War of the Tailgates

[rbattelle]

I couldn't find this discussed anywhere with the search feature, so I thought I'd bring it up here. I see lots of people in various trucks who drive around with their tailgate down. I believe this is in an effort to decrease drag and improve fuel economy.



However, driving around with the tailgate down actually increases drag on the vehicle (which decreases fuel mileage). With the tailgate up, a beneficial pressure "bubble" is created inside the bed, which acts to smooth the airflow over the cab. With the tailgate down, that pressure bubble is absent, and air pours over the cab in a turbulent mess. It takes energy to make turbulence, and that's manifested in drag.



Even worse are those Gator nets (nylon tailgate with all sorts of holes in it). The vortices you generate by having such a tailgate is like flying an airplane with slotted flaps fully extended.



Are people leaving their tailgates down for some other reason?

 

[Hohn]

I dunno-- I think they are mistakenly trying to get better mileage. A little basic Aero would help them see the error of their ways, you think?



If they want to reduce drag, then they should put on a camper shell and plaster the perimeter with AirTabs (which reduce turbulence and pressure drag).



I have often tried to explain (unsuccessfully) pressure drag to people (and why the dimpled golf ball flies farther as a result of reducing it). A turbulent boundary layer can reduce surface drag, but I am NOT about to drill thousands of tiny holes in the body of my truck and connect it to a vacuum system



Hohn

 

[Shovelhead]

Here's a little info from some College students.



http://mars.wnec.edu/~ehaffner/did.htm



Baseline condition Cd = 0. 432 (without mirrors)

Square Cap Cd = 0. 480 11. 11% Increase in drag

Replace tailgate with a cargo net Cd = 0. 461 6. 71 % Increase in drag

Removed Tailgate Cd = 0. 447 3. 47% Increase in drag

Lowered Tailgate Cd = 0. 414 4. 17% Decrease in drag

Tonneau Cover Cd = 0. 381 11. 81% Decrease in drag

New Cap Design Cd = 0. 302 30% Decrease in drag



Tailgate down

#ad




Tailgate removed

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Cargo Net

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[Hohn]

interesting results. It appears what is right in THEORY doesn't quite pan out in practice-- I should have known better.



Air likes a teadrop shape- blunt nose and pointy tail. In fact, the front end doesn't matter nearly as much as the back end does. It's all about reducing/eliminating separation. You want to keep the flow laminar.



I would expect their square cap to have more drag, because its trailing edges are very sharp. A more modern design would probably not be as bad. Airtabs might make it a lot better.



I found it most interesting that removing the tailgate INCREASED drag, but lowering REDUCED drag. Having that flat surface to aid in the flow streams rejoining appears to be more important than one might think.



Hohn

 

[Dkevdog]

I don't remember who, but I think it might have been Ken Lenger (Klenger) posted a link to some testing that was done on pickups in a wind tunnel with photos. The photos depicted that the aerodynamics were far better with the tailgate up then off or down.



You have to consider that the trucks were designed with the tailgate, and it was part of the airflow design characteristics.



Kev

 

[Hohn]

the links in this thread show windtunnel tests.



Against a tailgate-up baseline, t-gate down was better, t-gate off was worse.



Not what I would have thought...



HOHN

 

[illflem]

Originally posted by rbattelle





Are people leaving their tailgates down for some other reason?

I leave mine down sometimes so all the crud in the back can blow out.

 

[MCummings]

Originally posted by Dkevdog

You have to consider that the trucks were designed with the tailgate, and it was part of the airflow design characteristics.

Not sure if this is what the Big three were thinkin' in the '50's when they were first designing pick-up beds.



On another note,, I wish they would do Airflow tests with Dually trucks. . I want to know how the fenders affect the airflow, and if the airflow is any different with the tailgate up and down with the big dually fenders... .

 

[rbattelle]

Shovelhead,



Thanks for that post, because it completely contradicts what I have assumed for some time. However, that test has not convinced me that I'm wrong about the tailgate-up condition being lower drag. Plus, the fact that the no-tailgate condition resulted in more drag than the tailgate-down condition sounds a little fishy to me.



Perhaps there is another way to look at it. We can expect the drag from the bed will be a function of the density, velocity, and geometry (including areas, etc. ). With respect to the tailgate's interaction with the air stream from the top of the cab, it's distance from the cab will certainly play a role. I believe there is some critical distance behind the cab at which the tailgate is exactly far enough away to have no effect on drag (whether it's up or down). Move the tailgate closer to the cab, and the drag is reduced. Move the tailgate further away, and drag is increased (until you get infinitely far away, and then once again tailgate has no effect). The critical distance would depend on vehicle speed (higher speed, tailgate can be further back, and vice-versa).



Notice the test shovelhead quotes used a regular cab long bed pickup. I wonder if perhaps the distance from the cab to the tailgate at that speed might be enough to prevent formation of the pressure bubble, and the tailgate actually acts as a large forward-facing drag surface.



