Properly Reducing The Coefficient of Drag


There are actually all sorts of things that can be done into a vehicle to lessen the coefficient of drag. If you’re into cars, most of you can expect to recognise the 1999 Honda Insight’s faired-in rear wheels. They looked odd but were designed like that for a reason. The rear wheel arch was removed so air was no longer required to pass a spinning wheel in a gaping hole in the bodywork, when you are faired-in. That caused turbulence, which caused drag, which increased the drag coefficient. Honda made a more streamlined shape, more unlikely to cause turbulence, by making smooth body panels that covered the wheels. It’s worth pointing out that Honda didn’t get there first though. GM re-introduced this particular styling cue in 1996 on their fully electric EV1. Citroën got there a full 44 years earlier making use of their DS model, i say ‘re-introduced’ because in 1955.

Everything that creates turbulence will induce drag, so increase the coefficient of drag. For example taking off the rear view mirrors does two things – it removes two things that poke out from the side in the car which create turbulence, as well as reducing the frontal cross section. It’s not really wise for designers to simply remove rear view mirrors, even though both of these modify the overall aerodynamic drag formula above. Instead, they make smaller, more streamlined mirrors designed to present less frontal cross section, and importantly, to make less drag.

Another design and styling cue mimicked from your DS is the overall model of the car – wider at the front and tapering to a narrower rear, both in side view and plan view. Exactly why is this? Well it’s because that is how car designers begin to approach the most aerodynamic shape there is.

What’s probably the most aerodynamic shape?

A Drop-tank or Belly-tank racer A Drop-tank or Belly-tank racer

Think of what nature intended to best cut through water, it won’t surprise one to know that one of the most aerodynamic shape looks like a fish or a teardrop – the shape water naturally forms when it falls through air, if you think of air like a fluid. A wide, circular front moves the atmosphere aside, compressing it with minimal fuss, whilst a lengthy sculpted tail allows air to expand again in the low pressure area behind the shape without creating turbulence. If you’ve ever been to Speed Week about the Bonneville Salt Flats, you’ll have seen drop-tank or belly-tank racers. These folks take old world war 2 era aircraft drop tanks and make cars out of them. Those old drop tanks were designed to be added in to aircraft to increase range (by carrying more fuel) without a massive increase in drag (which would have negated the entire excercise). They’re the perfect aerodynamic shape. So vehicles like the EV1, the Citroën DS and the Honda Insight all attempt to access a usable version of the shape as best they are able to.

Picture credits:

Did Citroën really get there first?

The Aurel Persu car

For the history buffs, here’s a nugget of trivia : Romanian engineer Aurel Persu actually came to the actual final outcome that the teardrop was the ideal shape in 1922 and built a vehicle to prove it. He patented the shape in 1924 and both GM and Ford expressed an interest in buying the patent at the time, but as they didn’t want to agree to also building the car, Persu refused their offers. That vehicle had a drag coefficient of .22 – below most vehicles today – and it’s still completely functional and on display at the Romanian National Technical Museum in Bucharest. Since 2006, the museum has awarded an annual Aurel Persu Aerodymanic Automobile prize to the car manufacturer that produced a vehicle in the previous model year with a drag coefficient lower than .3. In 2006 the Mercedes S-Class took the prize and also in 2007 the prize was given to Toyota Corolla.