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Old 05-09-2023, 08:40 PM   #1
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Weight vs Wind Resistance

Here's something I have been wondering about....

Many users come to this forum to ask questions about suitable tow vehicles, and it always seems to drum up a lively debate.

When I say "lively debate", I am intentionally mispronouncing

"argument between those who believe that everyone needs a 3/4 ton, those who believe that you should never buy a 3/4 ton when a 1-ton is only a couple thousand more, those who swear that they have been towing their 12,000lb 5th wheel with a 1972 Toyota HiLux for 50 years and it works just fine, and the person who asked the original question - who swears that the trailer is "within spec" for his rig.

"Lively debate" trips much more easily off of the tongue.

Here is my question: does anyone know whether any tow vehicle manufacturer has quantified the additional wind resistance of a travel trailer/5th wheel/utility trailer - and applied a weight factor to it?

Let me try to put that another way - picture a 6,000lb bass boat on a 2,000lb trailer. Total of 8,000lbs. Now picture a 33 foot travel trailer that also weighs 8,000lbs. I believe that pulled behind the same truck at 60mph, the travel trailer is heavier than the boat. A *lot* heavier. That boat is basically out of the wind, as the truck is already pushing most of the air.

I am using "heavier" as a term to compare the difference in the amount of work that the truck is doing to move both loads at the same speed.

Pulling away from the stop sign, 8,000lbs is 8,000lbs. As speed increases though, wind resistance (drag) also increases - but in a quadratic relationship. Simplified, at 10mph you have 4 times as much wind resistance as you did at 5 mph. At 20mph you have 4 times as much resistance as you did at 10mph, and 16 times as much as at 5mph.

This resistance can (could?) be measured as weight pulling backwards on the hitch ball. The parachute packs on the tail of a top fuel dragster only weigh around 10lbs each going down the track, but the 3 of them put a heck of lot more than 30lbs of force on their mounting brackets when they are deployed at 300mph.

I know that some truck manufacturers have a "frontal area" specification, but it doesn't seem to be be linked to the tow weight rating in any way, and definitely doesn't seem to be de-rating the max in any way. I might be crazy, but I believe that 12,000lbs of gravel/dump trailer combo is going to be a very different towing experience behind that "F150 Max Tow" than 12,000lbs of "1/2 ton towable" 5th wheel that has 48 square feet of frontal area above the cab of the truck.

Should manufacturers qualify their "tow ratings" by trailer type/height/frontal area?
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Old 05-09-2023, 08:46 PM   #2
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Not so much the weight as it is "Surface Area" that has a bigger affect with wind resistance. A 35 foot long, 12 foot tall Trailer has a heck of a lot more resistance that a boat of similar weight.
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Old 05-09-2023, 09:04 PM   #3
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You wouldn’t assign a weight factor based on drag any more than you would assign a drag factor based on weight. They are two different things. You would need a rating that combines the two, such as maximum applied force or some such physics term.

To your point about the wind resistance increasing dramatically with speed, that’s true but what does the weight do? Once the vehicle is moving at a steady speed, the weight is no longer a load at all, since things in motion tend to remain in motion. The only real load is rolling resistance and aerodynamic drag. So as the wind and friction resistance loads increased, the force needed to accelerate the weight goes away as the cruise speed is reached.
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Old 05-09-2023, 09:08 PM   #4
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Quote:
Originally Posted by TheWolfPaq82 View Post
Not so much the weight as it is "Surface Area" that has a bigger affect with wind resistance. A 35 foot long, 12 foot tall Trailer has a heck of a lot more resistance that a boat of similar weight.
In this context, I think that it is useful to think of wind resistance as being basically additional weight.

Imagine if you could put something like a fishing scale between your hitch ball and the trailer tongue.

When you start moving, that fishing scale is going to measure whatever the weight of the trailer is, plus a small amount of rolling resistance from the tires.

As your speed increases, the trailer is still the trailer - nothing has changed there - but the faster you go, the more the wind resistance increases, and the more "weight" is going to be measured by that fishing scale.

