Trailer towing Range

[Deleted member 11233]

I wonder how efficient my Cybertruck will be when towing my little Casita. It's fairly aerodynamic, but my Ram 1500 Hemi averages 13 mpg towing it at 65 mph. I have never towed it with my MYP.

 

[CyberGus]

I wonder how efficient my Cybertruck will be when towing my little Casita. It's fairly aerodynamic, but my Ram 1500 Hemi averages 13 mpg towing it at 65 mph. I have never towed it with my MYP.

Fill it with helium balloons

 

[ldjessee]

Take a trailer with a flat front, but a tapered rear, then compare it to the v front, flat rear.
(Assuming the same height, width, and weight of the trailer)
The tapered rear will be much more ‘aerodynamic’ because the drag is significantly reduced. The v front does not reduce drag, it does not reduce the frontal area, and those two factors have significant influences on the ‘aerodynamics’ of a vehicle.

The little trailer is neat, but all the things sticking off of it or the disruptions to its lines… the roof top AC unit could be streamlined or put else where, the awning should be integrated into the interior, as well as make the door flush with the skin of the trailer.

Think of how some whales, dolphins, sharks, and such are shapedMoving thru water can be harder…

 

[Crissa]

The taper in the back does more than the front - but the taper in the front is still better than the flat front.

-Crissa

 

[PilotPete]

Take a trailer with a flat front, but a tapered rear, then compare it to the v front, flat rear.
(Assuming the same height, width, and weight of the trailer)
The tapered rear will be much more ‘aerodynamic’ because the drag is significantly reduced. The v front does not reduce drag, it does not reduce the frontal area, and those two factors have significant influences on the ‘aerodynamics’ of a vehicle.

I was agreeing with you, right up until you said “The v front does not reduce drag.” Yes, it does. In a trailer, its impact on drag reduction is not as great as the back (as you point out), but it DOES reduce some of the drag. The only possible exception to that would be if it were being towed by a van, the same size or larger, so that the trailer could sit in the “shadow” of the van.

 

[Luke42]

Take a trailer with a flat front, but a tapered rear, then compare it to the v front, flat rear.
(Assuming the same height, width, and weight of the trailer)
The tapered rear will be much more ‘aerodynamic’ because the drag is significantly reduced. The v front does not reduce drag, it does not reduce the frontal area, and those two factors have significant influences on the ‘aerodynamics’ of a vehicle.

The little trailer is neat, but all the things sticking off of it or the disruptions to its lines… the roof top AC unit could be streamlined or put else where, the awning should be integrated into the interior, as well as make the door flush with the skin of the trailer.

Think of how some whales, dolphins, sharks, and such are shapedMoving thru water can be harder…

It is possible to increase drag via the taper at the back.

For instance, the R-Pod campers got this just-wrong and they have a stall-strip installed as a hacky way fix their aerodynamics.

I agree that tapering the back is a good idea, I just need to point out that the implementation details matter for getting the effect you're looking for.

 

[fhteagle]

en.m.wikipedia.org/wiki/Drag_coefficient

Notice the "angled cube" vs "cube". Despite the angled cube presenting a wider total cross section to the wind at its mid point, because it gets to that maximum cross section and back more slowly and evenly, it actually has a lower coefficient of drag.

This is due to the "area rule" concept I talked of a few posts back.

https://en.m.wikipedia.org/wiki/Area_rule

There's actually one more shape missing from that chart, which is the "streamlined body" with the curves after it's maximum width point inverted. I remember papers on that shape claiming it was actually lower drag per unit volume. Apparently, detaching a toroidal wake at just the right size actually causes some air to push forward again on the rear end. This effect has been documented in fish, as an explanation for how they can stay mostly motionless in a flowing stream. Good luck utilizing that effect in the automotive world though, with all the complications like varying speed, cross wind, and the incompressible ground below you.

 

[LiveFreeOrDieBob]

en.m.wikipedia.org/wiki/Drag_coefficient

Notice the "angled cube" vs "cube". Despite the angled cube presenting a wider total cross section to the wind at its mid point, because it gets to that maximum cross section and back more slowly and evenly, it actually has a lower coefficient of drag.

This is due to the "area rule" concept I talked of a few posts back.

https://en.m.wikipedia.org/wiki/Area_rule

There's actually one more shape missing from that chart, which is the "streamlined body" with the curves after it's maximum width point inverted. I remember papers on that shape claiming it was actually lower drag per unit volume. Apparently, detaching a toroidal wake at just the right size actually causes some air to push forward again on the rear end. This effect has been documented in fish, as an explanation for how they can stay mostly motionless in a flowing stream. Good luck utilizing that effect in the automotive world though, with all the complications like varying speed, cross wind, and the incompressible ground below you.

Love that diagram and I feel like I intuitively understand the impacts of the different shapes with one exception - why is the half-sphere LOWER drag than the full sphere? The way I’ve always thought of aero, conceptually at least, is in terms of a “vacuum shadow” behind the shape moving through the air. In other words, if you had a wind tunnel and string/ribbons so you could see the air flow, at the point where the laminar flow detaches from the shape, the area between the boundary layer and the object represents a complex shape filled with different layers of vacuum pulling back on the object (drag). And filling in that shape with more “object” would effectively reduce the area of the vacuum shadow, reducing the drag. So basically the reason a airfoil has such lower drag than a sphere is that the vacuum shadow behind the widest point of the foil is “filled in” by the trailing structure of the air foil, so there is no vacuum shadow, at least at certain speeds. Given this, I’d almost expect the half-sphere going backwards (flat side first) to be close to the sphere‘s Cd, but higher, and the half-sphere with the curved edge going first and the flat side trailing would be much higher. Why is the half-sphere’s CD Lower than the full sphere? That makes no sense to me…

 

[ldjessee]

The way it was explained to me is that a sharp edge causes clean separation and so the air behind the flat surface tends to stay there in a turbulent air pocket, while the sphere causes irregular separation that can cause oscillations, but no static pocket of air remains, so drag is increased.
I could be misremembering or been informed badly… so please do not take my word for it.

