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SAFETY CONCEPTS

 

For Safety’s Sake 

 

August 1, 2004

 

Real and simulated crash tests have a long tradition and a bright future at DaimlerChrysler.

 

It didn’t seem fair to pit the tiny smart against the huge Mercedes-Benz: The match between the lightweight fortwo coupe and the S-Class, which is twice as heavy, was something like David versus Goliath. The outcome of this crash test, which was conducted by DaimlerChrysler engineers before the smart’s market launch, seemed to be a foregone conclusion.

 

It was only to be expected that the luxury vehicle, which weighs in at just under two tons, would pass the offset crash test with relatively good marks. But how would the smart, which weighs only 966 kilograms, look after the dust had settled? In particular, what types of injuries could passengers in the innovative two-seater be expected to suffer?

 

The odds seemed stacked against the automotive welterweight, partly because of the enormous weight difference ratio of 1:2. The smart’s lack of a protective long front end was another source of doubt before the crash test began. Although the collision was to take place with both contenders driving at a speed of 50 kilometers per hour, the huge difference between their weights meant that the impact would have very different results on each of them. For the heavy Mercedes, the impact would be equivalent to crashing against a barrier at a speed of 33 kilometers per hour. The lightweight smart, however, would have to absorb an impact corresponding to a crash against a stationary barrier at a speed of 67 kilometers per hour — double the speed of the Mercedes. In the end, however, the outcome of the crash test was much better than a first glance at the two smashed vehicles would suggest.

 

The measurements carried out on the four participating test dummies showed that potential occupants in the two vehicles would not have suffered any life-threatening injuries. The passengers in the S-Class would in all likelihood have walked away without a scratch, while the passengers in the smart would have suffered bruised ribs, and the driver would have also had some slight leg injuries.

 

Several different factors contributed to this relatively safe outcome. First of all, the smart — which is only 2.5 meters long — is equipped with a particularly rigid safety cell. This “Tridion cell” consists of an extremely firm steel frame that remains stable in a crash and thus protects the occupant cell. The S-Class, for its part, has a long and relatively “soft” front end that can absorb a great deal of impact energy. The solid core of the small car and the “soft” elements of the luxury sedan thus complement each other in a crash. This is known as compatibility.

 

DaimlerChrysler engineers have built compatibility into the company’s other models as well in order to guarantee maximum passive safety for vehicle occupants and those in any other car that might be involved in an accident. Rodolfo Schöneburg, head of the Passive Safety department at Mercedes-Benz Passenger Car Development, explains the meaning of compatibility here: “You can only say that a vehicle is compatible when it’s been designed for compatibility in terms of mass, deformation and structure.” In the case of the S-Class, achieving mass compatibility involved shedding 300 kilograms of the weight of the predecessor model, since this significantly improves the weight ratio in favor of lighter vehicles. At the same time, the little smart is designed in a way that ensures significantly less rebounding than one would normally expect after a head-on impact. This is achieved by something known as a mass damper — in this case, the compact rear axle drive module. In the event of a crash, this module, which consists of the engine and transmission housed in a single block, is guided by horizontally adjustable bearings. It impacts with the safety cell after a short delay, thereby compensating for the recoil, which is just beginning.

 

Deformation compatibility is most apparent in the S-Class in the high energy absorption capability of its long front end. In addition to an optimized flow of force brought about by a second side member plane, this compatibility is achieved by the short engine block for the V6 and the two front axle arms that have now replaced the lower triangular transverse links, which did not deform much. Deformation compatibility in the smart is exhibited in its very rigid front end, which is capable of deforming the front of the S-Class.

 

However, because of the rigid structure of the Tridion cell, there is only very little deformation in the smart (and thus a low level of energy absorption) in the event of its impacting an object other than a motor vehicle. “In order to

The force of the impact must be deflected so that the occupant cell remains largely intact. The illustration shows the energy-absorbing flow of force within the body of the new CLS-Class in a head-on collision.

 

ensure that occupants are subjected to a low level of energy impact, you have to provide them with an additional braking distance,” says Schöneburg. This is accomplished by a relatively large interior, with contact areas that give way in an impact, and a finely tuned restraint system.

 

The extremely rigid safety cell causes the smart to sharply decelerate immediately after a collision occurs. The control unit for the restraint system activates the seat belt tensioner correspondingly quickly, which means the passengers also participate in the deceleration of the body at a relatively early stage. The belt tension limiters then reduce the stress and the seat belts give way again.

 

Structural compatibility means that two vehicles in a collision are designed in such a manner that certain protective elements, such as the bumper brackets located behind bumpers, mutually activate themselves and absorb energy from one another. For this to happen, the bumper brackets in both vehicles must be located at roughly the same height. Otherwise there is an increased risk that the structures will miss each other in a collision or the structural interaction will not occur as desired. DaimlerChrysler engineers have been working on accident-compatibility issues for many years now. However, compatibility represents only one aspect of safety, which has been a central consideration at the German-American company throughout its history.

 

For example, the era of passive safety at Daimler-Benz began back in 1939, when developers at the Sindelfingen plant were assigned the new task of looking into the issue of safety in accidents. In 1951, the “structurally rigid

A scenario which people understandably dread, yet crash compatibility can be built into a wide range of vehicles. Here a Mercedes E-Class meets an  Actros truck

occupant cell, surrounded by crumple zones in the back and front,” was patented. The principle of deflecting impact forces into the vehicle floor by means of forked side rails also hails from this time. In 1959, the 220 series with its prominent tail fins was launched. This vehicle was the first in the world to be equipped with a rigid occupant cell and integrated crumple zones.