So I am using the word "weight" to encompass all of the forces that combine to resist the tow vehicle moving forward - trailer mass, rolling resistance, wind resistance, etc.
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Old 05-09-2023, 09:16 PM   #5
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Originally Posted by BehindBars View Post
I don’t see how or why you could assign a weight factor to an aerodynamic drag. They are two separate type loads on the tow vehicle. You could just as easily ask if a tow vehicle manufacturer has quantified a vehicle weight rating and applied a wind resistance factor to it. It doesn’t make sense since the two are not interchangeable.
But when you are pulling a trailer, all of those load are applied to the vehicle frame through the hitch ball.

The hitch ball doesn't know whether it is pulling a 3,000lb dump trailer with 3 tons of gravel or a 6,000lb Micro Lite with 3,000lbs of wind load. The ball is pulling 9,000lbs either way, but a trip to the scale would lead you to believe that the travel trailer is much lighter.

My point behind this thought experiment is that pulling a low, heavy trailer on the freeway at maximum tow spec (like a dump trailer full of gravel) would be a much different experience (for the driver *and* the truck) - than towing a travel (or other tall) trailer of the exact same scale weight.
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Old 05-09-2023, 09:17 PM   #6
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Originally Posted by BehindBars View Post
You wouldn’t assign a weight factor based on drag any more than you would assign a drag factor based on weight. They are two different things. You would need a rating that combines the two, such as total load, or maximum applied force.

To your point about the wind resistance increasing dramatically with speed, what does the weight do? Once the vehicle is moving at a steady speed, the weight is not a load at all, since things in motion tend to remain in motion. The only real load is rolling resistance and aerodynamic drag. So as the wind and friction resistance loads increased, the force needed to accelerate the weight goes away as the cruise speed is reached.
*on level ground
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Old 05-09-2023, 09:33 PM   #7
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Originally Posted by Qwkynuf View Post
But when you are pulling a trailer, all of those load are applied to the vehicle frame through the hitch ball.

The hitch ball doesn't know whether it is pulling a 3,000lb dump trailer with 3 tons of gravel or a 6,000lb Micro Lite with 3,000lbs of wind load. The ball is pulling 9,000lbs either way, but a trip to the scale would lead you to believe that the travel trailer is much lighter.

My point behind this thought experiment is that pulling a low, heavy trailer on the freeway at maximum tow spec (like a dump trailer full of gravel) would be a much different experience (for the driver *and* the truck) - than towing a travel (or other tall) trailer of the exact same scale weight.
Well I guess that’s why Ford, for example, acknowledges that the trailer frontal area will affect the performance of the towing vehicle. They probably don’t try to assign a value to that performance degradation because there are too many variables.

Example: https://www.ford.com/cmslibs/content...nger_Oct15.pdf
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Old 05-10-2023, 06:11 AM   #8
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Quote:
Originally Posted by BehindBars View Post
Well I guess that’s why Ford, for example, acknowledges that the trailer frontal area will affect the performance of the towing vehicle. They probably don’t try to assign a value to that performance degradation because there are too many variables.

Example: https://www.ford.com/cmslibs/content...nger_Oct15.pdf
Exactly-too many variables. It is a can of worms and everyone should know that speed of the vehicles (wind resistance) affects towability.
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Old 05-10-2023, 01:19 PM   #9
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Yep, uphill, downhill, no wind, headwind, tailwind, crosswind, pickup with wind deflector. Most big rigs I have driven had wind deflectors but light truck owners consider them too ugly even though they could save fuel.
Pulling strain on the hitch is far less than bending and twisting. Continuous pulling resistance has a big effect on cooling systems. The driver is the key.
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Old 05-10-2023, 04:51 PM   #10
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Well...

Quote:
Originally Posted by Qwkynuf View Post
In this context, I think that it is useful to think of wind resistance as being basically additional weight.

Imagine if you could put something like a fishing scale between your hitch ball and the trailer tongue.

When you start moving, that fishing scale is going to measure whatever the weight of the trailer is, plus a small amount of rolling resistance from the tires.

