Learn me: tiny box trailer aero improvements and the law of diminishing returns

Ook, so long story short, I have a very well built/sturdy aluminum box trailer, about 3.5-4' square and about 3' high (bottom of box to top of box) that I plan to use as a tire trailer behind my CSP Miata, and I want to improve the aero on it considerably to make it nicer to tow and lessen the mpg hit when towing it. I've got some ideas already, amounting to trying to replicate the old Gr.5 BMW 320 DRM bodywork , along with a full flat bottom, sizable "splitter" at the front, tiny wings to direct the airflow over the top, tiny wings to direct the airflow down the back to meet up with the air coming from underneath the trailer, a mild diffuser to help direct some of that bottom airflow up (and make some meaningful downforce hopefully), put a chin on the front of the fenders and give the rear of the fenders the full DTM treatment (vented, continues out to the back of the trailer, etc) and put wheel spats on. I know I'm insane, but is trying to out-aero a mid-'00s F1 car with a box trailer stupid, or is there at least some merit to it?

Slyp_Dawg wrote: I know I'm insane, but is trying to out-aero a mid-'00s F1 car with a box trailer stupid, or is there at least some merit to it?

Probably not much merit. A trailer that small has virtually no frontal area exposed when sitting behind the tow vehicle.

Turbulence, yes. Primarily between the vehicles. But I don't know how you'd accurately measure it.

Unless you are towing with a pedal powered car, I doubt you will be able to aero improve the fuel economy to any measurable level.

Regarding being "nicer to tow", the biggest discomfort with towing a small trailer is not the aero. It is the fact that they are too light, and tend to bounce. They don't stay planted on the road. It becomes a violent weight of several hundred pounds yanking back and forth on the bumper.

However, if you do all that to a mini box trailer and post pictures, I'll be the first to vote up your thread for the shear wacko cool factor alone.

What about tires? I've never looked/shopped for fuel economy tires but that should help since the majority of resistance, up until 50-60 mph or so, is rolling resistance. After that it's all wind resistance, as it's more exponential and the rolling resistance has already plateaued.

I looked up on tirerack, and they have tires labeled LRR (low rolling resistance). One thing to consider is tire load capacity but with such a little trailer, it shouldn't be an issue. Same with a speed rating.

Lower the trailer and make one of these (appropriately sized) for your miata?

The Aero of the trailer is 100% dependent upon the aero coming off the back of the vehicle towing it. A Miata might not punch a big hole on the air, and is probably generating a lot of turbulence to the trailer.

For what to do, I suggest looking at the big boys and see what they are up to.

FWIW

Now I want to see a trailer with box flares.

pinchvalve wrote: The Aero of the trailer is 100% dependent upon the aero coming off the back of the vehicle towing it. A Miata might not punch a big hole on the air, and is probably generating a lot of turbulence to the trailer. For what to do, I suggest looking at the big boys and see what they are up to.

I agree, however don't forget that the big boys are really happy when they can gain 0.25 mpg. Most cars can't even measure that.

We are doing diesel fuel additives. Our target is 10% improvement, but the starting point is 6 mpg. That's only 0.6 mpg improvement.

The difference is that we can prove it on a dyno and long term steady over-the-road tracking. You'll never drive a Miata a million miles.

How much of a reduction in fuel economy are you currently seeing when towing?

SVreX wrote:

pinchvalve wrote: The Aero of the trailer is 100% dependent upon the aero coming off the back of the vehicle towing it. A Miata might not punch a big hole on the air, and is probably generating a lot of turbulence to the trailer. For what to do, I suggest looking at the big boys and see what they are up to.

I agree, however don't forget that the big boys are really happy when they can gain 0.25 mpg. Most cars can't even measure that. We are doing diesel fuel additives. Our target is 10% improvement, but the starting point is 6 mpg. That's only 0.6 mpg improvement. The difference is that we can prove it on a dyno and long term steady over-the-road tracking. You'll never drive a Miata a million miles. How much of a reduction in fuel economy are you currently seeing when towing?

what kind of improvement are those guys seeing with the tail, skirts, super singles and mooneyes over the regular old trailer with DRW?

Sloping the front of the trailer box downwards will help a bit, more so than a v-nose from what info I have found out there. Also tapering the tail of the box will help, along with side skirts.

Here is an aero project for a semi with substantial improvements: http://ecomodder.com/forum/showthread.php/airflow-bullettruck-project-aerodynamic-modified-class-8-transport-20704.html While quite a bit bigger vehicle, the ideas can transfer over.

Those "wings" used to fill low-pressure areas on the back glass of rally cars and SUVs are NOT AS GOOD as actually filling in the bodywork there, like the "trialer tail" you see above. So give it a teardrop tail and pointy nose and those are the easy gains. Next maybe some "wheel pants" like small aircraft have on their non-retractable landing gears.

Got those itty-bitty trailer tires on there? They create a remarkable amount of drag

I've got a little harbor freight trailer with itty-bitty wheels, and a larger (but still small) box trailer with full sized truck tires. Pulling the little HF drops my gas mileage more, and I feel it much more.

foxtrapper wrote: Got those itty-bitty trailer tires on there? They create a remarkable amount of drag I've got a little harbor freight trailer with itty-bitty wheels, and a larger (but still small) box trailer with full sized truck tires. Pulling the little HF drops my gas mileage more, and I feel it much more.

