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The 3-point belt remains central in today’s and tomorrow’s safety solutions

8 January 2009

Many people believe the three-point safety belt has not changed over the past fifty years. They are right. And wrong. The belt’s ingeniously simple basic design is the same. The difference today is that the belt is part of a high-tech safety system under constant development.

The three-point belt has played a central role for occupant protection in all Volvo’s cars since its introduction in 1959. The inertia-reel mechanism, which made the belt more comfortable and flexible, became standard in 1969. Still it represents the most recent major change visible to the naked eye.

However, even if the changes are not visible, work on enhancing the safety belt has continued at Volvo to keep pace with the rapid advances in safety technology in general.

"The safety belt is unique in that it so effectively catches and restrains the occupant in the seat. Other systems can complement and help the belt do its job even better. For instance by providing information about the forces in action and by interacting with the belt during the collision sequence to provide optimum protection," says Lennart Johansson, head of the Interior Safety department.

Controlling collisions and forces
One example is the belt pre-tensioner, which when it receives a signal from the crash sensor tensions the belt against the body of the seat occupant if a collision occurs. This reduces the gap between the belt and the body caused for instance by thick clothes such as a winter coat. The safety belt pre-tensioner thus makes it easier for the belt to restrain the body as early as possible.

Seat occupants also benefit greatly from the force limiters that are integrated into the belts of modern Volvo cars. Thanks to a sensor that monitors how quickly the belt is being reeled out, the force limiters can for instance detect the dynamic mass - the seat occupant’s moving body - as it is thrown forward.

This in turn means that it is possible to tailor the force with which the seat occupant’s body is restrained and to optimise the absorption of the resulting dynamic mass. If the body is restrained by the belt with excessive force, the body may suffer injury. If on the other hand the force limiter is set too low, the body will be thrown forward too quickly against the airbag or instrument panel.

Force limitation can be exploited in different ways depending on the situation. For example, the force level in the belt may be higher at the start of the sequence, then switch to a lower level once the airbag takes over part of the task of energy absorption.

Sensors determine how the systems interact
The size of the seat occupant and the type of collision are the main parameters that determine when and how the belt pre-tensioners, airbags and force limiters will be deployed. In order to make the right decision in each case, the car’s on-board computer uses data obtained from thousands of pre-programmed collision scenarios and real-life accidents previously analysed by Volvo.

Scenarios spanning the entire range from high-impact head-on collisions to underrun crashes into trucks and difficult to interpret side impacts have been used in the system’s development. The type of accident determines how quickly and at what level the various systems are activated.

The information comes from sensors located all over the car. A main processor in the middle of the car collates the data and accordingly decides how the systems, including the safety belt, should interact.

Solution tailored to suit the occupant
If the car is fitted with an integrated child booster cushion in the outside rear seat, the belt protection system is tailored to suit and differs compared with the front seats. The reason is that this rear safety belt must also be able to protect a smaller, lighter person as effectively as possible

Development of the belt: for increased safety and usage
Today’s safety systems are optimised in relation to each other and the belt still plays a central role. But what of the future? Will we still be using the safety belt in 2020?

"At Volvo we are convinced that the belt will still be around in 2020 and way beyond that too. The belt may look somewhat different. It may have a four-point attachment instead of three. It will probably be designed so it is even easier to put on than it is today. Only when we have cars that automatically ensure that they are not involved in collisions can we do away with the belt. But that’s a long way off yet, even though there is a lot of research going on in that area too," explains Lennart Johansson.

Belt development follows two parallel tracks: one is to make the belt and the system in which it operates as safe as possible, and the other is to adopt a variety of measures to make the belt even easier and more convenient to use.

Four-point safety belt also possible in cars
The four-point safety belt has been discussed as a possible alternative by Volvo and other manufacturers too, and several solutions have been presented over the years. However, there is as yet no good technical solution that offers a suitable balance across the entire requirement specification.

The four-point belt has obvious advantages. For instance, it restrains the occupant more effectively if the car rolls over (one reason why rally cars are fitted with four-point safety harnesses or belts with even more attachment points). It also reduces the small risk of the seat occupant sliding out from under the three-point belt.

