Crunch time

New EU safety directives are poised to force car manufacturers to rip up the designs of some of their leading models and abandon cherished styling features.

Pedestrian safety regulations due to come into force in Europe from next year could drastically affect the appearance of sports cars, such as the BMW Z4 (above) executive saloons, off-roaders and small cars in the Smart or Ford Ka class.

A year-long study by MIRA, the automotive industry research body, suggests some manufacturers may even have left it too late to deal with the issue.

The regulations cover the impact that the front of cars has on pedestrians in an accident, setting out strict criteria that vehicles must meet in tests in terms of the injuries they are likely to cause.

Engineers and designers will face difficult trade-offs between packaging – fitting in all the components – and styling. Structural specialists will have to reconcile conflicting demands for structures that can absorb impact while retaining sufficient strength. Knock-on effects will hit everything from sightlines to aerodynamics and headlamps

Finding the right solutions will be critical for car makers. Gary Brown, principal engineer for pedestrian protection and crashworthiness at MIRA, which has been conducting in-depth R&D in this area, said: ‘Pedestrian protection is now driving styling. A poor-looking vehicle can break a manufacturer if it doesn’t sell.’

With the implementation date drawing closer MIRA has worked with a number of the big car makers on pedestrian safety issues, said Brown. ‘Some manufacturers are very well prepared. Others have not paid much attention and some are going to get caught out.’Adrian Guyll, head of vehicle safety at MG Rover, confirmed that the issue is high on the agenda. ‘It’s quite a challenge. It requires a fundamental rethink of the front of the vehicle and many cars could end up with much bulkier front ends.’

The requirements are being introduced under an EU directive in two phases. Phase one, which comes into effect in October next year, sets injury limits for three types of impact: lower leg to bumper, an alternative upper-leg-to-bumper test for off-roaders, and child/small adult head to bonnet. Two other types of impact are monitored: that is they have to be measured but no pass or fail level is specified. These are upper leg to the bonnet leading edge and an adult head to bonnet.

Phase two comes into effect in September 2010 and sets more stringent criteria for the leg-to-bumper impacts. Limits are set for a child’s head hitting the front of the bonnet and a heavier adult head hitting the rear, and for upper leg/bonnet leading-edge impacts. Meeting the specification for phase two would equate to scoring maximum points in the European New Car Assessment Programme (NCAP) pedestrian safety tests.

All the tests are based on an impact speed of 40kph. The requirement is to limit the force of the impact sufficiently to prevent breaking a pedestrian’s legs. Medical research says 5kN is enough force in a shearing action to break a thigh bone, while 300Nm is enough bending force to do the same. These figures represent the limits set by the new laws.

The regulations will apply to vehicles going through type approval after the dates specified, which means new cars and also those going through mid-life updates, since the front bumper and bonnet are the most common areas to be modified.

For designers the requirements pose two different sorts of problem. First, outer panels of the car need to be designed with enough flexibility so that in an impact they deform progressively to limit the forces acting on the pedestrian. Second, for this to work, empty space has to be built in behind bumpers and under bonnets to prevent secondary impacts with hard objects underneath, such as the engine.

This is not easy. Structurally there are difficulties in making panels flexible to absorb impacts at the same time as remaining strong enough to resist other loads ranging from air pressure at high speed to people leaning on them.

At the front of the car space is at a premium. Engineers trying to fit in an ever-increasing number of components and designers trying to maintain an aesthetically pleasing low bonnet-line already struggle to find acceptable compromises over packaging.

MIRA has carried out extensive modelling on all aspects of the pedestrian protection regulations, including a 12-month programme on upper leg/bonnet leading-edge impact. It has modelled real structures, materials and load cases to arrive at designs that can be manufactured. The organisation has also helped several manufacturers draw up guidelines for their designers.

From this work, and with the proviso that no two models are identical, Brown has been able to distil some general guidelines on the implications. First of all, he said: ‘It’s important to get the styling or packaging right at the initial concept stage – you must get it in at the very start of the programme or you won’t meet the requirements.’

To meet the phase one lower leg/bumper impact requirements around 60mm of space will have to be provided between the front bumper skin and the steel or aluminium cross beam behind it, rising to 80mm in phase two. This allows space for the bumper to deform, limiting forces on the leg.

