Automakers are making a final push to improve the aerodynamics of their biggest, boxiest vehicles. Here’s what they’re up to.
Pushing a large, square object through the air uses more energy than does pushing a smaller, more slippery shape along. Automakers want desperately to reduce the resistance from air so they can reduce fuel usage. Most of the easy things they could do to accomplish this are already done. Even the largest and most utilitarian vehicles, trucks and SUVs, already have angled, rounded edges, flush glass, sculpted mirrors, and other simple design elements to ensure they are as slippery up front as they can be.
However, look at the formula for the force it takes to push an object through the air and there is a clue to where the new gains in efficiency will be found, and also why it matters so much. You can see from the end of the formula that the resistance increases with the square of the vehicle’s speed. The upshot of that is that around town, about 20% of the fuel used goes towards pushing the car through the air, but at highway speeds, it accounts for 80% or more of the fuel used. Simply stated, aerodynamics is the only thing that really matters with regard to fuel economy once you are on the highway.
Force = 1/2 x Air Density x the Cross-Sectional Area of the Car x The Car’s Drag Coefficient x the Square of the Vehicle’s Speed
In that formula for a vehicle of any given size, the one thing automakers can change to reduce the force of air acting against the vehicle is the vehicle’s drag coefficient. The key here is to think of the drag on the vehicle as not just the drag from the front of the vehicle, that has already been formed into an optimum shape, but rather, the entire vehicle’s drag effect.
Designers working to optimize the all-new 2019 Chevy Silverado started at the back of the truck. They knew that the smooth laminar flowing air going over the top the cab was being disturbed by the cargo bed and causing significant drag. To smooth out this flow of air they added two small spoilers. The first is at the back edge of the cab. It directs air up and lengthens the air stream so that it will then hit the second spoiler on the top of the tailgate. Chevrolet found that this and other changes increased aerodynamic efficiency on the new Silverado by 7%. This despite the truck being longer, wider and having a higher seating area and more headroom.
The all-new 2018 Jeep Wrangler is a shape that raises a middle finger to the wind. Its cubist design has come to be loved by fans, but wind tunnel technicians get excited when the Jeep Wrangler shows up. The Jeep brand has many modern aerodynamic designs like the Cherokee and Compass, but the Wrangler’s iconic shape needs to stay constant. Still, designers needed to do something. In the end, they messed with one of the defining elements of the Jeep. They added seven degrees of additional rake, or tilt-back angle, to the new Jeep Wrangler’s windshield. This is almost heresy to Jeep fans, so the engineers didn’t just change the shape of the windshield. They also made it much easier to drop down (yes, having the windshield in place is optional on Wrangler) and also detached the mirror from the glass so that when the windshield is dropped down, the driver could still “check six.” Don’t forget to close your mouth if you put that windscreen down. Unless you like the taste of bugs.
The all-new 2019 Ram 1500 pickup isn’t just relying on static changes to its shape to improve aerodynamic efficiency. To reduce drag the Ram cheats the formula by using its air suspension to lower itself one-half inch at highway speeds. That effectively reduces its cross-sectional area. This is a trick that in the past was employed mainly by super-expensive European sport-wagons and SUVs. Now it is appearing on one of the most common models for sale in America today. Ram also has an active air dam that lowers at just 35 MPH to keep air from going under the truck where it becomes turbulent and adds to drag. Finally, Ram has designed a new generation of active air shutters for the truck’s grill. When cooling air isn’t needed they shut to redirect air away from the engine bay and up over the truck instead. Ram was able to reduce the drag of the new model by 9% compared to the previous model.