Tailgate Up or Down

[Malo83]

GMC says leave it up
http://www.foxnews.com/leisure/2013/...ntcmp=features

[Keith Seymore]

I figgered this was going to be a topic of discussion.

Diane Bloch is the aero engineer who was not credited with the work in your article.

DETROIT – The all-new 2014 Sierra full-size pickup truck spent more development time in a wind tunnel than any GMC pickup before it, resulting in design changes that benefit both fuel efficiency and interior quietness.
To achieve improved airflow, aerodynamic engineers like Diane Bloch examined every millimeter of the truck to find areas of improvement, debunking some popular myths along the way.
To study the way air passes over, under and around the Sierra, engineers used General Motors’ state-of-the-art Aerodynamics Lab, a 750-foot-long tunnel through which a 43-foot-diameter fan powered by a DC electric motor with the equivalent of 4,500 horsepower can generate winds of up to 138 mph. Aerodynamic advancement is one reason why the 2014 Sierra will be the most fuel-efficient V-8 pickup on the market.
“We can’t stop air; we can only guide it through the path of least resistance. It’s like electricity, without the shock,” said Bloch, GM aerodynamic performance engineer. “The biggest misconception is that it’s all about single components. But a certain side mirror design doesn’t create a certain amount of drag, its interaction with the rest of the vehicle does.”
For example, a new air dam below the 2014 Sierra’s front bumper successfully reduces drag because it directs air toward the ground and away from the truck’s rough underbody. And Sierra’s ducted flow path between the grille and radiator prevents air from swirling inside the truck’s front cavities.
Even the top of the Sierra’s tailgate and the center high-mounted stop light are optimized to guide air cleanly around the truck. And because Bloch’s team detected unwanted airflow between the cab and bed, new sealing has been added.
“We discovered that in the computational analysis we perform,” said Bloch. “The most harmful air between the cab and bed was coming over the cab and down through the gap, so we paid the most attention to that specific area.”
The pickup market has a great number of available aftermarket accessories, and Bloch says those have varying impact on aerodynamics. *Add-ons like bug deflectors on the hood, wider tires or aftermarket bumpers can raise the drag coefficient, which is the measure of how air pushes on a vehicle as it moves down the road. The result: added noise and increased fuel consumption.
A long-disputed topic among truck owners is whether a tailgate raised or lowered is better for aerodynamics, but Bloch says a tailgate in the up position is more aerodynamically efficient. As air flows over the truck, it falls over the cab and pushes forward on the rear of the truck. With the tailgate down, the benefits of that airflow are diminished.
“Replacing the tailgate with an aftermarket net is worse than having no tailgate at all,” Bloch said. “Imagine dragging a solid object and a fishing net through water. The net is going to require more muscle.”
So what accessories can truck owners add to help aerodynamics? Tonneau covers for the bed help smooth airflow over the truck, and Bloch says soft covers are more beneficial than hard covers because they form to how the air wants to flow. Running boards can also help air flow smoothly down the truck’s sides.
“Round, tube-style running boards can provide a minor improvement to the truck’s drag coefficient,” said Bloch, “Fully integrated, flush-mount running boards are even better.”

[BMERDOC]

Mythbusters did a test on this some time ago and proved "tailgate up" was the way to go and the results were verified by F@#D.

[INSIDIOUS '86]

They also said nets were the way to go but maybe that isn't so with the new truck the way it's designed they also showed taking the tailgate off altogether was an improvement fue to less weight. The one thing about a tunnel is it only measures drag and not really fuel mileage. Both are related but its possible a net may give better mpg just because its less weight and keeps the "bubble" somewhat in the back.
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[dieseldawg142]

driving around with the gate down just wears out the hinges & straps. in the late 80's early 90's, GM & i think gale banks took a s-10 to bonneville (have to dig out the magazine article). they ran half a bed cover- the back half only

[andrewmp6]

Id leave it up looks better and on a newer truck lock it they are super easy to steal off a newer truck.

[Keith Seymore]

Originally Posted by dieseldawg142

driving around with the gate down just wears out the hinges & straps. in the late 80's early 90's, GM & i think gale banks took a s-10 to bonneville (have to dig out the magazine article). they ran half a bed cover- the back half only

Driver was Don Stringfellow. And you are also correct about half a cover.

K

[bigd65]

Looks REDNECK to leave it down

[dieseldawg142]

Originally Posted by Keith Seymore

Driver was Don Stringfellow. And you are also correct about half a cover.

K

stringfellow!, thats right, & thats the s-10. thanks Keith

[KQQL IT]

Makes 0 difference at the drag strip at 110 mph
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[gchemist]

The company says this likely holds true for all of its pickups, not just the new model, but there are a few aftermarket add-ons that can effectively improve your MPG. Tonneau covers for the bed help clean up the air flowing over it, and properly-designed running boards can help as well.

"Likely"? Ha, still don't buy it without proof. Proof, I've daily driven LWB and SWB trucks. 8" beds with tailgate up are a drag on older pickups. On my 8" bed, I put a hard cover, softcover, and drove with the gate down. All improved milage. Truck was powered with a V6. My SWB 83 is covered with a softcover top. 18.5 MPG at 70 MPH is proof that covers do work. All new trucks have better milage because bedsides are higher. The high bed sides with aero front move air around a truck. Try putting something large or heavy into a tall bed side truck from the side.

Crewcab beds are only 5.5" or so. So they are shorter. In real world driving, air come at angles and not directly head on.

