high mountains with a trailer – 11 mpg.  That engine has a lot more reserve power, so the added load does not burden it like the example of the Subaru.

The Subaru engine was at the edge of capacity, so the car computer was feeding it everything to eek out top power.  Also, most cars have a fuel enrichment scheme near WoT (Wide Open Throttle), where they pour in more fuel to protect the engine systems and extract the last few horsepower.

The point?  If fuel economy is your goal, trailer aerodynamics will play a huge part, but make sure the tow vehicle is also up to the task.

Analyzing The Assertions

I once did an experiment with an enclosed trailer traveling the same 120 miles.  On one trip I filled the void between the trailer and the truck with large inner tubes contained in a tarp, and fuel economy went up about 0.8 miles per gallon.  Having stuff between was a pain, but it demonstrated the advantage.

One big caveat in the study of trailer aerodynamics is the effect magnitude.  If you do a ton of changes to improve fuel economy, it might not amount to much.  Significantly changing the Coefficient of Drag, Cd, is hard.  Second, things required to make a significant change are often expensive, or quite inconvenient.

For example, closing the gap between the trailer and the tow vehicle can create problems with turning, visibility, and/or backing.  Closing the gap makes access to the rear of the tow vehicle obnoxious.  A short tongue is another example that can improve trailer aerodynamics, but cause other troubles.  These are things we have to ballance.

Keep in mind . . . all these things have a minor effect, which still might not add up to much.  We talk about fantastic trailer aerodynamics as if it will make big changes in our fuel economy, but remember, the biggest effects on fuel economy are 1) Speed, and 2) Weight.  Weight doesn’t matter for aerodynamics, but it’s HUGE for fuel economy.

Ways To Improve Trailer Aerodynamics

To sum up much of what we’ve said above, let’s answer the question.  What can I do to improve my trailer’s aerodynamics?  Reduce the total air disturbance of your rig.  That can be smoothing, it can be making it smaller, but most of all, decrease speed.

Before we get into the things we can do to improve, let’s first address the shysters.

Gimmicks for Aerodynamics

Oh, there are a ton of things people sell to improve aerodynamics.  You can spend a bunch of money on gimmicks.  So, I always advise to look first at the science and see if it really helps disturb the air less.  If you can’t tell, it may actually cost you more to buy than what it might save.  Any big claims for huge fuel economy gains are likely bogus.

My favorite example of a gimmick is the large doghouse covers on top of RV’s to make vents more aerodynamic.  They probably do reduce the Cd some small amount as compared to having the vent full open, but they also add a bunch of frontal area.  Greater frontal area overshadows any gains in Cd.  It makes me laugh.  If you really want better aerodynamics, close the vents and stretch a super tough tarp (not the blue or green flimsy things) over the top of everything.  Winch the tarp really tight, and deal with the hot interior when you get there.  That’s a smoothing effect that doesn’t really affect frontal area.

What Things Will Improve Aerodynamics?

A valuable place for aerodynamic improvement is under the trailer and around the wheels.  See above.  I suggest the forum at ecomodder.com as a place to learn what other people have done.  You will have to dig through a lot of stuff to get to trailer specific things, but several of us have put information there.

Rounding front corners is good, as is a taper of some sort at the rear.  Both of these are much easier to do when building the trailer, but neither is super easy.  Rounding the front is easier, but has less effect.

Creating a Rear Taper is much harder (and makes some trailers less convenient to use).  This will make more effect, but don’t add a lot of weight, because that does the opposite.  Do what you can if it makes sense.

For Your Existing Rig

Anything you can take off the trailer exterior to make it smooth will help.

Look again at the black, white and blue trailer above in the section “Myths About Trailer Aerodynamics”.  That has a nice base shape, so if you remove the awning, and the roof-top units; close the vents, and remove or recess all the exterior handles; you can take advantage of the nice shape.  That might be more inconvenient than it’s worth – but that’s a decision you will need to make.

For commercial truck drivers, a few percent savings will add up.  For casual trailer-ers, conveniences might be worth more than aerodynamics.  Especially since the really aerodynamic trailers tend to be super expensive.  Don’t get fooled into thinking you’ll get huge mpg improvements by adding this or that.  The physics are against you.  Adding things increases weight which can have a stronger negative effect than the positive of aerodynamics.

Sorry if that sounds negative.  I just don’t want you chasing empty promises.  Again, the biggest effect you can have is to slow down.  Yeah, I don’t like that either.

What Have We Learned For Trailer Aerodynamics?

I hope the above information is valuable.  The field of aerodynamics is almost one of those black arts where only the magicians can figure it out.  I say almost, because we do know a lot about it.

As I have studied aerodynamics the one key bit I keep coming back too . . . nothing is sure, and nothing works independent of the rest of the system.

If we do something for a huge effect on trailer aerodynamics, it might affect fuel economy only a little.  Well, weight and speed have the biggest effect on fuel consumptions, so that makes sense.  Well, even if the improvements are small, they are still improvements.  Do it.

As mentioned above, the greatest effect you can make for your vehicle – trailer or not – is speed.  The faster you move through the air, the more disturbance you cause.  “Small, Smooth, Slow” – for aerodynamics.  Add “lightweight” for fuel economy.  Well, even if it’s true, it doesn’t sound very exciting.

Good Luck making your Aerodynamic Improvements.  Next up . . . Where is the weight in your trailer?

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