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What a drag

DON MILLS, Ont. - A wise man once said "spend your energy taking action on the things you can control not on complaining about the things you can't." As escalating prices at diesel pumps threaten to t...


DON MILLS, Ont. – A wise man once said “spend your energy taking action on the things you can control not on complaining about the things you can’t.” As escalating prices at diesel pumps threaten to take yet another chunk out of owner/operators’ profits, that’s advice well-worth heeding.

Reality is the price at the pump is controlled by international forces owner/operators have little long-term control over – no matter how many protests they stage.

The only part of the fuel price equation you as a truck owner do have control over is how much fuel your rig consumes. The more fuel efficient your driving habits, the more money in your pocket at the end of the day.

One of the factors with a direct impact on fuel savings is aerodynamic drag.

Any body moving through the air is subject to any or all of three types of aerodynamic drag, but trucks are impacted mainly by pressure drag. As your rig punches through the air, it’s fighting a great deal of pressure, but as Randolph Baerg, of Innovative Vehicle Testing, explains, it’s the flow of air as it moves over the sides and back of the truck that’s a greater concern.

“What’s happening is a separation of the air flow coming down the side of the truck. It’s shedding off vortices and swirling around back there,” Baerg says. Picture yourself standing at the side of a highway when a tractor-trailer whizzes by and you can understand what Baerg is talking about. For several seconds afterwards you are buffeted by wind, dirt and leaves kicked up and swirled about by the truck’s passing. Now imagine how big a fan would be required to create the same effect and how much fuel would be required to fuel that fan and you get an appreciation of how much fuel your rig must spend to punch through the air.

There are several variables that determine the degree of drag on your truck and one of the biggest is the shape and size of your vehicle.

Several tests – ranging from wind tunnel setups to computer models – are used to determine which truck designs slip through the air with the least amount of drag. The process is fairly complicated but to provide you with a rough idea, consider the figures provided by Baerg. A flat plate (similar to the design of long nose conventionals) has a drag coefficient of 1.95. Change that shape to a half-hemisphere and the drag coefficient is reduced to 1.42. Change it to a typical wing design and the drag coefficient is reduced to 0.007.

But, what’s the impact on fuel consumption?

“Take a 100-inch by 100-inch area, which amounts to a 10,000-square-inch frontal area on your truck, for example,” explains Baerg. “Say the resulting one hundredth of a psi pressure on the front creates 100lb of drag. At 100 km/h that adds up to 17 hp. That’s quite a bit of fuel when your truck is doing 100,000 miles a year.” n


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