Am I Screwed? (Tow rating question.)

Floyd

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Thanks, I looked that up. I wonder how much it is reduced if someone goes to 60 sq ft frontal area?
A lot!
I don't know all the math , but the fact is that flat frontal area is like pulling a parachute down the road and it is every bit as important as trailer weight.
Weight is most important for acceleration.
Aero is most important for maintaining speed.
So you can tolerate slower acceleration better than high cruising speed.

I tow a Scamp which is the best aerodynamic design in RV trailers.
My 2.5L TC , at rated towing weight limit in combo with my Scamp can achieve 25MPG at 60MPH.
18MPG at 65MPH (Average conditions)
I have a friend with a tall flat front 14ft cargo trailer... The TC would strain to tow it empty and not much more strain loaded.

Bottom line is that when towing... speed is most expensive, so keep the speed down and stay on the road longer between stops.
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BcP28

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A lot!
I don't know all the math , but the fact is that flat frontal area is like pulling a parachute down the road and it is every bit as important as trailer weight.
Weight is most important for acceleration.
Aero is most important for maintaining speed.
So you can tolerate slower acceleration better than high cruising speed.

I tow a Scamp which is the best aerodynamic design in RV trailers.
My 2.5L TC , at rated towing weight limit in combo with my Scamp can achieve 25MPG at 60MPH.
18MPG at 65MPH (Average conditions)
I have a friend with a tall flat front 14ft cargo trailer... The TC would strain to tow it empty and not much more strain loaded.

Bottom line is that when towing... speed is most expensive, so keep the speed down and stay on the road longer between stops.
*blows dust off fluid dynamics book*

Aerodynamic Drag force is defined by the equation:

Fd = 0.5 x Air Density x Velocity^2 x Coef. Drag x Frontal Aera

  • Assume Air Density is 0.002263 slug/ft^3 (1.17 kg/m^3 for you non-Americans)
  • Assume Velocity = mph * 1.46667 = velocity in ft/s;
    • We'll use 50mph for this example (industry standard for referencing and comparing vehicle roadloads)
  • Assume Cd = 0.600 - SAE recommendation for a square frontal trailer of ~55 ft^2
  • Frontal Aera: we can compare differences between 55ft^2 and 60ft^2

So at 50mph, a 55ft frontal aera will have a drag of 200.8 lb and a 60ft frontal will have a drag force of 219lb. To put into perspective the affect of speed, at 70mph, the forces are 394lb and 429lb, respectively. For reference, the Ranger has a nominal frontal area around 35ft^2, so a nominal areo drag force of around 160lb at 70mph, assuming a Cd of 0.375 (guessing on this one).

On the surface, that doesn't sound like a lot of force, so probably better to put it in terms of power/energy consumption. Industry standard for measuring vehicle roadload is to measure the vehicle energy in terms of horsepower. (not sure why it's horsepower and not Watts, or some other unit of energy, but that's just the way it is, haha)

Drag Power (hp) = Fd * Velocity / 550

  • Ranger aero roadload at 70mph = 29.2hp
  • 55ft frontal trailer = 73.5hp
  • 60ft frontal trailer = 80.2hp

Finally, to put this into terms everyone is familiar with, think of it in terms of fuel consumption. Google says a gallon of gas is around 114,000 BTU/gal (it can vary, depending on the fuel obviously), but entertain me...you can caluculate fuel consumption using the following:

  • 114000 BTU/gal = 44.8 hp*hr/gal
  • Assume the Ranger 2.3L has a nominal thermal efficiency of 30%
    • Pretty typical for a modern gasoline engine
  • Assume 70mph steady state
  • Assume the roadload we calculated above

mpg = (0.3 x 44.8hp*hr/gal x 70mph) / roadload

  • Ranger mpg at 70mph = 32.2 mpg
  • 55ft Trailer mpg at 70mph = 12.8 mpg
  • 60ft Trailer mpg at 70mph = 11.7 mpg

Keep in mind this is strictly energy due to the aerodynamic drag of the vehicle, there are a lot of other things that affect the vehicle roadload, engine performance, and ultimately fuel consumption including my pretty broad assumptions, but hopefully that puts it into perspective.

