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Car Aerodynamics

There are plenty of ways to alter a car’s aerodynamics. Below, we explore all things aero, from drag coefficients to splitters, diffusers, and giant wings.

Then and Now

Edmund Rumpler's Tropfenwagen - Julian Edgar on YouTube.com
Edmund Rumpler's Tropfenwagen - Julian Edgar on YouTube.com

It’s clear when looking back through automotive history that aerodynamics wasn’t always at the forefront of car design. Clearly, the Ford Model T, with its boxy shape and sharp lines, wasn’t built to cut through the air. That it wasn’t going very fast in the first place meant such considerations were largely moot as drag increases exponentially with speed. It wouldn’t be until cars got faster that aerodynamic design became relevant.

That wouldn’t happen until the 1920s and 30s. Edmund Rumpler’s Tropfenwagen or “tear drop car”, as it translates from the German, took its design from that of a tear drop, the most aerodynamic shape in nature with a drag coefficient of just .05. The car itself mustered a drag coefficient of .28, pretty good for 1921. Though it was the first mass produced car with a consciously aerodynamic design, only 100 of the Tropfenwagen were ever built.

Tatra 77 - Steering Media on YouTube.com
Tatra 77 - Steering Media on YouTube.com

Perhaps the most influential early aerodynamic design was the Czechoslovakian Tatra 77 designed by Hans Ledwinka and Zepplin engineer Paul Jaray. The Tatra 77 achieved an impressive .2455 drag coefficient. Ferdinand Porsche borrowed heavily from the Tatra 77 when designing his seminal Volkswagen Beetle.

Over the years car designers improved on aerodynamics, making racecars safer and consumer cars more efficient. Today, the most aerodynamic car is the upcoming Mercedes-Benz EQS with a drag coefficient of just .202, beating out the Tesla Model S by just .006. You’ll notice that both of these cars are electric. That’s not a coincidence. Electric car makers are looking to eek out every single bit of efficiency and range from their cars. Good Aerodynamic properties are central to that goal.

The Basics of Automotive Aerodynamics

Concept car in wind tunnel
Concept car in wind tunnel

So, what exactly is aerodynamics and why does it matter so much to the efficiency, and in some cases the safety, of my car? Put simply, aerodynamics is how the air flows around your vehicle as it moves forward. That air creates resistance, and because drag increase exponentially as speed increases, the faster you go the more drag, the more energy required to maintain that speed. This is why, during the Oil Crisis of the 1970s, the US highway speed limit was lowered to 55 mph, to save on fuel. The reduction in traffic accidents was just a nice added benefit.

Drag, the force of air against the car as it moves forward, isn’t the only force being exerted on your car. There’s also lift which is directed perpendicularly to your car. There’s the ironically named positive lift, which literally lifts the car up, which is actually very bad, especially at speed. And then there’s negative lift or “down force” which presses the car down, maintaining traction.

The pressure differential between the front and rear and top and underside of the car add to or reduce drag depending on the aerodynamics of the car. Keeping high pressure on top and low pressure on the bottom ensures your car stays planted on the road and cutting through the air with greater efficiency.

Drag Isn’t the Whole Story

Porsche 911 GT3 - porsche.com
Porsche 911 GT3 - porsche.com

Lowering drag coefficient isn’t the only important aspect of aerodynamic design, however. While less drag is valuable for greater efficiency, sometimes making concessions to increased drag improves downforce. Why is downforce valuable? Because it keeps your car planted on the road and increases traction. This may not be super important for your Toyota Sienna, but it is for your 911 GT3. At high speeds, positive lift begins to overwhelm negative lift which not only decreases traction (not good) but can lead to a car literally lifting off the ground (really not good).

This is why you see race cars and other performance cars with all manner of equipment designed to increase downforce. Spoilers, splitters, diffusers, and wings all contribute to downforce, keeping those cars pinned to the road. Tracing the evolution of aerodynamics in racing demonstrates their efficacy as nearly every early innovation was promptly followed by a ban as things like wings and side skirts and even fans proved an “unfair advantage”.

