Crash testing as the elite discipline in accident research
The goal of all developments of the Mercedes-Benz accident researchers in the late 1950s was to simulate a crash as comprehensively as possible. Consequently there was no way round the crash test – which was time-consuming, difficult to control and costly. This test procedure involved subjecting a vehicle to a controlled collision that resulted in corresponding deformation. The roles of the vehicle occupants were played by the crash test dummies.
At the first Mercedes-Benz crash test in 1959 a test car was accelerated head-on into a fixed barrier made of old press dies otherwise destined for the scrapheap. After this premiere the engineers paused for a while in order to evaluate the new procedure. Then crash testing was conducted again on three further test days between March and April 1960. This included the simulation of complete accidents – for example, the side-impact collision between a vehicle from the W 111 series and a saloon of the same series. Once again, head-on collisions were carried out and for the first time roll-over tests.
Pioneering tests with older cars made possible the design of a ramp that rotated the test cars about their longitudinal in mid-air so that they landed on their roof. This meant the safety experts were able to measure the reaction of the body in the event of a roll-over accident.
Another key element during this second year of crash testing was the company’s public relations policy on matters to do with accident research. Daimler-Benz invited the press to attend the day of testing on 11 April 1960 in order to give an official presentation of the latest approaches to safety research. In so doing the experts clearly ranked the importance of vehicle safety above previously held concerns about elaborating on the issue of accidents in customer communications.
Tests and technical development
Mercedes-Benz crash testing from 1959 onwards led not only to advances in vehicle safety. The technology used for these spectacular crash tests was also constantly improved. To begin with, for example, a winch was used to accelerate the cars, since this could not be done under the power of their own engines. The engineers borrowed winching technology used by the glider pilots at the Technical University to launch their aircraft. At Mercedes-Benz, however, the tow was used to launch brand-new saloons so that when they took off from the so-called corkscrew ramp at speeds of 75 to 80 km/h they rolled over in the air.
However, the winch system proved less than ideal for accelerating the test cars. And the head-on test also left much to be desired, as on occasions the test vehicles missed the barrier or stationary car altogether, resulting in a rescue call-out to the plant fire service. For in those days testing was conducted in the open air on premises belonging to the Sindelfingen plant. The area was bordered by a stream named the Schwippe – the final resting place for more than one vehicle during these pioneering years.
In 1962 Ernst Fiala came to the rescue. For the crash experiments conducted by Karl Wilfert and his team, Fiala designed a hot-water rocket that powers the vehicles without the use of a tow line. Fiala received critical advice for his design from the Institute for the Physics of Jet Propulsion based at Stuttgart Airport. Mounted on a single-axle trailer, the apparatus was attached to the rear of the test vehicle and consisted of a pressure tank, a fast-opening valve and a discharge nozzle. To create thrust, the tank was three-quarters filled with water prior to the experiment and heated until the water temperature reached around 260 degrees Celsius. When the valve was opened, this excess pressure accelerated the car and rocket ensemble to speeds in excess of 100 km/h.
The introduction of the hot water rocket in 1962 also led to improvements in the test track. Reinforced concrete was used for the runway, and the test vehicles and propulsion system were now guided on rails. A fence was also erected to prevent cars landing in the stream. Such clear improvements to the test facilities were not only important for in-house testing. From 1962 to 1967 Mercedes-Benz also carried out testing on new types of crash barriers on behalf of the State of Baden-Württemberg. In addition, in 1964 the test strip was lengthened from 65 metres to 90 metres in order that heavier passenger cars such as the 600 from the W 100 series could also be subjected to crash testing.
Dummies and data
The great advantage of crash tests over crash vehicle analysis lay in the opportunities these offered to record the actual events of the crash in every detail. The necessary analytical technology to do this had been developed in the years prior to 1959. This included the use of acceleration sensors fitted to the dummies and the test car itself, as well as high-speed film that could reproduce images in extreme slow motion for purposes of collision analysis.
The use of simulation dummies ensured very diagnostically conclusive results, since the dummies equipped with capsules for recording acceleration values – which had been in use in the United States since 1952 – for the first time produced repeatable data relating to the stresses to which the human body was subjected in a car accident.
Oskar, the VIP-type dummy produced by manufacturer Alderson Research, initially remained the only test dummy used for early crash testing in Sindelfingen. Front seat passengers were simulated more often than not using sandbags or shop-window mannequins. Even when other more advanced test dummies were acquired later (in 1972 eight units were in use altogether), Oskar continued to provide invaluable service – indeed this first ever test dummy went on to serve the Stuttgart brand’s safety research department for three decades.
However, the dimensions and data recording methods of the dummies soon began to differ from the VIP model. In the 1960s dummies were designed with body proportions in line with the average size of the typical man, woman and child. The precision with which specific test types were conducted was also constantly improved – in addition to dummy passengers there was also a test dummy for simulating accidents involving pedestrians. Dummies were also developed to measure specific accident scenarios with great precision, as well as highly specialised dummies for tests other than crash tests – for example, the climate dummy, used to establish the influence of air temperature and humidity on the human body.
Building of the new crash test facility
The increasing number of crash tests and the increased importance attached to the results of these tests revealed the clear capacity limitations and suitability of the old test track in Sindelfingen by the late 1960s. So a new accident research centre was built in Sindelfingen between 1971 and 1973. Initially the safety researcher installed a Bendix sled for crash simulation purposes. Then in 1972 construction work began on a new crash installation that would also facilitate head-on and side-impact collisions.
As a drive system to propel the test vehicle along the 65-metre test track the engineers opted for a linear motor with a thrust of 53,000 Newtons. This power unit accelerated the cars to the target speed over the first half of the track, then regulated the speed over the remaining distance to the desired value before decoupling in advance of the collision. In one direction of the track the test cars crashed into a 1,000-tonne barrier resting on a sensitive force plate. Roll-over tests were conducted in the opposite direction. Collisions involving other vehicles, on the other hand, were still conducted on an open-air site.
From rigid barrier to modern offset crash
From the outset, one goal of Mercedes-Benz crash testing was to produce results that were as realistic as possible. Increasingly, therefore, the head-on collision into a rigid barrier made of old iron or concrete was replaced with an offset crash. Research into offset collisions began in 1975.
In 1992 this offset front impact was conducted for the first time at Mercedes-Benz using a deformable barrier in order to produce results that were even more comparable with the behaviour of a vehicle in a real collision. A non-rigid barrier was subsequently developed for European car testing, the design of which had a significant influence on test results from the Sindelfingen safety centre. With the introduction of offset barrier testing, this new European test approach represented a further major advance in the direction of realistic accident research. In 1993, the offset crash against a deformable metal barrier with a honeycomb structure at 60 km/h and with a 50 percent overlap became the new Mercedes-Benz standard.
When the Mercedes-Benz Technology Center (MTC) was created in 1998 (the foundation stone was laid in 1995), the crash testing facilities at Sindelfingen also underwent modernisation. The acceleration strip was now lengthened to 95 metres, which meant that all types of crash tests could now take place indoors. In particular this applied to the offset crash, which in reality was a much more frequent occurrence than the head-on collision of a vehicle. Now, however, even head-on collisions between two vehicles could be carried out indoors, since the linear motor was also replaced with a cable winch system. High-speed film cameras were no longer used to record the tests; instead the crash process was recorded using video technology. The very high frequency of the image series remained unchanged so that the crash tests could be analysed in detail in extreme slow motion. During the rebuilding work, the extension to the test facility was also given a roof, which meant that tests with both passenger cars and commercial vehicles could now take place regardless of the weather outside.