Perhaps the same test performed on a short-bed pickup would reveal something more in line with my expectations?

 

[Shovelhead]

Here's the other pics.

Tailgate Up

#ad




Square Edged Cap

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Tonneau Cover

#ad

 

[QRTRHRS]

I know of one incident where a woman was killed when someones tailgate which was down flew off in an accident and went through this womans windshield. I have also seen some nice dings because the driver forgot his gate was sticking out.



I guess I will have to put a tailgate on my flatbed to get better fuel mileage

 

[Dkevdog]

Originally posted by MCummings

Not sure if this is what the Big three were thinkin' in the '50's when they were first designing pick-up beds.



On another note,, I wish they would do Airflow tests with Dually trucks. . I want to know how the fenders affect the airflow, and if the airflow is any different with the tailgate up and down with the big dually fenders... .

True- but then again, when I think of the 50's pick-ups aerodynamics isn't a word that comes to mind.



Besides, wind tunnel tests show only air flow. Does anyone actually have any real world mileage numbers?? I could lose nearly 1mpg with a flow through fifth wheel tailgate on my 96.

 

[MCummings]

Originally posted by Dkevdog

True- but then again, when I think of the 50's pick-ups aerodynamics isn't a word that comes to mind.

:-laf

 

[Robin]

Tailgate removal

At the public utility company from where I retired, the meter readers used 1/2 ton short bed trucks. They drive a lot of rough West Virginia back roads. To get better backing visibility, they took off the tailgates. Sooner or later, the sides of the beds cracked at the bottom and got wobbly. We used Chevy/Ford/Dodge trucks. All three manufacturers told us that the tailgate is a part of the structural integrity of the bed. If you examine where the tailgate closes up against the bed, most have parts that mate and lock the bed and tailgate together. Since the guys still wanted to leave the tailgates off, the garages fabricated braces out of steel channel. These large U-shaped braces went across the bed at the end and up the sides and were bolted or welded.



When the trucks were retired, the channel came out and the tailgates back in.



I have seen enough old pickups on the road with no tailgates and the sides of the beds flopping around to keep me from leaving the tailgate off my Dodge. I do use a ventilated fifth wheel tailgate when we are towing our trailer, but only to be able to back into the pin box with the tailgate up. I don't think the vents help a bit, but that is another controversial subject.



Bob

 

[TSuter]

I've read this post threw a couple of times, I can understand the theory of the wind tunnel experiments and all I can say is theory is great but reality sucks. I owned a 89 D-150 w/a 3. 9L v-6 5-spd while stationed in Germany. Me and a buddy would race everyday to work on the Autobahn, W/ my tailgate on and up my top speed was 119 with the tailgate off my top speed was 140 ( that's MPH) so I have a hard time believing that more drag is created by taking the tailgate off. Robin is right about the bed side wobble at those speeds even in a brand new truck I could see it in my rear view. BTW I didn't race the truck like that everyday it was usually my Basically Mechanically Worthless (BMW) that I abused.

 

[Big_Daddy_T]

I have done small tests with the tailgate down and up. Same course relitively flat. 40 mile round trip. I ran it 4 times back to back. With the computers instant readings it did about 1mpg better down than up(repeatable). The only thing that changed each run was time of day. And it was afternoon when i started so traffic and temp were pretty constant. Dead calm conditions. Wasnt scientific but it setlled it for me.

 

[Wheaties]

I put a tonneau cover on my truck hoping to see an improvement in the mileage. No such luck. Mileage was the same with or without. Oddly enough, this was not the case on my previous truck, a Toyota Tacoma, which got a consistent increase in mileage of about 2-3mpg with the cover. Both trucks are 4x4, but the Dodge/Cummins weighs only twice as much.



I suspect that while drag is a factor, it isn' t the most significant; keeping my foot out of the GO pedal might have more to do with it.



Matt

 

[Big_Daddy_T]

That in my mind means that the dodge has better wind flow characterists than the toyota did.

 

[Wheaties]

Big Daddy T, you could be right. Another possibility that crossed my mind is the amount of power the Dodge has over the Toyota. With just a 4 banger under the hood, I was in the habit of flogging it pretty hard to get the performance I wanted. This usually involved high revs and a good head start.



Even so, I can recall driving down the highway with some furniture in the back, pedal to the metal just to maintain highway speed. I couldn't imagine having that problem with the Cummins under the hood unless I had a small house in tow. I can accelerate to arrest me speeds with a light pressure on the throttle in a way that wasn't ever going to happen with the Toy.



So I guess my question is, is it the wind flow that makes the difference, or is it overwhelmed with the power under the hood?



Matt

 

[Big_Daddy_T]

The engine with more torque less sensitive(mileage wise) to decreased or increased resistance. Seems so in towing applications. Its definitely a good question.