Flat Tow vs Trailering - Any Engineers out There?

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RVJeeper18

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Flat Tow vs Trailering behind a Class C - Which Option should be "easier" from a power/energy required standpoint, get better MPG, and just feel easier to pull? My first though is flat tow is clearly the answer if for no other reason than a trailer adds significant weight - right? I ask because we do both regularly and the trailered vehicle just feels easier and seems to get better MPG. Doesn't make sense in my feeble little brain.... Maybe someone can explain the physics to me?

For context - Class C motorhome - If we are going off-roading we haul a modified 2004 Jeep Wrangler on a car hauling trailer. If we just need an extra set of wheels we flat tow the wife's daily driver 2014 Wrangler Unlimited. The 2014 is basically stock and set up correctly for flat tow (we have drug it several thousand miles so figure i would have blown something up if not..). The 2004 is heavily modified so probably weighs as much as the JK. We haul it on a good aluminum car hauling trailer.
 

Lou Schneider

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Trailers do have a slight advantage in rolling resistance as the only moving parts are the wheels and brake drums - four down you have additional parasitic losses from the rotating parts in the drivetrain between the wheels and the transmission. But I'd think this would be less significant than the additional weight of a trailer.
 

Ex-Calif

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IMO Lou is right. To understand you need an apples to apples comparison.

For example - Flat towing the 2004 is the baseline. You have a certain rolling resistance from the tires and drive train. The alternate system is to trailer tow the 2004. You've added what? 1800#+ of trailer minus the tow bar for the flat tow and reduced the rolling resistance slightly.
 

Ernie n Tara

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Flat tow has to be more efficient, and by a significant margin at any reasonable speed. First, the higher weight being towed will have an effect,but towed weight is a relatively small factor as is the extra friction in the drive train components.
The major factor is form drag! It is a larger force, even at low speed, and it increases exponentially (velocity increases result in squared drag force). Obviously the vehicle being trailored or towed does make a difference, but most vehicles are designed with some consideration given to reducing drag. Trailers do not have this advantage and when combined with a vehicle are unlikely to result in a low drag situation.
I would expect perhaps a two to one or higher advantage for the flat towed vehicle.

Ernie
 

Gary RV_Wizard

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I agree with all the factors that have been pointed out and doubt if we can generalize on this subject. With some trailers and vehicles, drag might be the primary factor, but maybe in some others rolling resistance might be important, e.g. fat tires and substantial mechanical friction. And neither may be significant enough to have much affect on a large motorhome.

I see claims that RVs lose 0.5 mpg while towing a car, and I remember numbers near that when we first towed (4-down with a small SUV). However in later years (bigger/heavier coach, newer but bigger SUV) I could only see a few tenths difference. Never figured out why. Nor worried whether I could save a tenth somehow.

Overall, I doubt if it's worth pondering the differences except maybe as mental curiosity. In my book, convenience is a bigger decider than some small difference in mpg.
 

RVJeeper18

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If the deciding factor is aerodynamics my trailer may have advantage. It has a 3’ tall wedge that follows the triangle shape of the tongue out to the full trailer width. What y’all think
 

Ro and Joe

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In my travels as a full timer, it seems that a very low percentage with a C or A actually tow a trailer. I will often disconnect my 4 down toad while DW is in the office if I know we have a backup site. You will limit your options on campgrounds and sites if you trailer. 4 down allows for much better flexibility. Campgrounds typically have a majority of back in sites.
 

Ex-Calif

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Barring wind tunnel testing (🙄) I doubt anything within 40 feet behind a 12 X 8 box is gonna add any aero drag at all.
 

Ernie n Tara

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I'll not bother to work the numbers, but I would note that the the eight ft by eight ft trailer is two thirds the frontal area of that ~8 by 40 foot motor home.

drag = 1/2 * p * v^2 * Csubd * A

where: p = .0023 at sea level
v = velocity in feet per second
Csubd = coefficient of drag
A = area in sq. ft.

A reasonable Csubd is around 0.3 for many cars and 1.0 for a flat front trailer.
At highway speeds (50 mph) that will result in around 800 lb. for the trailer (64 ft square) and 1/3 that for a car.
Note that drag will about double at 70 mph.
 

Skookum

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I also have a Wrangler. Only way it's ever coming with us is if it's on a trailer. I don't care about any MPG loss. The RV can do the extra weight. It's more about not dragging a heavily modified hobby vehicle around putting mileage and wear on very expensive tires, suspension, axles, etc. I'd flat tow the family SUV. Not my Wrangler.

The other gotcha... If you wheel it, and it has an issue, it's often easier to bring it home on a trailer than it is to do a field repair, if something significant fails.
 

HueyPilotVN

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If you do go with a trailer I would suggest an enclosed trailer.

I hauled two cars along for many thousand miles and they were not exposed to anything that would effect them.

This was especially important for the 32 year old Mustang Convertible with very low mileage.
 

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John From Detroit

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Well towing 4 down is lighter. By anywhere from 1 to 2 thousand pounds and towing on a trailer may exceed your tow limits... HOWEVER

As for Drag..That depends entierly on the vehicle.
My expierence with two toweds.
#1 was a Chevy Lumna APV IT tipped the scales at 4,000 pounds empty and we used it to store stuff so it was heavier... The Damon Intruder got the same MPG towing or bobtail.
Well that ride died and I replaced it (Eventually) with a 2001 Neon, about 2500 POUNDS cost me MPG..
Two things
One was transmission drag (The Lumina we "Unlocked" an axle so only the differential draged we towed in park,, the NEON I used a lube pump and towed in neutral so the tyranny dragged)

Air flow... My suspusicion is the Lumina "Spoiled" the air flow just enough to offset the added load.

It's impossible to predict things like that without ... as someone else said... a wind tunnel.
 

Domo

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I'm sure this must be an academic question - Here's my thoughts...

Flat tow:
Doesn't have the weight of the trailer added to "train"
Doesn't require separate registration (and possible insurance)
Doesn't need to be detached and parked "somewhere else"
Doesn't require two things (car plus trailer) to be hooked up when leaving

We've towed Wranglers (Rubicon and Sport) and currently a Jeep Cherokee Trail Hawk behind our MH. We have no concern about tire wear or other component wear on the Jeep. We get 8 mpg on a good day and find that acceptable given our equipment. To optimize fuel we drive sensibly in the MH's sweet spot which is 63 mph.
 
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Old_Crow

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Well, I'll find out about the mileage for sure in a while.
I've been flat towing my Wrangler for the 6 years I've been full time. I'm about to add a trailer because it dawned on me that without a co-pilot, if I get in a spot where I have to unhook, I no longer have a co-driver to move the Jeep while I move the motor home. With a trailer I'll, at least, be able to back out of trouble.
 

Old_Crow

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Look forward to hearing what you think. What kind of trailer are you getting?
Just a basic econo open car hauler. I'm thinking 16' will allow the Wrangler and a medium sized m/c across the front, but I won't know for sure until I get to the point of seeing them IRL. If I have to go 18' I will.
 

RVJeeper18

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Just a basic econo open car hauler. I'm thinking 16' will allow the Wrangler and a medium sized m/c across the front, but I won't know for sure until I get to the point of seeing them IRL. If I have to go 18' I will.
Assume you have a 2 door? I would recommend 18' for sure, gives you a little more room to balance load and have room to pull it front & back.
 

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