A safe seat for lightweight vehicles

The Working Group on Accident Mechanics has developed a low mass vehicle (LMV) with a curb weight of 650 kg, called “Cratch”. This experimental vehicle demonstrates that a high level of passive safety for the occupants of low mass vehicles is achievable in frontal collisions (Frei 97). The development of a car seat suited for use in LMVs has been a part of this project. The seat is an important element of the restraint system: In the case of a frontal crash, the initial position of the occupant is defined by the contour and position of the seat, and, during the crash, a part of the occupant’s kinetic energy is absorbed through deformation of the seat base. In rear-end impacts the seat represents the entire restraint system.

During a collision against a conventional car, the low mass vehicle, due to the fundamental laws of motion, is exposed to higher accelerations and a larger change in velocity than its counterpart (Niederer 93). The seat presented here was specially adapted to these severe conditions. Nevertheless, almost every feature of the concept could easily be adapted for use in conventional cars. The main focus of the development was on the improvement of the rear-end impact safety, which represents a substantial problem, also for conventional cars.

Compared to the considerable improvements of crash safety in frontal and side impacts accomplished during the last years, progress concerning the rear-end impact safety has somewhat stagnated. This may be related to the fact that rear-end crashes are often considered to be less dangerous, since there is a very high surviving probability for the occupants. In spite of this, it is very worthwhile to invest in rear-end impact safety since injuries caused by this collision type do not only cause high amounts of compensation costs but also can have very unpleasant consequences to the occupants involved.

Energy absorption

In addition to the functional mock-up, two crash-testable models of the seat have been built. They were used in the Cratch experimental low mass vehicle in a full scale frontal crash test with a delta-v of more than 70 km/h, and in a series of sled based rear impact tests.

Figure 4: The crash test model of the seat: raw structure and completed seats integrated into Cratch low mass vehicle. Seating position is more upright as in conventional cars.

The requirement for a geometrical adaptability for the spectrum ranging from the 5th to 95th percentile occupant alone is not sufficient; the energy absorption capabilities of the seat must also be made suitable for the whole group. This means that the seat must deform softly enough not to exceed tolerance limits for light persons but must also provide enough deformation space for heavy occupants. Since the amount of prototypes was limited, e.g. more than one test per seat specimen had to be performed, the seats had to be reusable, leading to a rather robust and heavy construction. Seats for ‘real world’ use do not have to fulfil the reusability requirement, allowing for a less heavy construction.

The seat has been designed to withstand an sled impact speed of 33.3 km/h. This corresponds to a situation in which a standing low mass vehicle is hit on the rear end by a conventional car of twice the weight travelling at 50 km/h. Based on a force-deformation curve of an existing car and an assumed characteristic for the Cratch (which has not been rear-end impact tested) an acceleration-time curve for the Cratch has been calculated and simplified for use in simulations and sled testing. The maximum acceleration level is 30 g. Since it is known that cervical spine injuries can already occur at much lower loads, impact speeds of 22.2 and 11.1 km/h have also been taken into account for the design of the seat. The corresponding acceleration levels for these speeds are only 20 and 12 g, because in these cases impact energy is considerably smaller and the deformation zones of the cars are not deformed to a degree that higher forces (leading to higher accelerations) are built up.

In order to find suitable stiffness characteristics for the different energy absorbing units of the seat, a simple computer simulation model was used in which the occupant is modelled by four independent masses. Realistic results with such a model can only be expected in case where there are no, or very little translational displacements between the body parts. For our purposes, this is not a real disadvantage, since the aim is to find a setting wich results in a minimum of relative translational deformations (at least in the upper body regions). In a first step, the model was verified through comparison with a well-tried rigid body simulation program. Unfortunately, there is no model of the cervical spine available yet that is able to exactly mimic the behaviour of a human neck.

Figure 6: Simplified rear-end impact model of occupant and seat, used for computer simulation. The deformation characteristics of the paddings have been evaluated by dynamic impact pendulum tests.

Energy absorption is performed both by foam paddings and by rotational yielding of the seat back. Yielding is controlled by a deformation element, which consists of a three point bending beam made of aluminium. The yielding moment is 3000 Nm. During loading, the element builds up deformation force in the elastic range only gradually. This is undesirable as it causes a faster backward movement of the head restraint in the first phase of the collision. Bolts have therefore been integrated in the construction to obtain a deformation characteristic that sooner reaches its energy absorbing level. The bolts shear off during the onset of the yielding process and cause higher forces at the beginning of the deformation process.

The replaceable deformation elements are the only structural parts of the seat that are supposed to absorb energy. Energy absorbing properties of other load bearing components are irrelevant. This means that the seat concept and the choice of material for the realisation are almost independent of each other.

The centre of rotation of the yielding back rest is positioned relatively high above the seating level. Yielding of the back rest is thus delayed and an earlier contact between head and head restraint is obtained. Because the pelvis is already in contact to the back rest at the beginning of the crash, no considerable relative velocities between the pelvis and the back rest arise during impact and therefore little deformation space is needed in this region.

The acceleration levels of the different body parts are mainly influenced by the stiffness characteristics of the foam paddings. The paddings have to be chosen such that relative movements between head, neck and thorax are minimised. A combination was found that works adequately under the conditions mentioned above.

