If you have a really big engine, very big fuel tanks, very low gears, and an unlimited budget, then you probably don’t give a hoot about wind resistance or the quality of the aerodynamic shape of your fifth wheel or trailer. Most of our frugal customers, however, are very interested in the cost of operation of their tow vehicles, and so should be concerned about aerodynamics.
So, how much force/power it takes to move the shape of the load down the road is a major concern of ours, and one of the big parts of our developed GS Specification for aerodynamics. Total weight of the load is important but many years of fuel economy and performance runs have proven to us that the shape of an RV is often more important that its weight when it comes to total rolling resistance.
We have all had experience with the type of resistance wind can create at highway speeds. When we were kids, holding our hand outside the the window of the car and "flying" our hand by moving our hand like an airplane wing was a common (and pretty fun) game. Remember what happened to your hand when you turned it palm straight forward? If you weren’t ready, you might have ended up with a bruised arm when it smacked the back of the door. Remembering this, it’s easy to see just what a tremendous resistance impeded air flow can cause at highway speeds.
Nature has taught us that perhaps the best shape we could use for a vehicle the size of an RV moving down shot of the oncoming air. The air traveling below the unit should be able to flow unimpeded from beneath the truck to beneath the trailer and out the back. This is why our GS specification calls for this panel to be above the lower part of the tow vehicle bumper.
Tests, such as "roll down tests", help prove the effectiveness of aerodynamic improvements (Roll down tests are the poor man's wind tunnel). Here is how it works. First, find an old WWII airport in the Western Desert that is very flat, has very long runways, and (this is an important one) no air traffic. Next, get a tow vehicle and it's RV up to 70 MPH and at a predetermined point on the runway, put the tow vehicle in "neutral" and see how far it will coast. Note how far you got. Now make a change to the RV shape and repeat the test.
If the improved RV coasts farther, then we must have done something right and we can calculate a percentage of change. If the RV doesn't coast as far as the previous test, then we did something wrong. This is a very simple but effective way to test RV aerodynamic improvements and we are not aware of many manufactures who go to the effort to make such improvements, or who even bother to test them.
Handling and Weight and Balance
Because a fifthwheel’s load is concentrated at the rear axle and not “levered” through the frame to the front steering mechanism, fifthwheels automatically handle better than a travel trailer.
In fact, the extra weight of a good handling fifthwheel should make your truck drive better, not worse. When towing a properly balance fifthwheel, unless you take a peek in your rear view mirror now and then, you may forget that your towing at all!
Of course, not all fifthwheels are good handlers. Over the years we have found that in order to get the best handling possible from our fifthwheels, we should put approximately 20% of the total fifthwheel trailer weight on the kingpin. We can do this by simply locating the axles forward or backward, just like a big teeter-totter. The kingpin (pin) weight on any fifthwheel could then be changed to 0 lbs, 500 lbs., 2000 lbs., 3000 lbs. etc. simply by changing the location of the axles.
A few months ago I was towing one of our GS35/6RL smoothly up a Tehachapi mountain grade the highway is the tear drop. The tear drop shaped Goodyear Blimp (which travels from football game to football game at speeds close to RV speed) would agree. Creating the front of the tear drop shape, the big nose of the fifth wheel, is not much of a design problem since a fifth wheel bedroom fits into that space just fine. However trying to live or store much in a pointed hind end is another story. I doubt we’d sell many fifth wheels with a hind end narrowing down to a point.
The pointy hind end of the tear drop shape allows the air to travel around the fifth wheel smoothly so as not to create turbulent air and perhaps even a vacuum behind the fifth wheel to slow us down. What we have found that works best as a compromise to total pointyness is to taper the roof to the rear wall area and angle the lower portion of the rear wall back toward the front of the RV. The resultant shape is not a point but it does help the air converge behind the RV progressively.
The problem with this design is that it costs more to build than a flat back, and many RV manufacturers have found the cost prohibitive. So in the interest of keeping costs down they have kept a flat and vertical wall that does nothing to help the air flow around the RV and reduce wind resistance. Since the rear end of an RV can create up to 50% of the total aerodynamic drag, this is an important point to consider.
Perhaps many RV manufacturers have been spoiled by today's big gas and turbo charged Diesel engines and think that powering through this wind resistance is good enough. Of course, those manufacturers don't have to pay the fuel bills.