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Blowin’ in the wind

INDIANAPOLIS, Ind. - Auto Research Center (ARC), a research group best-known for its aerodynamic testing of Formula 1, NASCAR and Indy cars, has expanded its capabilities to provide testing of heavy-d...

INDIANAPOLIS, Ind. – Auto Research Center (ARC), a research group best-known for its aerodynamic testing of Formula 1, NASCAR and Indy cars, has expanded its capabilities to provide testing of heavy-duty trucks. And the group says its early findings could revolutionize the way tractor-trailers are tested for aerodynamic performance.

ARC recently completed testing of tractor-trailers on two types of wind tunnel surfaces – stationary and rolling road. To date, stationary floors have been the standard when it comes to wind tunnel testing. But ARC has developed a rolling road facility that features a moving belt which causes the wheels to turn, more closely replicating the real-world operating conditions of a tractor-trailer.

ARC aerodynamicists conducted both stationary and rolling road aero testing on two scale model semi-trucks. They found that the rolling road wind tunnel testing provided more accurate results and in some cases conflicted previously accepted aero test results.

“We quickly learned that (some components) in fixed floor testing showed a drag reduction but when using the rolling road it showed a drag increase,” explained Mike Camosy, operations manager, ARC. “There’s a great potential for confusion and for developing trucks in the wrong direction.”

Gaylord Couthier, an ARC aerodynamicist, added “Real-world effects are the point of interest for any aerodynamicist. The air flow around the truck has to be managed for the actual road conditions which obviously include rolling wheels.”

ARC is suggesting OEMs begin using rolling road wind tunnel testing when developing new trucks and components. During a recent press conference, the company said it sees the potential for the industry to save more than $1 billion in fuel costs over the next three years thanks to an improved understanding of truck aerodynamics.

During its testing, ARC used two 1:8 scale model trucks. One was streamlined (to allow for a direct comparison to wind tunnel testing previously conducted by NASA) while the other was far more detailed, including an exact replica of the truck’s undercarriage and engine compartment. The model trucks were tested with the rolling road activated and also in stationary mode to provide a comparison.

Using a model with a detailed undercarriage revealed some surprising variations between rolling road and stationary road wind tunnel testing.

With the road not rotating, the landing gear showed a 0.64% increase in drag. However, when the road was rolling, the landing gear actually demonstrated a 0.38% reduction in drag.

“Without using the rolling road wind tunnel testing, you are getting the opposite and not seeing the actual drag that’s on these semi-trucks,” explained Camosy.

Other components also showed conflicting results when tested on both stationary and rolling surfaces. A spare wheel, for instance, showed no change when using a stationary surface yet revealed a 0.38% drag reduction when the wheels were turning. This made little sense to an admittedly unscientific truck writer, so I called ARC to find out how adding a component such as a spare wheel could in fact result in a drag reduction.

Henry Kowalczyk of ARC patiently explained that “In reality, you can’t predict interactions. Sometimes you add a piece and you think there would be less drag but because of interactions with the rolling road and the flows under there, you get an unusual result. What truly happens is that the base area right behind the trailer gets filled, so that’s why there’s a drag reduction. It’s very non-linear.”

The white paper ARC published on the subject explains that the significant variation between stationary and rolling road surfaces should serve as an alarm bell for OEMs.

“It is interesting to note that the combined effect of adding the landing gear, air tank and spare wheel in the road-off condition results in a drag increase of 0.42% while with the road on it results in a drag reduction of 0.95%,” reads a white paper published by ARC. “Not only is this a trend reversal, but it offers a variance of approximately 1.4%; this type of trend difference between stationary road and rolling road testing shows the importance of testing with the most realistic conditions possible.”

ARC also tested a variety of futuristic truck components such as underbody diffusers and vortex generators.

Once again, aerodynamicists found tests conducted with the rolling road activated showed vastly different results than those with a stationary surface. ARC’s conclusion was that “During initial testing of ARC’s first rolling road scale model semi-truck, it was very quickly discovered that without rolling road testing, semi-truck development could be misleading given the trucking industry’s current testing methodologies.”

Whether OEMs heed the advice and change their aerodynamic testing methods remains to be seen, but the ARC report certainly gives scientists and engineers plenty to talk about. For more information on ARC, visit

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