Vehicle mass does play a role, but is much more dependent on the coefficient of friction of the surface, tires, bearings, etc. The changes in force due to weight for a fixed frontal area will be much smaller than the changes mentioned above, but that would be another WAY too long post ?
 

Str8sh0otr

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Keep in mind this is strictly energy due to the aerodynamic drag of the vehicle, there are a lot of other things that affect the vehicle roadload, engine performance, and ultimately fuel consumption including my pretty broad assumptions, but hopefully that puts it into perspective.

Vehicle mass does play a role, but is much more dependent on the coefficient of friction of the surface, tires, bearings, etc. The changes in force due to weight for a fixed frontal area will be much smaller than the changes mentioned above, but that would be another WAY too long post ?
This is an excellent post as an engineer myself I can appreciate. However, I am electrical by degree, so if I weigh in on this and am wrong I have a built in excuse :sunglasses:

As you point out at the end this is very nuanced. We would have to know the Cd for the entire rig, not just the trailer. I learned from Tom Cruise in Days of Thunder that the drag of two vehicles travelling in close proximity is less than the sum of the two when separate. When looking at it that way, does the 5 more sqft have the same impact than if looking at just the trailer as you are? I believe Cole Trickle would say no. (apologies if you haven't seen this movie...and my attempt at humor) Also, rather than just an "area" which the automotive industry assumes is all us mouth breathers can understand, it would be better to know the area, as well as theoretical drag and weight of the setup to make decisions. If the towing guide is taken at face value, you could argue that an 8' x 8' x whatever empty cargo trailer would be less appropriate than something slightly smaller at 7,500lbs. You could also argue that nobody with a Ranger should ever tow a travel trailer other than a pop-up. Bet that would ruffle some feathers.

Bottom line, the frontal area is presented in the towing guide as a consideration, not a hard and fast rule. For the weight police who are concerned with sway and stopping, it is not really a factor and may even help the latter. I believe it is presented for longevity of the powertrain, not safety. When towing up a grade heavily loaded...your frontal area is a consideration. You may also consider not pinning your foot to the floor, slow down, move in line with the semi's, and put on your flashers.
 

P. A. Schilke

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This is an excellent post as an engineer myself I can appreciate. However, I am electrical by degree, so if I weigh in on this and am wrong I have a built in excuse :sunglasses:

As you point out at the end this is very nuanced. We would have to know the Cd for the entire rig, not just the trailer. I learned from Tom Cruise in Days of Thunder that the drag of two vehicles travelling in close proximity is less than the sum of the two when separate. When looking at it that way, does the 5 more sqft have the same impact than if looking at just the trailer as you are? I believe Cole Trickle would say no. (apologies if you haven't seen this movie...and my attempt at humor) Also, rather than just an "area" which the automotive industry assumes is all us mouth breathers can understand, it would be better to know the area, as well as theoretical drag and weight of the setup to make decisions. If the towing guide is taken at face value, you could argue that an 8' x 8' x whatever empty cargo trailer would be less appropriate than something slightly smaller at 7,500lbs. You could also argue that nobody with a Ranger should ever tow a travel trailer other than a pop-up. Bet that would ruffle some feathers.

Bottom line, the frontal area is presented in the towing guide as a consideration, not a hard and fast rule. For the weight police who are concerned with sway and stopping, it is not really a factor and may even help the latter. I believe it is presented for longevity of the powertrain, not safety. When towing up a grade heavily loaded...your frontal area is a consideration. You may also consider not pinning your foot to the floor, slow down, move in line with the semi's, and put on your flashers.
Hi Folks,

I think this is not about absolute values nor how to calculate them, It is to provide the actual math behind the lay person's understanding that the larger frontal area of a trailer the likely degradation on fuel economy and increased horse power required. It is very difficult to provide scientific principles in lay terms.

Another back story,

I managed Ford Truck's Off Road Racing for 25 years of my 30 year career as a "side job". I was on TV quite a bit of time. I quickly learned that what I assumed was average customer understanding was way off. I filmed a commercial for Ford Tough through Off Road Racing. What I found was what the script had me say and what I really wanted to say conflicted. I was sat down and it was made clear that what I would relate as a normal customer's understanding was way, way too technical. Lightbulb came on and I followed the script. Yep...there was me on the tube for however long the commercial ran on TV. I did refuse to wear a white coat, as this is for Doctors, not Engineers...Sheesh.

