Mercedes reveals how it took the drag out of the CLA

Mercedes' new compact four-door coupe, the CLA, will be available with special aerodynamic tweaks which can reduce CO2 emissions by as much as 20g/km at speed.
A reduction of just 0.01 in a car's drag co-efficient, or Cd, brings down CO2 by 5g/km at motorway pace, say Mercedes aerodynamics engineers. But the CLA range will include a BlueEfficiency model which exhibits a drag reduction of four times this - 0.04.
This not only brings down CO2 but also reduces fuel consumption and makes the interior quieter, they add. To achieve the same CO2 reductions through weight-saving, the car would have to be 140kg ligher.
The CLA 180 BlueEfficiency, powered by a 120bhp 1.6-litre turbocharged petrol engine, has a world record low drag figure for a production car of 0.22. Although this is bettered by the 0.186 of Volkswagen's teardrop-shaped hybrid XL-1 revealed at the Geneva show last week, that car is not a regular production model, with only 50 scheduled to be made.
It took 220 hours in the Mercedes wind tunnel and 15 special tweaks to be able to achieve the CLA BlueEfficiency's super-slippery shape. But models without these aerodynamic additions and with bigger wheels and tyres still achieve class-leading figures, says Mercedes aerodynamics engineer Norbert Fecker.
The BlueEfficiency will have 15-inch wheels with streamlining covers rather than the 18-inch rims fitted to every other model in the UK. It has adjustable radiator shutters so that the engine receives only as much cooling air as it needs and special cowlings beneath the engine, rear suspension and exhaust system, while the engine cover extends further back than normal.
Other aerodynamic tricks employed on the CLA include serrated edges ahead of each tyre and vents in the wheel arches to minimise turbulence, a diffuser at the rear to prevent air travelling under the car from causing a vortex, specially shaped door mirrors, a flush-fitting boot lid with a built-in spoiler and even tiny fins on the rear lights.
The CLA's shape benefits from what Mercedes calls computational fluid dynamics (CFD) - digital testing before a single clay model is built so that aerodynamics engineers can predict how and where air will cling to the car, causing drag. The company's wind tunnel can replicate airflow around the car at speeds of more than 160mph.
Mercedes has had a wind tunnel since the 1930s and has been using CFD since the 1990s, but this is now much more sophisticated as a result of vast increases in computing power.
Fecker says the CLA's low drag figure would not have been possible without the co-operation of the car's designers, who were prepared to compromise on some design details to help.