GMC Pickups 101: Busting Myths of Truck Aerodynamics
2014 Sierra gains fuel economy, quietness from time in wind tunnel
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
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
"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."
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