As your speed increases, the trailer is still the trailer - nothing has changed there - but the faster you go, the more the wind resistance increases, and the more "weight" is going to be measured by that fishing scale.

So I am using the word "weight" to encompass all of the forces that combine to resist the tow vehicle moving forward - trailer mass, rolling resistance, wind resistance, etc.
Well, you would have to think of it as a variable weight, which varies as a function of speed and frontal area (as others have said). But the weight doesn't vary with speed. Weight affects rolling resistance which doesn't change. And the rolling resistance doesn't change much with respect to weight.

Note that the drag goes up as a function of the square of the speed. That is, if the drag at 15 mph is X, the drag at 2x the speed (30 mph) is 4X, and the and the drag 4x the speed (60 mph) is 16X. (Ever wonder why the mileage is so bad at highway speeds?)

Here are links to several calculators that do this:
Horsepower & Aero Drag Calculator
https://www.omnicalculator.com/physics/drag-equation
https://www.engineersedge.com/calcul...orce_14729.htm
Horsepower & Aerodynamic Drag Calculator
https://www.engineeringtoolbox.com/d...ent-d_627.html
https://x-engineer.org/aerodynamic-drag/

So it's easy to find a point where the drag is sixteen times what it was at a lower speed. You couldn't find a corresponding weight where the rolling resistance was sixteen times what it was at another speed. You would have to load so much weight on the trailer that the tires would flatten, wheels would crush, springs would break, and spindles would shear.
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Old 05-11-2023, 02:28 PM   #11
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It is a factor but when you get stopped at the DOT scales, they don't say the wind is blowing 10 mph so you can only haul 38000lbs instead of 54000lbs.
Also if you happened to read about, I can't think of his name, but he rented a ford lighting and only got 90 miles out of a charge because the engineers did plan for wind resistance pulling a camper.
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Old 05-11-2023, 09:43 PM   #12
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Of course just more EV misrepresentation. That’s with 100% charge? But as always charge time rated on 80 or sometimes 70%. Haha, what an rv machine.
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Old 05-11-2023, 10:40 PM   #13
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Of course just more EV misrepresentation. That’s with 100% charge? But as always charge time rated on 80 or sometimes 70%. Haha, what an rv machine.
That's why, at this time, using an EV as a tow vehicle, isn't worth discussing. Unless you are towing a teardrop, for short distances.

It'll be sometime before the EV technology can improve the poor battery performance, when towing a real trailer. There's no argument about EV towing power but battery charge and distance are their real Achilles Heel.

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Old 05-12-2023, 08:06 AM   #14
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Weight vs Wind Resistance

Quote:
Originally Posted by Qwkynuf View Post
But when you are pulling a trailer, all of those load are applied to the vehicle frame through the hitch ball.



The hitch ball doesn't know whether it is pulling a 3,000lb dump trailer with 3 tons of gravel or a 6,000lb Micro Lite with 3,000lbs of wind load. The ball is pulling 9,000lbs either way, but a trip to the scale would lead you to believe that the travel trailer is much lighter.



My point behind this thought experiment is that pulling a low, heavy trailer on the freeway at maximum tow spec (like a dump trailer full of gravel) would be a much different experience (for the driver *and* the truck) - than towing a travel (or other tall) trailer of the exact same scale weight.


There is a difference in the direction the load is applied and what the limiting cases are. Tongue weight affects load on the tires/axels, tongue weight is directly dependent on total weight (10%-15%). Wind resistance is not in this direction so it doesn’t contribute to the max tongue weight.
As others have pointed out, driving at a steady speed the weight of the trailer shouldn’t matter. Where weight comes more into play is in maneuvering, acceleration, and braking. Wind resistance should only impact power to maintain speed, it doesn’t contribute to the limiting cases that drive the weight rating.
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Old 05-18-2023, 12:10 PM   #15
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The equations for computing aerodynamic drag (wind resistance) do not have weight in them at all.

The equations for computing rolling resistance have little variability with weight - much more to do with bearing quality and tire material and shape.