 

[Crissa]

No, that's a fine layman's explanation ^-^

Another thing that trips up is that it's different at different speeds.

-Crissa

 

[PilotPete]

No, that's a fine layman's explanation ^-^

Another thing that trips up is that it's different at different speeds.

-Crissa

Yep, and quick look at the actual chart on Wikipedia indicates that the shapes all have the same Reynolds number. But that number is calculated from four values, 3 of which are based on the fluid through which the object is passing. As pointed out, velocity is one of those values. And the two publications from which the numbers appear to be derived, are Dynamic response of pipe rack steel structures to explosion loads and Explosion Hazards and Evaluation. The former is a masters thesis, the latter is a published book. If you’re looking at lift and drag during an explosion, you’re looking at supersonic flow, where the behaviors are much different. I didn’t buy the book, but I did go over the paper. This chart appears to bd derived from a chart in the paper which is evaluating the backside load on small pipe and other construction materials when compared to the shock load on the frontside.

Shock wave formation and interaction are not an issue with any street legal vehicle.

 

[ldjessee]

I was taught some basic aerodynamics and theory of flight when going thru training to be a helicopter mechanic.
I guess some of it stuck.
The rest I learned from others on ecomodders.

 

[PilotPete]

I was taught some basic aerodynamics and theory of flight when going thru training to be a helicopter mechanic.
I guess some of it stuck.
The rest I learned from others on ecomodders.

Helicopters are proof that with enough power applied, anything can fly.

 

[ldjessee]

Helicopters are proof that with enough power applied, anything can fly.

Helicopter definitely beat the air into submission… but one show of weakness, and down you go…

 

[TyPope]

While we don't know the actual battery size and range of the Cybertruck, we can make some good guesses based on the efficiencies of other Teslas and some good old physics work. I'll copy my post here so you don't have to go link-hopping.

PHYSICS IS PHYSICS. It takes a set amount of energy to move a mass.
You can just read the bold Blue at the bottom if you don't want to know the physics behind the assessment.

Here are the calculations based on physics:

Physics dictates some things:
Pulling a 14,000# trailer with a frontal area of 60^2 feet 100 miles with 1 mile of positive elevation change at 75 mph and assuming the CT and passengers weighs 6,500# and a drag coefficient of 0.30 will take 170.5 KwH. That same trip at 65 mph takes 145.8 KwH. That's a lot of battery for just 100 miles of range towing a really heavy trailer. But, it is also going uphill a LOT. Let's look at towing without elevation change because most people make round trips so the downhill leg makes up for the uphill leg a lot.

Spoiler: Spoiler Alert

You won't be towing a heavy trailer very far between charges, unfortunately.

Good news? If you aren't towing uphill (ie not going up in elevation 1 mile),
Going 75 mph:
You'd consume 170.5 KwH minus the 40.8 KwH gravity penalty which means it would take 129.7 KwH at 75 mph to pull that trailer 100 miles with no elevation change and you can tow 154 miles on a full battery.

Going 65 mph:
You'd consume 145.8 KwH minus the 40.8 KwH gravity penalty which means it would take 105 KwH at 60 mph to pull that trailer 100 miles with no elevation change and you can tow 190 miles on a full battery.

There. I mathed it for everyone.

Obviously, without a 14,000# trailer, its aero, rolling resistance, and gravity penalty, you get 500 miles of range. So, there's a sliding "Miles per full battery" that will run between 500 miles (not towing) and 190 miles towing 14,000# at 65mph.

Since you shouldn't pull MORE than the 14,000# and will NEVER pull less than zero pounds, physics shows that your range will be always fall between 190 and 500 miles at 65 mph.
The 190 mile range is for a 14,000# trailer being pulled at 65 mph flat. (assuming 200kwh battery)
The 500 mile range would be pulling nothing at 65 mph flat. (assuming 200kwh battery)

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STOP READING HERE IF YOU DON'T WANT MORE PHYSICS IN YOUR LIFE!
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some calculations for the technical inclined:
Aero = 1/2 (0.002377) (80.667)^2 (0.4) (40) = 123.74 Ft Pounds, or (123.74)(0.001989) = 24.61 KwH
That's (0.5) x (coefficient of air times velocity in feet squared) x (drag coefficient) x (surface area of entire system) ---- surface area is if you looked at the CT and trailer head on and figured the size hole it would punch through a wall as it drove through...

Rolling resistance =Total weight x 0.015 = 13,500(0.015)(0.001989) = 40.27 KwH
That is (weight) x (coefficient of rolling resistance of a tire on pavement)

Gravity: 0 (only comes into effect when traveling UP HILL on average)

Add those three forces together and you get 64.88 KwH to take a 7,000 pound trailer 100 miles.

You'd be able to drive 324 miles with that 7,000 pound trailer and 500 pounds of occupants if you maintain 55 mph and we assume a 200 KwH battery.

In short, figure on only going 1.62 miles per kWh if you are towing a regular size, 7,000# cargo trailer at 55 mph.

You heard it here first!