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”.

Naturally I asked several questions about brakes dragging, unusual tire wear, etc. but he had all the right answers.  Then he came up with the REST OF THE STORY—something I won’t soon forget.  He said “You know where I-24 crosses Monteagle east of here, there are seven miles of 5% grade down the other side.” Both were facts I knew to be true.  Then he said, “On the way back from Sturgis, I deliberately sped up to 80 miles an hour at the top of Monteagle and when I started down the other side, I kicked the Suburban into neutral.  Would you believe it slowed down to 20 mph going downhill with a motorcycle and a whole lot of other stuff on board.”  Not surprisingly, he is now the proud owner of an Ironhorse painted to match his Suburban and much happier with his gas mileage.

At any rate, that got me thinking about what really determines how hard a trailer is to tow and what role the design of the trailer play.  After a few hours surfing the net and talking to some of the folks from the U.S. Air Force Arnold Engineering Development Center right down the road from our place, I dug up some pretty good info. I’m going to share with you via a series of blogs.  This is the first of three.

How hard a trailer is to tow is a function of the total forces trying to keep it from moving forward:

Weight X Acceleration
+    Rolling Resistance
+    Grade (< 0 for downhill, > 0 for Uphill)
+    Wind Resistance
———————————————————
=    Total Resistance

Let’s look at the forces that try to keep a trailer from moving forward one by one.  The first one is weight multiplied by acceleration.  The bottom line here is that, generally, fiberglass and aluminum cycle haulers are much lighter than steel ones of similar size and shape.  So everything else being equal, fiberglass and aluminum cycle haulers are easier to tow than steel ones.

The second force that tries to keep a trailer from moving forward is rolling resistance.  Imagine taking a tire and rolling it down a long hallway.  As long as you continue to push it occasionally, it’ll keep moving.  But if you stop pushing, the tire will coast to a stop.  Rolling resistance makes that happen.  Now imagine you were rolling two tires down the same long highway.  Clearly you would have to keep pushing both of them to keep them rolling.  Two tires have roughly twice as much rolling resistance as one and four twice as much as two.  So everything else being equal, trailers with tandem axles exert more rolling resistance than trailers with single axles.

The third force that tries to keep a trailer from rolling forward is grade.  It can work for you or against you.  Think about that long hallway again.  If it slopes downhill enough, the force of gravity will overcome the rolling resistance and the tire will move faster and faster as it rolls down the hallway.  The reverse is also true.  If the hallway slopes uphill  enough you can roll the tire forward but it’ll stop pretty quickly and roll back toward you.  So everything else being equal, your trailer will experience less resistance going downhill than up.

The fourth force that tries to keep a trailer from rolling forward is wind resistance.  That’s where it gets really interesting because with respect to towing ease and fuel economy, the 800 lb gorilla in the room is wind resistance.  Exactly why that’s true and how it relates to the kind of trailer you have is a little more complicated than the things I’ve talked about so far.

That’s enough for now.  Next time we’ll explore what determines wind resistance.

(To be Continued)