box trailers, ironhorse trailers

Which is a Box Trailer so hard to Tow!

Since we’ve been in this business we’ve met some interesting characters and heard some pretty wild trailer towing stories.  My favorite was the guy who couldn’t believe the gas mileage he didn’t get pulling his shiny new low hauler box trailer from Tennessee to Sturgis with a Suburban.  I asked him how bad it was and he said he wasn’t sure but he would check on the way home.  A few days later, back in Tennesse, he showed up and said he got a little less than seven miles per gallon on the way home.

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)

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