300 SL Mercedes Gullwing in a modern wind tunnel

by Bernd

in Mercedes SL-Class, News

300 SL Mercedes, 300 SL W194 and 300 S tested in a wind tunnel

300 SL Mercedes

In the so-called good old days, cars were styled by intuition, not by wind tunnels. That way we have great cars such as the 300 SL Mercedes Gullwing, styled by Karl Wilfert or the equally important 300 Adenauer and its sibling the two door 300 S, styled by Hermann Ahrens of 540K fame.

Naturally intuition plays an important part today too, but it is just one part of the process, the more dominant aspect is the wind tunnel.

A few months back Mercedes engineers had the idea to put cars from the early 1950s through wind tunnel tests in order to see, how they would compare with more modern contemporaries. The results were published by Mercedes in mid June 2012.

Three cars have been selected, which all have some technical relationship. The first of the three cars is the 300 S, introduced in 1951 and closely related to the slightly earlier introduced 300 Adenauer. Both cars share part of their drive system with the first Mercedes sports car development after the war, the fabulous 300 SL Mercedes W194.

300 sl mercedes gullwing, mercedes sl, mercedes coupe

1954 Mercedes 300 S W188

The 300 SL Mercedes W194 had its first public appearance with chassis no. 1 on March 12, 1952 on the autobahn between Stuttgart and Heilbronn. Unfortunately, this car does not exist today anymore. Right next to it the 300 S can be seen in the picture below. The indicators of the 300 S are interesting, as they were not part of the standard production car.

300 sl mercedes gullwing, mercedes sl, mercedes coupe

Picture courtesy of Daimler AG

300 sl mercedes gullwing, mercedes sl, mercedes coupe

300 SL no. 3 was selected for this test, here seen at the Carrera Panamericana (picture courtesy of Daimler AG)

The street-going version of the W194 is the 300 SL Mercedes W198 Coupe, which was pushed by American car importer Max Hoffman into its public life. So in one way or another one can claim that the 300 and 300 S, although vastly different from the two sports cars,  have been instrumental for the birth of the W194 and W198.

300 SL mercedes gullwing, mercedes sl, mercedes coupe

Coming back to our test now, it is the early morning of a cold and snowy January 23rd 2012 and those three cars are being gently pushed into the vast Mercedes wind tunnel facility. For the next eight hours they will be tested for their relationship in terms of surface and air resistance value. The large 5-megawatt electrical motor will propel huge fan blades in order to drive the vast amount of 9,000 cubic meters of air.

As the cars are normally being driven without underbody paneling in order to ease access to their service points, all three cars are also being tested in the wind tunnel without these panels. Todays’ wind tunnel program includes ten different measurement points in order to establish the correct cD-values at wind speeds of 130 km/h (80.8 mph) and 200 km/h (124.3 mph).

300 sl mercedes gullwing, mercedes sl, mercedes coupe

The test routine is about to commence (This and all following pictures courtesy of Daimler AG)

The 300 S

The first car to be tested is the big red 300 S. Its prominent, almost teutonic, chromed radiator is a proud reminder of designs from the 1930, where a long hood with a large three pointed star at its far end was testament to its proud owner’s status.

300 sl mercedes gullwing, mercedes sl, mercedes coupe

Today such a radiator is compared with a brick being pushed through the wind and logically everybody, who participated in this test assumed the worst for a car with a frontal area including exterior mirrors of 2.28 square meters. But the old girl surprised everyone and it became clear that good old-fashioned design does not necessarily translate into poor wind tunnel efficiency. At a wind speed of 130 km/h, the average drag coefficient cD was 0.468 and at a wind speed of 200 km/h it was 0.482. Such values would be unacceptable for modern cars (one might wonder though, how the G-Wagon of 2012 will fare), but when compared with other cars from the 1950s and even 1960s, such values were quite reasonable to say the least. A 1955 W121 190 SL for example achieved a drag coefficient cD of 0.461 with hard top, while the beautiful pagoda 230 SL W113 had a drag coefficient cD value of only 0.515. It was always rumored that Bela Barenyi’s safety roof was particularly strong and looked unique, but that it was from an aero-dynamic point of view a slight disaster. No wonder that no car manufacturer in those days chose to copy the pagoda roof concept.

The cD × A value of the 300 S Coupe, which is the product of the frontal area A and the drag coefficient of the vehicle body cD, results in 1.067 at a wind speed of 130 km/h, and 1.100 at a speed of 200 km/h.