Even with an automatically adjusted head restraint, for comfort reasons there will remain some initial distance between the head and the head restraint, causing a delay of the acceleration of the head in comparison to the thorax. A layer of a very soft foam applied in the thorax region reduces the acceleration of the thorax in this first phase of the impact (Muser 94) and thus helps to synchronise movements of the head and the thorax (as tests by Svensson (96) have shown).

Assembly of energy absorbing foams in the seat back. A hard foam type (Woodbridge Enerflex) and two softer foams (Dow) have been used. Empty spaces in front of protruding structural components prevent excessive compression of foams and increase of forces in these regions.

Renault is hoping supermini buyers will be ‘Vel Satisfied’ with the look of the new Clio, which is revealed by Auto Express with this world exclusive spyshot picture.The distinctive supermini borrows its bold front-end styling from the controversial Vel Satis, and is charged with storming straight to the top of its competitive market sector when it goes on sale in the UK in September priced from �7,500.

Destined to go head-to-head with the forthcoming Ford Fiesta and the still-secret Peugeot 107, the radical Clio will boast an innovative new range of engines and gearboxes. These include direct-injection petrol and diesel powerplants, and new CVT transmission plus a revised five-speed manual. Both the three and five-door editions will be available from launch, and the duo will be instantly identifiable from the existing cars thanks to bold new headlamps and an aggressive grille. Wing mirrors, sills and side rubbing strips are also revised, as are the edges of the bumpers, which now blend into the slippery lines more effortlessly.

At the rear, revised lights share centre stage with a huge Renault badge. And in keeping with company policy, the manufacturer’s name will no longer appear on the bootlid. Major interior changes will place greater emphasis on safety. It’s thought that the Clio will now include curtain airbags, following the lead set by Vauxhall’s Corsa, and it will aim to gather the full five stars in the revised Euro NCAP crash-test series.

Equipment levels, too, are expected to be more generous than at present, and will probably include satellite navigation for the first time. Keyless start, as fitted to the Laguna (see photo) and Vel Satis, will not feature, however.

The engine line-up will be largely familiar to buyers of the current Clio, as the powerplants are among the newest in the business. The company used to have a reputation for poor-quality, outdated motors, and so has worked hard to transform its entire range into the most sophisticated in the business.

First in the line-up will be a new 1.2-litre 16-valver offering 75bhp and class-leading fuel economy, followed by the 98bhp 16v 1.4-litre. The top-of-the-range luxury Clio will be powered by the 1.6-litre unit which features 110bhp, while the 172 tuned by Renaultsport will continue to use the fire-breathing 2.0-litre engine – ensuring it keeps its place at the top of the hot hatch tree. Renault will also keep pushing its diesels, which are enormously popular in mainland Europe. Two options will be available, both based on the company’s state-of-the-art new 1.5-litre dCi oil-burner, with either 65bhp or 85bhp.

Gearbox choices will be limited to a five-speed manual and a conventional auto at first, although the firm is experimenting with Nissan’s acclaimed CVT automatic from the current Micra. Renault’s own ‘Easy’ clutchless manual was dropped due to slow sales and is unlikely to be revisited.

Of course, the new look isn’t only reserved for the mainstream models. Renault’s stylists have also interpreted it for the fire-cracker hot hatch, the Renaultsport 172. And our spy photographers were able to bring you an all-round tour of the model which must uphold the honour of the craziest hot hatch on the market today. Inside has been treated to a mild spruce-up. You can clearly see new metallic-effect sports trim on the facia, and Renault insiders say that there will be revised specification levels for the UK. However, the main architecture remains unchanged.

Visually, the new face has a meaner stare, with an aggressive front airdam that’s open rather than fluted. The deeper rear valance comes with an air-vent groove beneath the bumper line. New five-spoke alloys, a chunky Renault logo on the tailgate, colour-coded side-rubbing strips and a new slash of silver trim complete the picture.

As you can see, the styling tweaks lift the Clio’s profile slightly upmarket, giving it a more mature and sensible image – but don’t be fooled… The tuned 2.0-litre four-cylinder 16-valve engine remains one of the most entertaining hot hatch motors of its kind, and the power output will stay at 172bhp, guaranteeing the same performance.

However, if enthusiasts were hoping that the wildest hot hatch ever – the Renaultsport Clio V6 – would get the same changes, they’ll be disappointed. As it’s such a specialised, low-volume machine, it is remaining unchanged throughout its lifetime. Not that we are complaining! Something as anti-establishment as the Clio V6 isn’t about to appear dated just because the car it was loosely based on has had a subsequent facelift. Of course, the standard model’s fresh look is designed to keep the Clio riding high in the sales charts until an all-new successor arrives in 2003. This will share its platform and engines with the next Micra, although they will have different interiors and styling to reflect their diverse characters.

On the whole, these latest revisions to the range look set to keep interest levels bubbling away for the already successful Clio family. And what’s more, the timing couldn’t be better, as there is a whole host of new metal lurking on the horizon…

With the likes of the stylish MINI here this summer, a Fiesta replacement being unveiled in September, the next-generation Peugeot 106 and Citroen Saxo due next year and Volkswagen’s Polo arriving in 2003 as well, the Clio must face up to some stiff competition.

But by welcoming its baby to the latest family look, Renault is out to prove that it hasn’t only been concentrating on larger products such as the Laguna, Avantime, Vel Satis and next Espace. There’s more than enough fight left in the supermini – especially as insiders say that the update won’t affect the pricing strategy.