[dieseldawg142]

dont make much of a difference if sb or lb. throw a little bit of garbage in the back of your truck & go for a drive, tailgate up, tailgate down. where does all that garbage end up? migrates right up behind the cab.( anyone with a rear sliding window can attest to this) the syclone lsr s10 hit 210 mph with the gate up & a bed cover from tailgate to just the wheelwells, you would think that that would be like a giant parachute, but aerodynamics beyond me prove otherwise. a tonno is good, even a smooth canopy will help even out the flow (but the weight will probably negate any gains)

[gchemist]

Ya know why the half cover worked? Cause the front half cover captures air to fill up the bed. Once full of "air", all air goes over the tailgate. Air resistance over air is less than air over hard or soft cover.

[Keith Seymore]

Originally Posted by gchemist

Ya know why the half cover worked? Cause the front half cover captures air to fill up the bed. Once full of "air", all air goes over the tailgate. Air resistance over air is less than air over hard or soft cover.

- or - the air shoots off the top of the cab, over the front half of the pickup box and doesn't engage until it hits the back half of the box.

I've spent a little time in the GM wind tunnel and the one thing I've learned is this: ya just don't know til you smoke it.

The airflow over these surfaces is not intuitive, is affected by the most minute changes, and does the zaniest, most unexpected stuff I've ever seen.

You simply can't imagine what affect a change will have until you verify it experimentally.

K

[dieseldawg142]

ok, both make sense to me. alls i know is i see a truck do 210 with the gate up...& never open your rear slider when leaving the beach

[wraprail]

Originally Posted by INSIDIOUS '86

They also said nets were the way to go but maybe that isn't so with the new truck the way it's designed they also showed taking the tailgate off altogether was an improvement fue to less weight. The one thing about a tunnel is it only measures drag and not really fuel mileage. Both are related but its possible a net may give better mpg just because its less weight and keeps the "bubble" somewhat in the back.
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Yeah right....sure. OK.

Originally Posted by Keith Seymore

- or - the air shoots off the top of the cab, over the front half of the pickup box and doesn't engage until it hits the back half of the box.

I've spent a little time in the GM wind tunnel and the one thing I've learned is this: ya just don't know til you smoke it.

The airflow over these surfaces is not intuitive, is affected by the most minute changes, and does the zaniest, most unexpected stuff I've ever seen.

You simply can't imagine what affect a change will have until you verify it experimentally.

K

Which one is you Keith?????
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[Keith Seymore]

The distinguished looking gentleman behind the smiling guy holding the Motor Trend "Car of the Year" trophy.

Gotta let all the bosses be up front.



K

[tucsonjwt]

I remember driving away from the local dump with the tailgate down on my old 73 C20. Hitting the bumps caused the tailgate to bounce up and down and before I left the dump one of the tailgate latches snapped in half.

I had a soft tonneau cover on my 2000 Silverado V6 and I think that may have helped gas mileage about 1-2 mpg - I drove mostly city under 45 mpg.

I hope the new GM trucks have a good lock for the tailgates, since they are popular items for theft around here.

Generally speaking, if you do only city driving I think it doesn't matter much what the tailgate configuration is.

I always installed a bug shield on my new trucks and SUVs to stop the stone chips on the hood and windshield. The cost of reduced gas mileage was not enough to offset the damage to the body that would be caused without the bug shield. The new designs of bug shields are very aerodynamic looking - but I don't know if they are actually aerodynamic in a wind tunnel.

[cory d]

short bed, long bed, short step side, even the uncommon long stepside. try them all

[Jonboy]

A hose clamp over the open side of the hinge is a pretty good theft deterrent for the tailgate, too- and you don't have to remember to lock it.

[kevinr1970]

Originally Posted by Keith Seymore

The distinguished looking gentleman behind the smiling guy holding the Motor Trend "Car of the Year" trophy.

Gotta let all the bosses be up front.



K

Where's the rabbit ears Keith?!?

I find it amusing that people think that this one time they drove their truck this way and another time they drove it that way with no way to control the variables, seem to be arguing the results with aerodynamic engineers...

[hyatt]

Originally Posted by Keith Seymore

- or - the air shoots off the top of the cab, over the front half of the pickup box and doesn't engage until it hits the back half of the box.

I've spent a little time in the GM wind tunnel and the one thing I've learned is this: ya just don't know til you smoke it.

The airflow over these surfaces is not intuitive, is affected by the most minute changes, and does the zaniest, most unexpected stuff I've ever seen.

You simply can't imagine what affect a change will have until you verify it experimentally.

K

Love the volt Drive one every day

[wraprail]

Originally Posted by Keith Seymore

The distinguished looking gentleman behind the smiling guy holding the Motor Trend "Car of the Year" trophy.

Gotta let all the bosses be up front.



K

Dam bosses awalys hogging the spotlight. lol


Great picture Keith and congrats.

[INSIDIOUS '86]

Originally Posted by wraprail

Yeah right....sure.
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Surely you must know so much more.. Please share your infinite wisdom
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[beamn7]

Hmmm, so the majority of us have 8 cylinders, 4 barrels and heavy iron with the aerodynamics of a drive-in movie screen and we want to discuss whether the tailgate up or down gives us better fuel mileage? I really dont care what I get for MPG, I (like the rest of us here) like driving a square. There is more mods you can make under the hood to better your MPG than lowering the darn gate. Besides, looks better with the gate up.

Just sayin'............