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Travel Trailer Aerodynamics: A Mid-summer Update

Houston to Alaska – 5,128 Miles

John P. Bridge (7-21-2007)

In mid-May of 2007 my wife Patti and I set out from Houston, Texas, to cover a very large part of North America in a heavy-duty Dodge Ram pick up truck, pulling a 32 foot travel trailer. I had rebuilt the nose of the trailer to improve its aerodynamics. (Please see Travel Trailer Nose Cone Aerodynamics for a full account of the design and construction of the nose.)

Leg 1 – Houston to Phoenix

The first 400 miles of the trip were somewhat disappointing. I had attained an average of 9.1 miles per gallon on a very short road test prior to departing, and on this I had estimated that across the relatively flat areas of the Southwest I might be able to approach 10 mpg over level stretches and average 9 mpg overall. But for the entire first day of travel we were faced with a very brisk crosswind that veered from the quarter (approximately 45 degrees) to about 90 degrees off the nose. So for my first two fuel stops I logged 8.2 and 8.1 mpg respectively, and at that rate I would be hard pressed to maintain even an 8 mpg average overall. Even though 8 mpg was a fair improvement over the 7 mpg pre-modification average, and knowing that the wind conditions had been unfavorable, I was a bit disheartened at the end of that day.

Next morning there was little or no wind, as is usually the case, but I knew it would pick up about mid-day and continue into evening. I sailed along during the morning and logged a run at about 9 mpg, which meant that if the afternoon run was anything like the previous day’s work, I could expect a daily average of around 8.5, far short of the 10 mpg target. But to my surprise, when the wind did pick up it came not from the front quarter but from the rear, and the tail wind held throughout the following day. I logged 10.9, 11.1, 10.0, and 9.8 mpg successively. And over that period my gloom was replaced with mild elation. I knew that a 9 mpg average was at least possible, and in fact, the run from Houston to Phoenix (about 1,200 miles) showed us a fuel economy rating of 9.7 mpg, representing a 38 percent increase over pre-modification rates.

John and Pat Bridge upon arrival in the Phoenix area.

Leg 2 – Phoenix to Seattle

There are, of course, hills in West Texas (along I-10), but they are not particularly daunting, and cruising through Arizona, one can expect mostly level road. Entering California is another matter, as there are some fairly steep grades in the south and a number of actual mountains in the north. Oregon and Southern Washington are hilly. It is in this type of terrain that the weight of the vehicle becomes more of a concern. I have not weighed it, but I estimate the Puma weighs about three and one-half tons, and I probably have a quarter-ton of gear in the back of the pick-up. With passengers, then, my payload is right at four tons. This plus the weight of the truck must be pulled up hills by a 5.7 liter Hemi V-8 engine. The Hemi is equal to the task, but it demands extra fuel on the way up. Even so, I managed a fuel economy rating of 8.7 mpg on this leg.

The desert crossing from Phoenix to Los Angeles is notorious for its strong crosswinds. Crosswinds slow a vehicle by causing added friction to its sides, top, bottom, front and rear, and I have come to the conclusion that these winds are more detrimental to good fuel economy than those coming straight from the front. On that segment of the run we managed about 8 mpg.

All in all, I was well pleased with the 8.7 mpg. It represented an increase of 24 percent over pre-modification rates, a substantial savings in my fuel bill.

RVcampground at Fort Worden State Park, Port Townsend, Washington.

The terrain from Seattle to Alaska (Anchorage by way of Fairbanks) can be described as hilly. Even though much of the trip is through the Rockies, the passes are fairly low. The highest point reached, as I recall, is around 5,000 feet. But as I mentioned, hilly conditions take their toll.

Additionally, one has to consider general road conditions in much of Canada and Alaska. We had good roads until we reached the Cassiar Highway in Northern British Columbia. From that point onward we encountered multiple frost heaves, rough pavement and lack of pavement altogether. There are many construction areas where the roadway is dirt or gravel -- much starting and stopping and periods of continuous driving in the lower gears. The Dodge will not shift into overdrive at speeds lower than 45 mph.

There is an advantage to driving at lower speeds, though. Wind resistance becomes much less of a factor, and that condition offsets the loss of momentum. As a result, I never incurred a reading of less than 8.2 mpg during the Cassiar run (about 450 miles).

The Cassiar intersects the Alaska Highway just west of Watson Lake in the Yukon Territory. The road there is good, and I began thinking my troubles were over. I had learned from fellow travelers that the final 100 mile stretch before the Alaska border had patches that were under construction, but I looked forward to a couple hundred miles of good road until then.

It was not to be. About 30 miles west of the Cassiar-Alaska Highway intersection severe frost heaves became apparent, and they continued unabated until we reached the construction areas nearer Alaska. So there were many stretches along this segment where cruising speed – 50 to 60 mph – could not be maintained. Still, it was better than the Cassiar, and I was able to log runs at around 8.5 mpg.

Frost heaves are a problem everywhere in the north country, and they remained a problem after leaving Canada and entering Alaska. There seem to be fewer construction areas in Alaska, though. From the border through Fairbanks and on down to Anchorage we managed runs at well over 9 mpg. The final tally showed a fuel economy reading of 8.9 mpg for the segment from Seattle.

Delta Junction, Alaska – the final mile of the Alaska Highway. Mile One is at Dawson Creek, British Columbia. We’ll head that way when we leave Alaska.

I calculate the average for the entire trip from Houston to Anchorage to be 9.3 mpg, three-tenths of a point over my goal of 9 even and a 33 percent gain over the pre-modification 7 mpg. The reading was attained over a run of more than 5,000 miles in all types of terrain with road conditions that ranged from excellent to putrid. Winds came from all directions, but since the trip was taken during summer, winds were easterly and southerly much of the time. Crosswinds were always a factor, though.

I can’t be more pleased with the result of my trailer modification. I expected much interest in the design at RV parks and other stopping points, but people seem not to notice. Or if fellow RVers do notice the front of my trailer, it’s very possible they simply consider the adjustment to be reasonable but unremarkable. Only two or three people have asked me whether I did the work myself or had it done professionally. I think there would be more interest if people knew the modification saved me $719 in fuel purchases over the course of the trip. This is based on the assumption that without its new nose the trailer would have rendered fuel usage rates of 7 mpg, but it is certainly possible that fuel consumption would have been greater in the hilly western regions of the continent. This is to say that my fuel savings estimates are probably on the conservative side. We actually spent $2,179. That figure would have been $2,898 or more without the nose modification.

Would I change the design? Possibly, but not being an aeronautical engineer, I’m not sure. I have no means of wind tunnel testing at my disposal.

I’ve thought about the angle of attack. My design incorporates a center member that enters the wind at about 45 degrees from vertical and two side members that enter at a considerably lower angle. The result is a warped presentation with a contoured top. This shape, I think, directs wind up over the top of the trailer but also off to the sides. Were I to raise the side members to 45 degrees the result would be a single broad plane that would enter the wind at 45 degrees. I don’t know whether this change would be significant one way or the other, but I know it would be much easier to build.

Of the three members which form the top of the nose, if the two outer ones were raised a plane would be formed that would enter the wind at a higher angle of attack. The nose itself would then be a rectangle instead of a triangle.

We still have over one-half of the summer trip before us, and once out of the Rockies the terrain will be generally flatter. If we experience a significant increase in fuel economy over that portion of the trip, I’ll update the article again. Until then, happy trailering.

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