Not since the Porsche 356A of 1959 has a sports car been as aerodynamic as the new Mercedes-Benz SL.
The Porsche had a drag coefficient (Cd) of 0.29 and, over the last 50 years, the trend has been higher and higher drag as wheels and engines became bigger and bigger, according to Mercedes-Benz aerodynamicist Teddy Woll.
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Now that trend has been reversed.
The new SL is the most aerodynamically efficient sports car on sale with a claimed Cd of 0.27 for the 350 version.
Mercedes is producing cars with even lower drag, said Woll. The B-class with eco package gets down to 0.24 but reducing drag is a constant battle.
For years the industry has either largely ignored or at best only paid lip service to aerodynamic efficiency. But the need for ever-lower CO2 is giving new impetus to the craft of the aerodynamicist.
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By GlobalDataThe SL’s impressive figure has been achieved without having to resort to tricks like a spoiler or boot lid lip, for example.
“If we don’t need it the stylists are happy,” said Woll.
Achieving low drag is a painstaking, little-by-little approach, he said. There are the obvious tricks, such as smoothing the airflow on the underside and over the car.
The shape of the SL helps with long front and rear overhangs.
Then there are the not so obvious.
Getting the right shape to reduce noise and drag for the A pillars was a challenge. There was the need to stop water flowing off the windscreen on to the side windows while still allowing smooth airflow. The solution: a small channel that traps the water and sends it up and over the top.
Similarly, there is a groove in the top of the door mirrors that traps water coming off the front and sends it down to run off the bottom edge instead of back onto the side windows. The nodule that guides the water creates a little extra drag “but is worth having'” said Woll, noting that managing water flow around a car can’t be done by computer.
“We have to have a full size mock-up to do that,” he said.
How much more aerodynamically efficient can cars become? “A fish or a bird are about as aerodynamic as possible”, said Woll. “A fish has a drag coefficient of about 0.05. Stick a set of wheels on it and it immediately jumps to 0.15. The shape of cars will have to change to get much lower drag coefficient than today.
“The look would be unacceptable by today’s standards,” he said, adding: “But it will come.”
