2017 Chevy Bolt EV Is Less of a Drag Than Originally Believed

It turns out that the Chevrolet Bolt EV is better at slipping through the air during high speed driving than earlier reports had indicated.

When the Chevrolet Bolt’s 217-mile highway range estimate was published last September it surprised many who thought its boxy aerodynamics would drag down its efficiency at speed.

After all, the car’s lead designer had been quoted a month earlier by Automotive News as saying “It’s a disaster for aero.”

The article went on to quote him as saying the car’s coefficient of drag was measured at .32 versus the Toyota Prius with a Cd of just .24.

Stuart Norris, the managing director of design for GM South Korea, had actually been referring in that article to the general category of squat and wedge-shaped hatchbacks as being “a disaster” but some news accounts reporting the Automotive News interview implied that he was referring specifically to the Bolt EV itself.

Car and Driver magazine then published a detailed review of the Bolt in its October 2016 issue in which a road test showed the car could drive a surprising 190 miles with cruise control set to 75 mph and climate controls set to 72 degrees.

How could this be?

We now know at least one part of what enabled the Bolt’s unexpectedly good highway range.

In an interview last week, Norris said the .32 coefficient quoted by Automotive News last summer was actually the original engineering design target for the car rather than the final measured result.

When pre-production prototypes were revealed in January 2016 the wind tunnel testing was coming in at .312, as Car and Driver magazine reported at the time. With the final production headlight lenses and other exterior parts in place on the car it has since fallen to .308.

Coefficient of drag combined with a car's width and height describes its resistance to being pushed through the air. Drag has an adverse effect on efficiency as vehicle speeds increase so it does not matter much in city driving but does play an increasing role at higher speeds.

Coefficient of drag combined with a car’s width and height describes its resistance to being pushed through the air. Drag has an adverse effect on efficiency as vehicle speeds increase so it does not matter much in city driving but does play an increasing role at higher speeds. The larger Bolt is actually sleeker than the 0.326-cd Spark EV.

Other factors like powertrain design also play a role in good highway efficiency. For example, the Bolt’s large 60 kWh battery may allow it to more efficiently discharge the power levels needed to maintain faster highway speeds where a smaller battery might struggle a bit. In addition, the Bolt’s motor is also geared to run at a relatively slower rpm at highway speeds than some other electric cars.

According to Norris, computer analysis guides the design process but final surface shape optimizations are done with physical models and wind testing.

He said the Bolt EV was put through 6 full-scale rounds in GM’s wind tunnel to optimize its slipperiness and counterbalance the inherent drag of its space-efficient utilitarian shape. Typical car designs may see the wind tunnel 4 times or less, he said, but even conventional gasoline-powered cars are getting increasing attention now as companies stretch to meet regulatory fleet mileage goals.


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