[TestDrive]

AutoSpeed: Low Drag Car Aerodynamics
Originally appeared March 8, 2008.

Nice introduction to the aerodynamics of automobiles.

When a car moves, air is deflected above, below and around it. The point at which the air splits to pass above or below the car (termed the stagnation point) is important in deciding how slippery the car will be. The lower the stagnation point, the better, because then less air runs into the (usually) rough underside of the car. However, unless the car has a front spoiler extending almost to the ground, air will pass under the car. This has caused some manufacturers to start adopting low drag undersides for their cars. There is also another reason for entraining this air into a smooth flow – creating less lift, which we’ll get to in a moment.

Even quite minor changes in rear hatch angle can cause major changes in drag. Tests carried out by Volkswagen have shown that the Cd of the car can vary from 0.34 to 0.44 as a result of slight alterations to the rear hatch angle. At one angle (30 degrees to the horizontal in this case) the airflow separation point jumped back and forth from the end of the roof to the bottom of the hatch, depending upon the curvature at the rear edge of the roof. It was this 30 degree rear hatch angle that produced the highest Cd value. This sort of substantial change in the car’s drag coefficient will have a large influence on the car’s top speed and fuel consumption. In some cars even a 10 per cent reduction in drag will decrease open road fuel consumption by 5 per cent.

The current automotive fashion is for manufacturers to pay only lip service to aerodynamics. In fact, there has been little improvement in CdA figures in the last decade. However, at highway speeds, most of the power being developed by the engine is being used to push air out of the way. For cars of the future to improve their open road fuel consumption, CdA figures will have to once again fall: it is simply a physical requirement of efficient car design.