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Old 04-07-2010, 04:27 PM   #1
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Trailer Drag

At the coffee shop today I was reading Road and Track and they had an article on vehicle drag. I thought it was interesting. Here are the highlights.

The drag caused by a vehicle is proportional to 1. cross-sectional area times 2. its shape (titled drag coefficient) times 3. its Velocity squared.

Cross-sectional Area.
The cross-sectional area of the typical Sunline is 8 ft wide by 9 high or 72 square feet. The cross-sectional area of the smaller Sunlines is 7 ft wide by 8 feet high or 56 square feet. The larger trailers have about 25% more area. This translates into 25% more drag.

This compares to a VW Bug at 19 square feet or a model T at 28 square feet. Trailers provide a big profile to the direction of travel. As well we frequently add things to the roof that increase the cross-sectional area like air conditioners and Maxx Air vents. Beyond increasing the cross-sectional area these additions are disruptive to the smooth flow of air.

Shape.
The Shape is very important. Ideally we want a shape that allows the free flow of air across and around the object. Since most trailers are designed for max internal space, little thought has been given to the exterior shape. Most people, at least as children have held their hand out a car window and have felt the effect of wind on your hand flat to the air and on edge to the air, a rather dramatic difference.

The flatter the surface the higher the drag coefficient. A barn door would have a drag coefficient of about 1 with it's flat side the the direction of travel. A water droplet is about 0.1 or 10 percent of a barn door. Good cars are around 0.3; a model T was about .8, near a barn door. Definitely the typical trailer is closer to a barn door than a water drop.

Newer trailer designs seem to take shape into consideration via the general shape, encapsulating things like tanks and making flat bottomed trailers. Some motorhomes no longer have roof air conditioners.
The ideal shape is closer to the small tear drop trailers. Scaling this up would produce something closer to the ideal.

Velocity.
This is the most important factor because the amount of drag is related to the square of the velocity. With a constant shape and cross-sectional area, the drag for 60 mph is about 50 % greater than 50 mph; the drag for 70 mph is about 100% greater than 50 mph.

Simply compare 50 times 50 = 2500 to 70 times 70 = 4900.

Since Model Ts did not go very fast they could have a large coefficient of drag and a large cross-sectional area without much penalty. Of course the price of gas was low and people did not drive very far.

Since we like our Sunlines, the cross-sectional area and the shape are defined; choosing our speed is all we can control to reduce our drag. As a long distance traveler we reduce the cost of fuel by not covering long distances in shorts amounts of time (we do no more than 150 miles a day and average a lot less), and by keeping our speed down.

On our yearly trips of 8,000 miles, we do it over 6-7 months reducing towing gasoline consumption to 60 gallons a month or about 1200 miles – cost $180 a month for towing fuel. We typically burn another $120 a month for non towing miles. This represents about 10% of our monthly budget. We can see this growing to 15-20% over the years as gas prices continue their rise – the price of oil has gone up 69% in the last year.

Of course all this information cost me two cups of coffee and two muffins - $8. Now that's about 3 gallons of gas or 60 miles of travel. Life costs.....
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Old 04-08-2010, 03:50 PM   #2
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Close to the perfect shape.

This site has a beautiful trailer about the size of our Sunline.

Very Charming.

http://www.trailerworks.biz/1200.html
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Old 04-08-2010, 08:33 PM   #3
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We have a cap on the bed of our PU TV.
That means that the truck has already punched through a good hunk of the air hitting the frontal area of the trailer.
I think ideally the trailer should be much closer to the TV, as in drafting like the racers do.
Since the truck cap has roof rack rails, I have wondered if there would be any benefit for having a deflector mounted near the aft end of the cap, sort of like the class 8's have on the roof of their cabs.
May be the stuff on the roof of the trailer negates much of the potential benefit.
I have never seen a deflector like this on a TV with a camper behind it, and I wonder if anybody makes one.
It would have to be easily lowered for when you don't tow.
Just thinking out loud here.
Roar
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Old 04-08-2010, 09:05 PM   #4
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Quote:
Originally Posted by Viking
Since the truck cap has roof rack rails, I have wondered if there would be any benefit for having a deflector mounted near the aft end of the cap, sort of like the class 8's have on the roof of their cabs.
May be the stuff on the roof of the trailer negates much of the potential benefit.
I have never seen a deflector like this on a TV with a camper behind it, and I wonder if anybody makes one.
It would have to be easily lowered for when you don't tow.
Just thinking out loud here.
Roar
Mooney (Greg) has one! It was a pretty neat invention too. It mounted to the top of his cap and would fold down flat for when the trailer isn't behind. It was adjustable too I believe.

I believe he got it through JC Whitney if I remember correctly. Hopefully he'll come in with the details.

I do have a picture if he doesn't have.

I remember talking to him about it at the M&G...I'm pretty sure he said he did get a mileage increase, but I can't remember how much of one, maybe 1 mpg?

Jon
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Old 04-09-2010, 07:25 AM   #5
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Thanks Jon!
So there are other people thinking in the same direction.
Hope Mooney chimes in.
I did a quick calculation.
If I tow 10.000 miles and get 12 MPG without a deflector and 13 MPG with one, and Diesel fuel is at $3 per gallon, the cost savings would be less than $200.
That makes it questionable if it is worth it.
Roar
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Old 04-09-2010, 11:39 AM   #6
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Roar, you are correct but gas prices in the UK just hit $9 a gallon. The price of oil has gone up almost $40 a barrel since the election, the value of the dollar decreases every day with climbing debt, and that cap and trade bill is still breathing in the senate that will add more taxes to fuel and energy costs.

I see US gas prices climbing and I don't see the end in the near term. The environmentalists fight everyone who wants to drill and the government does nothing to help them drill.

As RVers we at least need to consider what will help us maintain our lifestyles. As for me, I drive slower, probably the most cost effective thing a retired person can do with plenty of time.

At $4 a gallon, it will soon cost $40 in fuel to go 100 miles. I'm afraid at those prices a lot more RVs will be for sale.
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