As the coefficient for lift at a wind speed of 130 km/h for the front axle (cAF = 0.268) is virtually identical to that for the rear axle (cAR = 0.270) there is no pitching moment. Which means that the driver of the 300 S had a very stable vehicle attitude even at higher speeds. When tested at a wind speed of 200 km/h, the 300 S experiences a stronger load relief of the front axle with a resulting front axle lift coefficient cAF = 0.293 and rear axle lift coefficient cAR = 0.267. However, both these results are purely theoretical, as the 1951 300 S had a top speed of “only” 175 km/h (108.7 mph).

The 300 SL Mercedes W194

The second car tested was the 300 SL Mercedes W194, the number 3 of ten cars for the now famous 1952 racing season.

At just 1.784 square meters, its frontal area is 22% lower than that of the 300 S. Due to financial constraints in 1951, Mercedes engineers had to use as many parts from existing cars such as the 300 series as possible. The problem was that they had been developed for use in every day street cars, so the weight was not of concern to the engineers. Consequently the engine and front and rear axles were heavier than on competing racing cars. In order to compensate for this, reduction of the drag coefficient was one of the key development points. So it comes as no surprise that its cD values, (0.376 at 130 km/h and 0.383 at 200 km/h) are almost 20% better than those of the big 300 S.

300 sl mercedes gullwing, mercedes sl, mercedes coupe

Back in the 1950s, a 1:5 scale clay model of the W194 was tested with a drag coefficient value cD of just 0.25. These results show how far from reality these results have been and how great an influence for example the air flow through the engine compartment (which of course the clay model did not have) has on the overall air resistance of the car.

Another interesting result is the lift coefficient at different speeds, where the 300S showed surprisingly stable numbers. The value at the front axle showed a cAF = 0.305 at 130 km/h and 0.318 at 200 km/h. At the rear axle, a cAR = 0.149 and 0.140 respectively was measured. This shows a significant load transfer from the front axle at higher speeds.

The 300 SL Mercedes W198

The third and last car is the 300 SL Mercedes Coupe, the street going version of the W194. With its large bumper and side mirror it is expected that it will fare worse than its racing predecessor. Yet the wind tunnel measurements held again a little surprise in store. With a frontal area of 1.757 square meters the car is slightly better than the W194 (1.784) and can thus – with the cD × A value – partially compensate its less favorable cD value. Those values were 0.389 at 130 km/h and 0.397 at 200 km/h and the resulting cD × A figures 0.685 and 0.697 respectively.

The lift coefficients of the 300 SL Mercedes Gullwing Coupe show an even greater load transfer from the front to the rear axle at increasing speeds than the W194 race car. At 130 km/h the cAF is 0.334 and at 200 km/h it is 0.358 (W194: 0.305 and 0.318, 300S: 0.268 and 0.293). The cAR figure is 0.125 at 130 km/h and 0.108 at 200 km/h (W194: 0.149 and 0.140, 300S: 0.270 and 0.267).

300 SL mercedes gullwing, mercedes sl, mercedes coupe

The tests show that the two 300 SL Mercedes models generate a fairly strong lift at the front axle that increases at higher speeds. This makes both cars susceptible to side winds, which is also confirmed by another test  that indicates a high yaw moment under a diagonal incident wind-flow direction of 20 degrees.

For the 300 S Coupe the test shows a strong flow break at the top edge of the radiator grille, a separation which remains perceptible as far back as the windscreen. By contrast, the transition from the windscreen to the roof turned out to be better than expected. However, the airflow breaks away again some 30 to 50 millimeters above the rear window and meets the vehicle body again on the boot lid.

300 sl mercedes gullwing, mercedes sl, mercedes coupe

Both SL models show a relatively good airflow over the front area. However, the lower edges of the vehicle body are too high, in particular in the case of the W194 race car. This leaves the vehicle underbody unprotected, which results in the high lift forces at the front axle. Airflow around the windscreen, the A-pillars and the roof of both SL models is good, but with the W194 the airflow breaks off at the rear window rubber surround, to later hug the car again at the boot lid.

Are those tests necessary? Not really, but they reveal that the Mercedes “stylists”, as they were called in those days, and the engineers had some sixty years ago a fairly good feeling for efficiency. That is why the W194 created such a stir among competitors, when it was revealed in 1952. And who would have suspected that the 300 S would be under side wind conditions the safer car to drive at high speeds.

If you want to read much more about the 300 SL Mercedes Coupe, the 300 S and other Mercedes models of the 1950s, all of that is covered in my books and e-books and can be ordered from my website. They offer next to the cars’ history the experience to own or drive one of these great vehicles and have plenty of color pictures, most of which have never been published in a book before. I am sure you will enjoy.

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