In addition, to help keep the lower leg vertical and limit bending stresses in it, a lower stiffener must be incorporated into the bumper, effectively bringing its lower leading edge forward so it is more or less vertically below the front-most point of the car.

This relatively simple-sounding change actually has a potentially big knock-on effect on the aerodynamics. The front undertray is designed to channel air over the radiator and brake discs, and this will have to be reoptimised. Moving the lower edge forward will tend to reduce lift at the front, changing the aerodynamic balance of the vehicle, so this will have to be compensated for. In turn, these changes could add drag, affecting fuel consumption and emissions. None of this is particularly difficult, but it is not an insignificant exercise.

Not all the extra packaging need be added directly to the length but Brown estimates front overhangs will increase by 20-40mm in phase one. Coupled with the lower stiffener, this means the ramp angle – the slope the vehicle can negotiate without the bodywork catching on the ground – will also have to be watched.

The alternative test for upper leg/bumper impact is intended for sport-utility vehicles, where ramp angles in off-road use are more critical. No lower stiffener is required, but Brown reckons the extra space needed will be roughly double that needed in the normal test.The head/bonnet impact rules raise another set of problems. In phase one the main injury criterion only applies to two-thirds of the bonnet area, with a less severe requirement over the remaining third. This allows the bonnet to be stiffer round the edge, and is a concession to car manufacturers so that bonnet design does not yet have to be changed radically.

Bonnets have problematic hard points at their hinges, latches and bump stops, and all the way along the edge meeting the wing, because the wing has to be strong enough to carry other loads.In general, said Brown, bonnets are too weak in the centre, allowing secondary impact with components underneath, and too hard around the edge. ‘You need to stiffen the bonnet with extra bracing in the centre and make it weaker round the edges. But it still has to resist the forces imposed on it by being opened and closed.’

There is an additional complication: ‘Styling lines on bonnets are an enormous problem,’ said Brown. ‘There is a local stiffness change and a sharp edge. There’s a lot more work to be done on the influence of styling lines. They have finite element engineers pulling their hair out.’

He recommends 35-55mm as a good starting point for the empty space to be provided between the bonnet and the top of the engine. In most cases, he reckons bonnet lines will have to be raised by 20-40mm in phase one. This presents particular problems for sports cars with large engines at the front, such as the BMW Z4 or Honda S2000.

‘Sports cars are likely to suffer more from phase one than any others – there’s a danger of compromising the occupants’ field of view.’

Increasing the height of the occupant to compensate means raising the height of the roof, which in turn affects the whole design of the body and its aerodynamics, drag and even crash performance. ‘The seating position is the hub of the car: everything starts from there and is designed around it,’ he said.

Things get even more difficult in phase two, when he estimates an extra 80mm of packaging space will be needed under the front of the bonnet for the child’s head impact, and 100mm at the rear for the adult head. ‘There will be many more problems with occupants’ field of view: much more of the vehicle fleet will be affected,’ he said.In addition, the less severe requirement around the wing edge will no longer apply, which means clamshell bonnets will become the norm. ‘We’ve tried to design a wing edge to meet the head impact criteria which doesn’t collapse when you lean on it, and the two stiffness requirements don’t add up,’ says Brown.

But perhaps the most problematic of all is the phase two requirement for upper leg/bonnet leading-edge impacts. The test specifies the amount of energy in Joules to be absorbed in the collision, and is very sensitive to bonnet height. High, flat-faced vehicles cause more severe contact with the upper leg/hip and abdomen than low, sleeker vehicles. The higher the bonnet, the more energy, and the more space has to be packaged to absorb it. In addition, there is again the difficulty of combining the need to be flexible in an impact and strong enough to resist other loads at the same time. For cars at the top of the range of 700J, 200-250mm of space will be needed.

Initially it was thought that this requirement would be a problem mainly for SUVs, but Brown said: ‘Even a small car with a bulbous nose to meet engine packaging needs can find itself in the 700J range. SUVs will suffer badly, but small cars are even more of a problem. For SUVs it’s difficult, but not as bad as finding 200mm on a Ka or a Smart.’

Executive cars, which also tend to have high bonnet lines, will face the same problem. Brown said: ‘Originally we thought that not just the packaging space but also the stiffness requirement would be a problem. I had doubts about finding acceptable solutions over 450J. But we now know how to engineer a vehicle for pretty high-impact energy. We’ve identified all the problems, though we haven’t overcome them all.’