Downforce Devices

1970 Plymouth Superbird spoiler - carsforsale.com
1970 Plymouth Superbird spoiler - carsforsale.com

Spoilers and Wings – The airfoil shape of these devices (basically an upside-down airplane wing) produces a difference in pressure between the slower top air and the faster moving low air. The result is greater downforce. A spoiler by itself is fairly modest, doing more to prevent lift than actively producing downforce. A wing, by contrast, usually can contribute significantly, at least at higher speeds, to downforce.

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2020 Chevrolet Corvette Stingray splitter - carsforsale.com
2020 Chevrolet Corvette Stingray splitter - carsforsale.com

Splitters – Located at the front of the car, splitters prevent air from moving under the car and instead sends it up over the car adding to the high pressure above the car.

Side Skirt - BoostedWhipz on YouTube.com
Side Skirt - BoostedWhipz on YouTube.com

Side Skirts – Similar to splitters, side skirts prevent air from moving under the sides of the car and creating lift.

Front Undertray - B is for Build on YouTube.com
Front Undertray - B is for Build on YouTube.com

Undertray – The undertray is a panel or series of panels made of metal or plastic covering the underside of the car that allow air to pass easily under the car. Many consumer cars feature undertrays as they aid in reducing drag.

Rear Diffuser - vicrez.com on YouTube.com
Rear Diffuser - vicrez.com on YouTube.com

Diffusers – Diffusers are located under the rear bumper and underside of the car and often feature “straights” which pinch and direct the air moving out from under the car. Diffusers operate under the Venturi Effect that says fluids, or in this case air, moves more quickly when moving through a narrow passage (think a raging river in a canyon). In the case of a diffuser, the straights pinch the air causing it to move faster from under the car, contributing to downforce.

As critical as these items are, they do come at the expense of added drag. The average F1 car, which is basically a collection of downforce devices with wheels and an engine, has a drag coefficient of anywhere from .7 to 1.0 or two to three times that of the average car on the road.

Best of the Best

2022 Mercedes-Benz EQS - media.mbusa.com
2022 Mercedes-Benz EQS - media.mbusa.com

For those of us who don’t own an F1 car or a McLaren, drag is still the paramount consideration when it comes to your car’s aerodynamics. That Mercedes’s new flagship EV, the EQS, can claim the best drag coefficient of any production car shouldn’t surprise us. In fact, their lineup already specializes in low drag cars like the A-Class and S-Class, each of which has a drag coefficient of .22, the E-Class at .23, and the C-Class at .24. Other super low drag cars include the Ludic Air EV at .21, the Porsche Taycan Turbo and the BMW 5-Series at .22, the Tesla Model 3 at .23, and the hybrid Toyota Prius and Hyundai Ioniq at .24.

2021 Toyota RAV4 - pressroom.toyota.com
2021 Toyota RAV4 - pressroom.toyota.com

For comparison’s sake, the most popular vehicles in America, the classic full-size pickups from Ford, RAM, and GM run from the Ford Raptor at .56 to the RAM 1500 at .59. Size, even more than shape, has a major effect on drag. This is why you want a parachute and not an umbrella when jumping out of a plane. Small yet still boxy, the Jeep Wrangler comes in at .45 drag coefficient. The Toyota RAV4, while still roughly the same size as the Wrangler has a much more aero conscious design and comes in at .32.

Whether built to minimize drag or maximize downforce, aerodynamics is a critical component of your car’s design.

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Chris Kaiser

Chris’ greatest passions include topiary, spelunking, and pushing aging compact cars well past 200,000 miles on cross-country road trips. His taste in cars runs from the classic and esoteric to the deeply practical with an abiding affection for VW Things, old Studebakers, and all things hybrid-crossover.

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