Rolling resistance plus aerodynamic drag give the total resistance for towing on level surface at a constant speed. Rolling resistance is almost always very small in comparison to aerodynamic drag, which is why you can move a trailer by pushing it by hand.

Trailer weight becomes a real factor going upgrades, acceleration, and deceleration (braking).

Many vehicles used to have a frontal area tow limit as well as a weight limit as a way to get at the limit for aerodynamic drag. However the question was always how to measure frontal area. Is it measured from the ground to the top of the vehicle, or is the distance from the ground to the frame deducted? The intent of the frontal area limitation was to stop a modestly powered vehicle from trying to tow a full height travel trailer - forcing the owner to go to a pop-up of some kind to reduce frontal area.

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Old 05-18-2023, 01:41 PM   #16
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1. Air resistance is the primary resistance to towing once you get moving.

2. Air resistance is exponential. Twice the speed produces four times the resistance. Here's what that means in quick number: Resistance at 70mph is twice what it was at 50mph.

Air resistance can be calculated by taking air density times the drag coefficient times area all over two, and then multiply by velocity squared. Since everything other than the velocity is gonna remain unchanged for your rig the only thing that counts is the velocity squared. (Change trailers and the frontal area will change too.)

3. Just slow down.

The Ford "frontal area" notation is trivia not a restriction of any sort. It's there for those surprised by the dismal fuel economy towing giant trailers.

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Old 05-18-2023, 06:27 PM   #17
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All of this is fascinating, and mostly is just restating what I said in the original post.

And none of it addresses the actual question - which I apparently did a terrible job asking.

Let me try to simplify down to two sentences:

1) Any given truck is going to be doing a *lot* more work towing a 10,000lb travel trailer than the same truck pulling gravel in a dump trailer with the same 10,000lb weight.

2) Truck manufacturers only publish a single "max tow" specification despite this difference in the amount of work the truck is doing to move the same mass.

Why don't they make a distinction based on trailer type? Or height? Or frontal area?

And the point behind my question, if anyone cares, has to do with the steady stream of folks coming onto forums like this, complaining about their towing performance, swearing that their Ranger can tow 7,500 pounds and their new 33' travel trailer only weighs 7,400, so they are totally within the ratings....etc
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Old 05-18-2023, 06:56 PM   #18
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Quote:
Originally Posted by Qwkynuf View Post

1) Any given truck is going to be doing a *lot* more work towing a 10,000lb travel trailer than the same truck pulling gravel in a dump trailer with the same 10,000lb weight.


2) Truck manufacturers only publish a single "max tow" specification despite this difference in the amount of work the truck is doing to move the same mass.

Why don't they make a distinction based on trailer type? Or height? Or frontal area?

And the point behind my question, if anyone cares, has to do with the steady stream of folks coming onto forums like this, complaining about their towing performance, swearing that their Ranger can tow 7,500 pounds and their new 33' travel trailer only weighs 7,400, so they are totally within the ratings....etc
As has been well-stated already, the work done depends on speed, wind, grade, etc.

SAE standard J2807 sets out requirements to develop tow ratings. It is interesting. And all trailers will fit within the appro 13 ft tall, 8 1/2 ft wide envelope with infinite variations in that envelope. Do you want truck mfrs to design according to every possible trailer shape and size?

Some guys expect their big diesels to idle uphill at highway speed, while others might be happy putting along in their 4-banger 1/4 ton with the mirrors folded in so they don't see the line of vehicles they are leading. The range of driver preferences is infinite. So I think the key to your query is depends on the owner and what they find to be "quick enough".
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Old 05-18-2023, 09:57 PM   #19
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The amount of time I spend pondering this kind of stuff decreases in direct proportion to the amount of wine consumed around the campfire.
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Old 05-19-2023, 06:06 AM   #20
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Quote:
Originally Posted by TheWolfPaq82 View Post
Not so much the weight as it is "Surface Area" that has a bigger affect with wind resistance. A 35 foot long, 12 foot tall Trailer has a heck of a lot more resistance that a boat of similar weight.
Isn’t that (essentially) what the OP said - but he used more words and technical terminology.
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