However, the four-point belt also has disadvantages. It should preferably be designed as a cross, forming an X pattern across the body. It is across the ribcage that the human body is strongest and has the best chance of absorbing incoming collision force. The challenge therefore is how best to effectively attach the upper point of the belt to the car where there is no natural attachment point in the bodywork.

Another challenge relates more to usage: for the past 50 years now, people have acclimatised to the three-point safety belt. How would a new solution be received? Is the possible benefit of better anchorage in the seat sufficient if at the same time usage actually drops? These are issues that are being studied by Volvo Cars, and the company is by no means excluding the possibility of future cars being fitted with four-point belts.

Motorised belt that responds to potential hazards
The motorised belt is an exciting new technology that tightens the belt and places the driver in the right position in potentially hazardous situations. For instance, the system could register if the car is being driven more actively, with more steering wheel movement. In such a situation, there may be a benefit from having the support of the belt. The belt may also receive a signal from the car’s collision warning system that an obstacle is approaching. Or that the car has noted that the driver is beginning to be drowsy or inattentive.

If so, the belt can provide a warning and alert the driver to the situation by pulling tight and positioning him or her in the seat. One benefit of this system is that it can be activated an unlimited number of times without being used up, unlike for instance pyrotechnical belt pre-tensioners.

Belts that are easier to use
Far too many drivers still do not use their safety belts. Solutions that make usage more natural and convenient are therefore constantly being discussed.

One alternative may be a belt buckle that rises from its place between the seats when someone sits down. This makes it easier, particularly in the rear seat, to find the buckle and use the belt. Other ideas involve showing the occupant how to use the belt with the help of strobe lights, or by sewing an illuminated strip into the belt to make it easier to find in the dark.

Tests have been conducted on fully automatic systems where the belt is placed across the seat occupant and then fastened. The challenge with such solutions is not primarily technical in nature but rather the logic of how they should be used.

When exactly should the belt be put on? When the occupant sits in the seat? But perhaps he or she is not intending to drive off just then. When the door is shut? When the ignition key is turned? And what will happen if one of the car’s occupants has just sat down and is holding a delicate bouquet of flowers or a big ice-cream? Interesting challenges that the development engineers still have to solve.

Changes in the belts themselves
Volvo and the other car makers today use belts from a small number of manufacturers. There may be variations in the belts’ stretching properties, but their structure and width are the same. One might imagine that a broader belt would offer better protection. However, since the force tends to gather in the middle of the belt, the additional width only offers marginal benefit. It is also more comfortable, particularly for women, to place a slimmer belt diagonally across the chest.

Making the belt inflatable and giving it some form of force limiter is another solution that has undergone limited testing by some manufacturers.

Allowing new technical solutions to interact with the belt can also improve its efficiency. In conjunction with the launch of the Volvo XC60 in 2008, Volvo Cars introduced the Pre-Prepared Restraints (PRS) function. PRS uses the same laser sensors as the collision-avoidance City Safety function. The laser sensor interacts with the airbags and force limiters so that the latter can be regulated more effectively in response to the severity of the collision.

Volvo Cars’ work to ensure the very highest occupant protection
Based on a relatively simple but highly effective mechanical design, the three-point safety belt, Volvo Cars has developed a high-tech safety system that provides the best possible protection for the car’s occupants. The examples in the list below show the journey - so far - from that groundbreaking innovation in 1959 onward:

1959 Three -point safety belt in the front, a Volvo innovation
1967 Safety belts fitted as standard, rear
1969 Three -point inertia-reel safety belts, front
1971 Safety belt reminders, front
1972 Three -point safety belts, rear
1986 Three -point safety belts, rear middle seat
1987 Mechanical belt pre-tensioner
1991 Automatic height adjustment for safety belts, front
1992 Pyrotechnical belt pre-tensioners, front
1993 Three -point inertia-reel safety belts in all seats
1996 Force limiters in safety belts, front
1999 Pyrotechnical belt pre-tensioners in all seats
1999 Force limiters in hip belts, front
2003 Safety belt reminders in all seats
2003 Pyrotechnical belt pre-tensioners for hip belts, front
2003 Adaptive force limiters, front
2007 2-level force limiters for children and adults respectively, rear
2008 Adaptive force limiters for low-speed impacts (PRS), front

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