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Better Cars for People Who Walk

While there have been enormous advances in automotive engineering over the last couple of decades, nothing has quite equalled the resources that have been put into reducing exhaust emissions.

While there have been enormous advances in automotive engineering over the last couple of decades, nothing has quite equalled the resources that have been put into reducing exhaust emissions. What was considered impossible 20 years ago has not only been met but also substantially exceeded. Tough though they have been, these challenges almost pale into significance beside those that the motor industry now has to think about: pedestrian safety.

At first glance, pedestrian safety does not arouse much interest. Ensuring that greater protection is afforded a human being in an impact with a vehicle does not sound too much of a stretch, bearing in mind the laws of physics. After all, there are only certain things that can be done. This, though, is far from the truth. The proposals that are likely to come into effect over the next few years are not only set to change the way cars are made, but also the way they look.

As things currently stand in Europe, there are two important factors at play. The most significant one is something called "Working Group 17" that has been set up by the EEVC–the European Enhanced Vehicle-Safety Committee–to draw up and recommend proposals on pedestrian safety that can be presented to the European parliament as the basis for legislation. It is working on a timetable that would see any new proposal applied for new vehicle types from about 2008.

As a counter to this, the European Automobile Manufacturers Association (ACEA), the body that represents the interests of the European carmakers, has drawn up its own proposals that will be adhered to by the European manufacturers in a voluntary/compulsory sort of way following negotiations with the European Commission. Divided into two stages, the first commitment is to bring new vehicle types into line with the recommendations of the Commission Joint Research Centre (JRC) with effect from 1 July 2005 with the second covering new vehicles over a period from 2010 to 2014. This is subject to a feasibility study to be conducted by 1 July 2004. Although it is too early to state categorically, inside opinion is that it is the latter option that will prevail, especially if they are also adopted by the Japanese and Korean manufacturers associations (JAMA and KAMA), who are in separate negotiations with the Commission.

It is translating these proposals into fact that has been exercising the minds of those involved in launching new models in just over three years. What at first sight seemed to be a relatively straightforward exercise is an extremely complicated one in which a huge number of issues need to be resolved. These include decreased front-end stiffness and styling, increased weight (and thus increased CO2 emissions) and packaging. Chief amongst these, though, is the interconnectivity of many components and subassemblies in the vehicle to many other assemblies and functionalities. Seemingly unrelated parts can have significant effects on the performance of others, not just physically but in terms of meeting targets.

As pointed out by Stuart Smith of TNO Automotive at a conference organised by Ricardo last year, the choice of powertrain affects compatibility and the front structure. This, in turn, affects the ability of the sensor diagnostic module to meet the legislative criteria of deploying the airbags in high-speed impacts, as it is also affected by the low-speed pedestrian protection performance. This, in turn, affects the hood style and therefore the NVH performance, which is dependent on the driveline characteristics, design, and so on.

Stiffness is also an issue. For leg impact it is important to achieve a proper balance in stiffness between the hood's leading edge, the fender and spoiler. Apart from the sheet metal itself, there is also the immediate infrastructure that needs to be considered. Hood locks, for example, are often tweaked to reduce hood flutter at high speeds, while there are many load areas designed in to withstand severe impacts so as to maintain a controllable collapse of the hood.

Gary Brown, engineer with responsibility for pedestrian safety at MIRA, mentions another incompatibility–in order to comply with legislation, the upper leg must be able to pass through the headlights. The trouble is, though, that lights have their own stiffness requirements, added to which the next-generation versions are becoming bigger and heavier. "The new active adaptive headlights are going to be a great safety bonus for the driver to avoid accidents," he says, "but they're not going to be so good for the pedestrian if they are hit by one!" Again, more package space is needed for the pedestrian impact. "The end result," says Brown, "is likely to be an increase in the use of aluminum in hoods because its lower stiffness-to-mass ratio and strain rate sensitivity are better suited than those of steel." In other words, the faster a steel panel is hit, the stiffer it becomes while there is only a small difference in the strain rate in an equivalent aluminum panel. There is a downside for the automakers, though, and that is in the increased cost.

According to Brown, the days of the sleek, handsome sports car may be a thing of the past. "As things stand, there is simply no space between the hood and the engine as this has been minimized to its full extent. However, if a car is to comply with pedestrian safety requirements, a space must be created. It is unlikely that the engine can be lowered any further, so the only alternative is to increase the height of the hood. This, in turn, means that in some cases the height of the car's occupants has to be increased, otherwise the field of view might be obscured, which means that the height of the roof has to be increased. Now, of course, you are talking about major changes to the body-in-white."

Packaging is a huge issue, one in which the question of whether the size of a gap is 35 mm or 40 mm can be argued over between engineering teams for hours, if not days. Legislation is going to demand that the front of a car must absorb up to 700 Joules of impact energy on the front edge of the bonnet. "This means for high front vehicles such as SUVs there needs to be about 300 mm from the surface before anything stiff is hit," says Brown. "With space in a car being at such a premium, it is difficult to know how this is going to be achieved. The problem is," he continues, "that the maximum practical impact energy absorption as things stand is about 500 Joules before solutions become unfeasible. The 15-kN impactor load seen on many current vehicles must be reduced to 5 kN, which means that the internal target is therefore just 4 kN going by the traditional 80%. If you take a short car, for example, it will probably need an increase in front overhang of between 150 mm to 200 mm made of a soft material that will allow the car to meet the upper leg requirements. However, a soft material without support undermines the integrity of the car in other areas, such as hood torsion and bending stiffness needed for driving at high speeds."

With such issues needing to be addressed, especially with the changes that will be needed to the body structures, Brown says that it is important that the advanced engineering teams are aware of as many facts as possible at the start of the programme. "Pedestrian protection is all on the surface of the vehicle as it's not possible to lower the engine or suspension by even 30 mm because that's already been done."

While pedestrian safety has not yet entered the public domain as a burning issue, there is no question that over the next few years it will do so. As the introduction of the EuroNCAP rating system five or six years ago so transformed the public perception of vehicle occupant safety, so the new tests, which are based on the EEVC guidelines, will do for pedestrian safety. The face of the car in Europe is going to change dramatically over the next few years. 

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