The penalty is that there are big styling implications, which haven’t yet been fully worked through. ‘Manufacturers will have to go through a big styling exercise to see if they can make it work,’ he said.

Greatest concern is now over small cars. For vehicles such as the Ka or the Smart, on which the front slopes up in a more or less continuous line to the windscreen, the bonnet leading edge in effect becomes the bottom of the windscreen. This is problematic because key elements of the car’s structure meet at this point – the windscreen bottom rail, a bulkhead, the beam supporting the instrument panel. The windscreen bottom rail is a key structural member and contributes to torsional stiffness and crashworthiness.

Brown added: ‘Most small cars have a bulbous front end to accommodate the powertrain and cooling. You can’t lower the bonnet line because there’s not enough packaging space; you can’t push the cooling system down because there’s too little ground clearance.’

Car makers may therefore be faced with increasing the front overhang to make the bonnet longer and reduce the height of the leading edge — provided designers can make this look acceptable.There is just a chance car makers could be given some breathing space. A review of the phase two requirements is currently in progress to consider whether any alternative test method would result in as good or better injury protection while retaining more design freedom for car makers. It is due to report later this year, but few have great optimism about the likely outcome.

The other possible escape route is to use active systems, which car makers are also researching. The European SAVE-U research project is due to test a multi-faceted sensor system this month. This combines infrared, radar and vision-based technologies to detect an impending impact with a pedestrian.

The hardware needed is all relatively well-known. More difficult is devising robust algorithms and demonstrating that the systemwould work in real conditions.

Could it correctly recognise pedestrians of different sizes in allweather conditions, and not fire unnecessarily?

If the sensing system could be perfected, it could even apply the brakes and possibly avoid the impact in the first place.

It could also lead to the practical deployment of the ‘pyrotechnic bonnet’. This would use small explosive charges, familiar from seat-belt pretensioners, to raise the bonnet in an impact crash, in principle providing the crush space without affecting styling.

<b>Model motors: the Honda Civic and MG TF</b>

The Honda Civic five-door gained the highest-ever NCAP pedestrian safety score in 2001, narrowly missing out on four stars. (Since then the system has changed so star ratings from 2002 are not directly comparable with those up to 2001).

The Civic’s performance was the result of a policy decision to address the issue. Honda began introducing its enhanced pedestrian safety design from 1998. It believes its current technology will be enough to meet phase one needs.

Features that contribute to the rating include impact-absorbing wing supports, bonnet hinges and windscreen wiper pivots. The outer bumper shell is designed to absorb leg impacts. The bonnet leading edge is low, but the occupants sit quite high, permitting a bonnet line with room for empty space above the engine.

MIRA’s Gary Brown says: ‘They wouldn’t be able to do this on a smaller vehicle or sports car. The Civic would still need a significant amount of development to meet phase two. But it’s given Honda good experience, and they’re probably a long way ahead of everyone else.’

The MG TF achieved three stars in 2003. It started with the advantage of a mid-engined layout, but MG Rover head of vehicle safety Adrian Guyll says that during the facelift of what used to be the MGF the opportunity was taken to rethink the front.

The front bumper/nose section is made of polycarbonate, which has good energy-absorbing properties, and the nose is low and ‘non-aggressive’. Brackets under the bonnet and wing edge were re-sited and components such as the horns moved. The headlamps are set back behind polycarbonate lenses, an arrangement also adopted on the new facelift of the Rover 45 and MG ZS saloons.

Guyll said there was a ‘juggling act’ to implement these features while keeping good performance in the front offset impact test, but he believes there could be a trend towards a mid-engined layout in sports cars. ‘You get excellent dynamics as well as the benefits for pedestrians.’

<b>Hard Spots</b>

Headlamp design will also come under the microscope. Headlamps are heavy and unyielding, and to meet the legislation they are likely to have to be made to detach in a controlled way. Alternatively they will have to be designed to absorb energy, or, as in MG Rover’s approach, be set back behind softer polycarbonate lenses.

A lot of development work remains to be done, especially as some cars may have several lighting options (halogen, xenon, and adaptive), and one result might be to hasten the development of LED lights.

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