Sidebar was that I had to fly to California to the garage of one of my off road teams. I had to stare into the Sun. Blinding...so they had me stare at the sun with eyes closed for about 10 minutes and then shot the segment. Then I had to fly to Nevada where the filmed a few more seconds. Glad I did not have to wear makeup.... This "glamour" is a lot of work. I go a bunch of ribbing at work when the ad played on Sunday Football and other TV spots..... Sheesh!

Best,
Phil Schilke
Ranger Vehicle Engineering
Ford Motor Co. Retired
 


Deleted member 1634

Hi Folks,

I think this is not about absolute values nor how to calculate them, It is to provide the actual math behind the lay person's understanding that the larger frontal area of a trailer the likely degradation on fuel economy and increased horse power required. It is very difficult to provide scientific principles in lay terms.

Another back story,

I managed Ford Truck's Off Road Racing for 25 years of my 30 year career as a "side job". I was on TV quite a bit of time. I quickly learned that what I assumed was average customer understanding was way off. I filmed a commercial for Ford Tough through Off Road Racing. What I found was what the script had me say and what I really wanted to say conflicted. I was sat down and it was made clear that what I would relate as a normal customer's understanding was way, way too technical. Lightbulb came on and I followed the script. Yep...there was me on the tube for however long the commercial ran on TV. I did refuse to wear a white coat, as this is for Doctors, not Engineers...Sheesh.

Sidebar was that I had to fly to California to the garage of one of my off road teams. I had to stare into the Sun. Blinding...so they had me stare at the sun with eyes closed for about 10 minutes and then shot the segment. Then I had to fly to Nevada where the filmed a few more seconds. Glad I did not have to wear makeup.... This "glamour" is a lot of work. I go a bunch of ribbing at work when the ad played on Sunday Football and other TV spots..... Sheesh!

Best,
Phil Schilke
Ranger Vehicle Engineering
Ford Motor Co. Retired
There are quite a few stupid people out there. As a fellow engineer who absolutely loves to turn everything into a math problem or excel spreadsheet, it is frustrating when others aren't on my level of caring or understanding for a topic. In college we used to have scientific debates and rap sessions on very mundane and normal topics, because it was fun to work our brains. One of my favorite things from going to a college filled with so many like-minded people!

Also, as far as I'm concerned, an engineers "uniform" is jeans and button-down shirt (maybe a polo in warm weather), usually plaid or flannel. haha Can't wear something too nice because we still do some hands-on work and can get dirty in the shop or on the production floor, but also have to look a little official and give off some status. haha
 

Floyd

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There are quite a few stupid people out there. As a fellow engineer who absolutely loves to turn everything into a math problem or excel spreadsheet, it is frustrating when others aren't on my level of caring or understanding for a topic. In college we used to have scientific debates and rap sessions on very mundane and normal topics, because it was fun to work our brains. One of my favorite things from going to a college filled with so many like-minded people!

Also, as far as I'm concerned, an engineers "uniform" is jeans and button-down shirt (maybe a polo in warm weather), usually plaid or flannel. haha Can't wear something too nice because we still do some hands-on work and can get dirty in the shop or on the production floor, but also have to look a little official and give off some status. haha
As Phil explained... sometimes it is best to convey the practical application, even if you arrived there with the math and can read.
In my job, I too had to deal with stupid people and smart ones.
The smart ones were teachable.
The stupid ones were not.

That included unskilled and skilled labor and engineers with degrees.
I learned a lot from each and taught a lot to each.
 
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Jimmy637

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As Phil explained... sometimes it is best to convey the practical application, even if you arrived there with the math and can read.
In my job, I too had to deal with stupid people and smart ones.
The smart ones were teachable.
The stupid ones were not.

That included unskilled and skilled labor and engineers with degrees.
I learned a lot from each and taught a lot to each.
And the, as you call them, Stupid ones, that thought they were smart.
 
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Glocker

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You can turn that all on with for scan though. It’s not a physical difference, just software.
Actually, I don't have the tow package and I have the option to turn this on when scrolling through the screen between the gauges. It's pretty cool!
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