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Why is Your Box Trailer so Hard to Tow? Part III
When it comes to towing ease and fuel economy, it’s hard to over-emphasize the importance of the frontal area and the drag coefficient of a particular trailer design.
The frontal area of a trailer is simply how tall it is multiplied by how wide it is. But the drag coefficient can only be approximated by some terribly complex mathematical calculations, or by wind tunnel testing.
Yes, but what about the fact that there’s always a tow vehicle pushing the air out of the way in front of your bike hauler? Yep, that cuts down on the wind resistance your trailer meets—some, but not as much as you might think. Just remember how much the wind buffets your bike when you tow it on an open trailer. A trailer with a smaller frontal area will still meet less wind resistance than one with a larger frontal area.
Where can you get the drag coefficients for different trailers? Don’t hold your breath waiting on trailer manufacturers to supply them. And unless you work for a well known racing team, you probably don’t have access to a wind tunnel. So where does that leave you? Take a look at these published drag coefficients I dug up.
Shape Drag Coefficient
Round nosed bullet .29
Sphere .30
2003 H2 Hummer .57
18 wheeler with fairing .65
Prism (triangle with depth) 1.14
Rounded cube 1.20
A smooth brick 2.1
Square-corned cube 2.2
So how can you use this information? Stand 100 feet back from the front of the trailer you already have or are thinking about buying. If it’s seven feet wide by seven feet tall, it may be called a low hauler or low profile motorcycle trailer, but its frontal area is still forty nine square feet and it has a drag coefficient near the brick end of the scale.
Yeah, but what if its nose is not flat? When it comes to frontal area, shape doesn’t matter, but remember the shape of the nose primarily determines the drag coefficient of trailers. And if the trailer in question has a flat nose, it’s just like a big shiny not-so-smooth brick (too many corners, rivets, handles, etc.). So you may as well plan on keeping your fuel card on the console next to you because you’ll need it too often to bother putting it away.
What about V-noses? Hmm, a v-nose sounds a lot like a prism shape to me and that suggests a drag coefficient somewhere in the 1.1 to 1.3 range. What if it has a downward sloping nose? Suppose forty percent of the height of the trailer is sloped. Then the top forty percent of the trailer will have a drag coefficient in the 1.1 to 1.3 range and the bottom sixty percent will still be greater than 2.0
A bulbous add-on to the top half of the front of a trailer has much the same impact as a sloped nose. The top half has a reasonably conservative drag coefficient—probably a little less than 1.0, but the bottom half still has a coefficient greater than 2.0
Before you go, take another look at the scale above. Notice that a round-nosed bullet has the lowest drag coefficient (.29) of all the things listed. There’s currently only one widely available, purpose-built motorcycle hauler shaped much like a round-nosed bullet—Ironhorse Motorcycle Carriers. No wonder their owners are so happy with their gas mileage and keep calling us saying—“you don’t even know it’s back there”.
