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Why is Your Box Trailer so Hard to Tow? Part II
The force required to overcome wind resistance is given by:
Air density factor (another constant)
X Trailer air speed squared (a variable)
X The frontal area of the trailer (a variable)
X Trailer drag coefficient (a constant)
——————————————————————
= Force required to overcome wind resistance
If you look closely at the arithmetic above, a couple of things become clear. First, air density is not a function of your trailer design or your driving habits. It’s simply a constant for whatever air you happen to be traveling through—lower at higher altitudes and greatest at lower altitudes. It’s not really something you can mess with. For a given location, with minor differences for time of day, it is what it is. No more, no less.
Second, the other three factors that determine the total force required to overcome your trailer’s air resistance are much more interesting. In fact, you don’t have to be a mathematician to figure out that:
A trailer at 100 mph on a still day requires 100 times the force the same trailer requires to overcome wind resistance at 10 mph.
A trailer with twice as large a frontal area as a second trailer requires twice as much force as the second trailer requires to overcome wind resistance.
A trailer with twice as great a drag coefficient as another trailer requires twice as much force to overcome wind resistance.
Clearly wind resistance should be a primary consideration in choosing trailers and tow vehicles. Once you choose, you’ve pretty much determined how much gas mileage you are going to get at a specific air speed. Keep in mind that air speed is ground speed plus or minus wind. So maintaining 70 mph with a 30 mph hour headwind is as hard as maintaining 100 mph when the winds are dead still.
The implication for gas mileage is clear. No matter what you drive or tow, the one thing that you can do to get better gas mileage is slow down! At sixty miles per hour, for example, about 70% of the gas you burn is used to overcome wind resistance, and 30% rolling resistance. And the faster you drive, the worse it gets!
That’s enough for now. Next time we’ll get back to exploring how a trailer’s design determines its frontal area and its drag coefficient.
(To be continued.)