 

The first crash tests, which were unusual at that time and therefore attracted a lot of attention, confirmed the innovative design concept and made drivers conscious of the importance of safety for the first time. The “core” of passive safety systems at DaimlerChrysler today are quite literally the “highly rigid occupant cells,” which feature high- tensile steel members and sheets, and pass even the most demanding crash tests with flying colors. Last year, for example, the Mercedes-Benz E-Class received the highest marks in various consumer-protection tests, such as the European New Car Assessment Programme (Euro NCAP) and the Side Impact New Car Assessment Program (SINCAP) in the U.S., as well as in tests conducted by the renowned Insurance Institute for Highway Safety (IIHS). “These independent crash tests,” says Dirk Ockel, responsible for Safety Communication at the Mercedes Car Group, “include a side impact involving a mobile, deformable barrier, a side impact into a pole, and the so-called 40-percent offset impact against a deformable barrier, which represents one of the most frequent types of accident.”

 

Doing more than what’s required

 

Like all other models, the newly developed sedan also underwent an additional series of crash tests from the Mercedes-Benz program, which are not only more demanding than those of the consumer protection organizations but also go beyond the legal requirements in Europe, the U.S. and Japan.

 

The internal tests include collisions in the mid-speed range that are common in everyday driving. Accident researchers are convinced that special measures are required to mitigate the effects of such crashes. When conducting these tests, DaimlerChrysler pays attention not only to safety but also to easy reparability. “The current E-Class isn’t the only vehicle that’s been designed so that light or medium damage can be repaired at relatively low cost,” says Ockel’s colleague, Bernhard Holzapfel. One of the “crash specialties” at DaimlerChrysler is the “roof-impact test,” in which the engineers examine the behavior of the occupant cell in extreme stress situations, such as a rollover.

 

The most important point here is keeping the occupant cell intact. This is not a problem in closed vehicles, which have a rigid roof. It is more difficult, however, with convertibles or roadsters, which need to have a rollover bar to ensure occupant safety. Mercedes-Benz developed an elegant solution that uses a fold-out rollover bar in the SL-Class and CLK convertible (illustration, right). Here, a sensor system registers early on the possibility that the car might tip to the side. It releases the pre-stressed spring that deploys the roll bar within 0.3 seconds.

 

Despite the importance of the ratings from the consumer protection organizations, DaimlerChrysler engineers feel that just passing those tests is not enough when it comes to safety. Decades of experience at Mercedes-Benz have shown that standardized tests do not completely reflect the variety of accidents that actually occur. “That’s why we evaluate our vehicles’ behavior in a crash, and the protective measures we have installed, on the basis of real accidents rather than just tests and calculations,” says Schöneburg.

The automatic rollover protection system in the CLK convertible consists of two recessed roll bars made from steel tube. Should they ever be needed, pre-stressed springs deploy them within 0.3 seconds.

 

 

Systematic analysis of accidents

 

As early as 1969, DaimlerChrysler established a special Accident Research department. Since then, employees there have systematically analyzed and reconstructed thousands of accidents in which Mercedes-Benz vehicles were involved and which resulted in injuries. Their goal was and remains to identify areas for improving occupant safety. One of the first results of internal accident research was the realization that vehicle safety should not be judged solely on the basis of standardized crash tests. At the end of the 1960s, legislation went into effect in the U.S. and Europe calling for standardized crash tests, including a completely overlapping frontal impact against an object. However, analyses from the Mercedes-Benz Accident Research department at the time showed that this type of accident was extremely rare.

 

“Head-on collisions in which two vehicles coming from opposite directions crash into each other with partial overlapping in what is known as an “offset accident” are much more common,” says Dieter Scheunert, head of Accident Research at Mercedes-Benz Passenger Car Development. “What’s more, a vehicle that performs outstandingly in the original head-on crash test can easily exhibit flaws when it comes to the second, far more common type of accident,” he adds.

 

For this reason, Mercedes-Benz incorporated the offset crash into its internal testing program in 1979 and has continually optimized it ever since. The test has also been legally required in Europe since 1998. The history of crash tests goes back half a century. A big change took place about 15 years ago, however, when virtual tests were introduced. Simulations on super computers now make it possible to determine how a vehicle will behave in a crash more quickly, in greater detail, and above all less expensively than with a real crash test. “Nevertheless, our intention is not to replace one with the other but instead to exploit the advantages of both,” says Jürgen Kohler, head of the Calculations unit. “The combination of simulations, crash tests and accident analyses provides the information we need to develop even safer vehicles.”

 

One of the advantages of crash simulations is that they enable engineers to test vehicles that haven’t even been built yet. This significantly shortens development times for new models. For example, engineers can now assess and compare variants of a specific design within just a few hours, without having to build individual components or even entire vehicles beforehand.

 

Pursuing the vision

 

Today’s high-powered computers even make it possible to crash two cars into each other virtually in various types of simulated accidents. Moreover, engineers can eliminate components that impair their view onscreen, thereby enabling them to observe the deformation behavior of parts of the vehicle that would normally be hidden to them. Nevertheless, simulations only depict the behavior of an ideal vehicle and are therefore not capable of completely reproducing reality.

 

The introduction of crash tests and the stable occupant cell made DaimlerChrysler a pioneer in safety. The company has played this role well ever since, and intends to keep on doing so.

 

It will never be possible to completely prevent accidents, but in order to mitigate their effects, DaimlerChrysler developed PRE-SAFE — the first safety system that can recognize a critical situation at an early stage on the basis of driving dynamics data and then take measures to protect vehicle occupants. Future updated versions of PRE-SAFE will help reduce the risk of injury in an accident even further. Nevertheless, as has been the case for decades, there’s nothing like an alert driver when it comes to avoiding a crash.

 

 

 

Source: DaimlerChrysler press release, August 1, 2004.