wind drag/weight conversion

A person that delivers rv trailers has posted numerous times, the WORST mpg trailer is a ball mount with corrugated aluminum sides. THe same trailer with smooth fiberglass sides weighing upwards of 700 lbs more, will net better mpgs. Not much, but better. Still worst than a front bedroom slide 5w, not as good as a non front bedroom slide 5w, nor as good as an airstream! ANY trailer with the aluminum siding is worst than its heavier counterpart with smooth fiberglass walls.

Also, back in 92 when I bought my TT, on the wall of the dealer, was a poster showing what Ford's recommendations were at the time. Generally speaking max trailer with a 460 was 10K lbs "IF" the trailer had less than 80 sq ft of frontal area. from 81-100, you were to lose 2500 lbs, or max trailer of 7500, 101-120 was reduced another 2500 lbs to 5000 max. And it was not recommended you tow a trailer etc over 120 sq ft.

While a lot of folks will not keep frontal area, or its ease of going thru the air as a factor in towing performance, it should be a factor just as weight is. Tow ratings are generally speaking, performance ratings, ie will it pull the load up a 12% grade as in the J2087 specs in first gear. WIll it hold at least 35mph on a 5% grade in 100F temps with AC on and no overheating etc. If you are like me, and pull 20-25% grades locally frequent enough that 12% grades are a ho hum grade, I will be stalled out on some of these grades at gcwr! as the spec is not stiff/high enough for how and where I tow at. So I do use some forumulas to make sure what ever light duty truck I buy since my first stall out on a 20=% grade at 12K with a 16K gcwr 1 ton/454/th400 trans/4.10 gears. Since that time in 1991, I have yet to trust a manufacture gcwr to get me to where I need to go! My 96 6.5td with a measly 12500 gcwr, could pull 20K up a 30% grade. The 05 dmax, all of 25% at 20K! so less grade. BUT, the dmax at 20K on a 4% grade would be doing some 20 mph more than the 6.5 due to more HP. But the torque multiplication with the NV4500 was better than the allison behind the dmax. Hence why the one went up a steeper grade at slower speeds.

More than one way to look at what is a good tow rig.

marty

Missing is the drag coefficient as Ron mentioned. Meaning a squared
frontal area will have much more drag than one shaped like a bullet

This should also include the ground to trailer bottom...as the more
this gap is....the more turbulance to cause underbelly drag



Drag coefficient

Automotive drag coefficient

More to it I think.
Old TT, 5000lbs, 9'5" high, 10mpg average
New 5th, 9000lbs, 12'6" high, 12mpg average

Old TT had flat angled front, new 5er has rounded front cap. What about differences in drag between a TT behind the truck and a 5th over the bed all else being equal?

dockmasterdave wrote:
---For every foot the trailer extends above the roof line of the truck, it approximates another thousand pounds of towed load.
Has anybody else heard this ratio, or does it sound close to you?

It depends on how you're moving the added load.

According to Aerodynamic Drag, the drag force at 60 mph on a frontal area of 8 sq ft (1' high by 8' wide) with a drag coefficient of 1.2 is about 88 lbs.
The extra power required to overcome this added force at 60 mph would be about 14 hp.

Now, let's ask how much additional load could be carried inside a trailer on a level surface at 60 mph so that the additional power required to overcome the added tire rolling resistance would be 14 hp.
Assuming a rolling resistance coefficient of 0.01 for a truck tire on a smooth surface (see Rolling resistance), the answer would be about 8750 lbs.

Or, we could ask how much additional load can be moved up a 6% grade at 60 mph with a power expenditure of 14 hp. The answer would be about 1250 lbs.
About 2 hp would be used to overcome the added rolling resistance, and about 12 hp would be used to move the extra 1250# up the 6% grade.

So the ratio between added height and equivalent added weight based on horsepower can vary widely depending on how you are going to move the added weight.

If you're concerned about the extra power required by additional frontal area, you can achieve a significant reduction in required power by reducing speed.
Power required to overcome aerodynamic drag force at 55 mph is about 50% of that required at 70 mph.

Ron

Mvander wrote:
I always figured weight affected getting it going or climing grades. Aero affected how easy or difficult it was to maintain hwy speed.

My experience also.

And yes, a strong tail wind makes a huge difference. I got over 16mpg one time but it was a 25mph tail wind the whole way.

path1 wrote:
And with her stuff loaded only dropped mpg another 1 1/2, so wind much more important than weight of load.

Oh, that is definitely true. I once towed a 20+ ft enclosed car hauler for a friend that had a frontal space about like my TT. I only got about 1 mpg better on the empty return trip (3500# car inside). A few months later, I towed a Suburban on a flat bed that probably weighed 8000# or more, but didn't stick up as high, and got better mileage than I've ever had towing my TT (max 6500# loaded).

I always figured weight affected getting it going or climing grades. Aero affected how easy or difficult it was to maintain hwy speed.

Carrying my truck camper gives me almost the the same mileage as towing a three times heavier toy hauler bumper pull trailer. Towing the enclosed trailer behind the truck camper gets me to the same weight as my toy hauler with about the same mileage. Towing only the enclosed trailer, which weighs twice as much as the truck camper, gets me better mileage than just hauling the lighter truck camper.

If one Dow some googling, you might find an actual formula for figuring this out. I have it at home from a how to spec an hdt truck. But off the top of my head I can not repeat it. It includes things like tire type and construction, road bed, ie asphalt vs green vs cured concrete vs crushed rc vs spongy material.

Figuring out just how good a manufactures over for your useage is quite fun. I even have a formula that can tell you how steep.of a grade you can go up before stalling.v out.


Marty

Thanks blt2ski : That is basically what I was wondering. I usually figured the factory tow rating x @70% was a comfortable tow weight.
I had never heard a simple formula like his.
Basically a 5000# wet trailer, with 3.5 feet above tow vehicle roof will tow like it weighs 8500#.