We’ve all heard of cars being called ‘as aerodynamic as a house brick,’ but have you ever wondered just how aerodynamic a house brick really is? Well, the coefficient of drag (or cd for short) is used to measure how easily a shape slips through a viscous medium, which in the case of a car means the air, and I can tell you that a smooth house brick has a coefficient of drag of around 2.1. Back in the 1920s, a figure of 0.7 would have been good going for a car, but by the 1980s somewhere between 0.3 and 0.4 was more normal, although extra-slippery shapes such as Citroën’s CX achieved 0.33 as early as 1974. By the early 2000s most cars were between 0.32 and 0.38.
All of this is important because although mechanical upgrades can improve a car’s performance in terms of both speed and efficiency, they quickly become an expensive way of going faster or further. That is why weight reduction can be so important to anybody serious about going fast, as well as anybody wanting to travel more efficiently. (Which is another way of saying ‘use less fuel,’ and who doesn’t want to do that?) That’s because when you are accelerating, a car’s mass is the biggest single factor consuming horsepower and holding it back and any weight lost is essentially free horsepower gained.
However, by the time you are cruising at anything over 50mph, then 70% of the engine’s power is devoted to overcoming aerodynamic drag. And while any increase in speed will be linear, this drag increases exponentially, which is why building a 16-speed gearbox won’t allow you to simply keep accelerating through the sound barrier. It’s also why manufacturers spend huge amounts of time in the wind tunnel when designing new cars. However, that still leaves a whole host of steps you can take to improve the aerodynamics on your own car, but only if you understand some of the principles behind the science. So let’s start by debunking some of the myths surrounding drag, particularly the coefficient of drag so often used to measure it.
Which would you think has the lower cd figure out of a Lamborghini Countach and a VW Type 2 van? Well, the very fact that I am asking this question will have aroused your suspicions that the obvious answer is