That makes zero sense, something else HAS to be at fault. Do they both have some large air brake-like structure behind the wheels that the large wheels are "plowing" for by any chance?

foxtrapper wrote: Got those itty-bitty trailer tires on there? They create a remarkable amount of drag I've got a little harbor freight trailer with itty-bitty wheels, and a larger (but still small) box trailer with full sized truck tires. Pulling the little HF drops my gas mileage more, and I feel it much more.

Probably more wheel bearing quality than size of the wheels, but that's just a guess.

Radiused belly pan and nose, plus wheel pants. Basically make the front face of the trailer slightly smaller than the back of the Miata and cover all of the hadrware that is hanging out in the breeze. The belly pan will help plant the relatively light trailer to minimize sway and bounce on hard suspension. The wheel pants are similar to the wheel spats mentioned by the OP, but cover the wheels everywhere except where the ires need to touch the road (these should run all the way down to the bottom edge of the rim to give some flat safety).
The tail treatment can be a pretty simple extension to start the teardrop shape. It does not need to be very long, just enough to give the air a direction to the back of thetrailer instead of a sharp edge.

GameboyRMH wrote: That makes zero sense, something else HAS to be at fault.

Itty bitty wheels spin like mad compared to big wheels when going down the highway. A good bit of energy is eaten up making that spin. All that spin generates more heat in the bearings as well.

Itty bitty wheels have a much harder time getting over the bumps and lumps in the road, while big wheels just glide over them.

Shape it like a Teardrop?

foxtrapper wrote: Itty bitty wheels spin like mad compared to big wheels when going down the highway. A good bit of energy is eaten up making that spin. All that spin generates more heat in the bearings as well. Itty bitty wheels have a much harder time getting over the bumps and lumps in the road, while big wheels just glide over them.

Bearing friction shouldn't make any significant difference. The better ability to roll over bad roads is a strong possibility though.

Heh, I used to get better milage when driving my Miata with the HF trailer attached than I did without it, but that's because I was forced to drive a significantly lower speed. :)

GameboyRMH wrote:

foxtrapper wrote: Got those itty-bitty trailer tires on there? They create a remarkable amount of drag I've got a little harbor freight trailer with itty-bitty wheels, and a larger (but still small) box trailer with full sized truck tires. Pulling the little HF drops my gas mileage more, and I feel it much more.

That makes zero sense, something else HAS to be at fault. Do they both have some large air brake-like structure behind the wheels that the large wheels are "plowing" for by any chance?

Actually, if you think of the tire as a lever, and consider bearing drag, it kinda does... a larger tire has a longer lever arm to act against the bearing drag. If that doesn't help, think of how much easier it is to break loose an axle nut with a 4' section of pipe on the breaker bar versus just a bare 1' breaker bar

And yes, the trailer has itty bitty little 16" diameter tires on 8" rims, they are so adorable

Strizzo wrote:

SVreX wrote:

pinchvalve wrote: The Aero of the trailer is 100% dependent upon the aero coming off the back of the vehicle towing it. A Miata might not punch a big hole on the air, and is probably generating a lot of turbulence to the trailer. For what to do, I suggest looking at the big boys and see what they are up to.

I agree, however don't forget that the big boys are really happy when they can gain 0.25 mpg. Most cars can't even measure that. We are doing diesel fuel additives. Our target is 10% improvement, but the starting point is 6 mpg. That's only 0.6 mpg improvement. The difference is that we can prove it on a dyno and long term steady over-the-road tracking. You'll never drive a Miata a million miles. How much of a reduction in fuel economy are you currently seeing when towing?

what kind of improvement are those guys seeing with the tail, skirts, super singles and mooneyes over the regular old trailer with DRW?

they must be seeing quite a bit of improvement, given that i see more and more of them on the interstate every week.. i have noticed that they do leave a smaller "hole" in the air behind them when they've got all that aero stuff on them than when they don't- my Camaro gets buffeted worse behind one of the aerodynamic rigs than it does behind the big rolling bricks, but the rolling bricks kick out a bigger wall of air around the sides than the sleek rigs do, which you notice as you go past the door on the cab as you pass them.. these are the kinds of things that you notice just from driving around them at 75 mph in a relatively small car..

GameboyRMH wrote:

foxtrapper wrote: Itty bitty wheels spin like mad compared to big wheels when going down the highway. A good bit of energy is eaten up making that spin. All that spin generates more heat in the bearings as well. Itty bitty wheels have a much harder time getting over the bumps and lumps in the road, while big wheels just glide over them.

Bearing friction shouldn't make any significant difference. The better ability to roll over bad roads is a strong possibility though.

the better ability to roll over bad roads probably adds up. It's like taking a skateboard wheel and rolling down the sidewalk as opposed to a BMX tyre.. the Bike tyre will not get "jarred" with each small imperfection as it has to switch directions from rolling to also lifting over the bump.

As for the big trucks. When I was driving commercial, the truck came to us without any aero. I slowly started adding some. The factory front airdam went on first and then a "spoiler" on the roof of the cab to help the air up and over the box. MPG went from 11 to 13 on those two additions alone

With the manufacturing quality on some HF stuff being what it is, you might also check the wheel alignment of the trailer. IE, relative toe. If thats off it could mess with efficiency.

Relube the axle with better grease?

Any aero stuff would probably be easily evaluated with a string test.

I am intrigued by the idea of a metallic brown Miata

looking at the miata airflow pics.. could you make a small "spoiler" bubble on the front of the trailer that overhung the trunk on the miata to take advantage of the air flow off of the convertable/hard tops?