Despite the constant development in technology, the automotive world's most important safety mechanism – the safety belt – has remained much the same since it was first used fifty years ago. The three-point safety belt's ingeniously simple design has stood the test of time and the only difference is that these days the belt is part of a larger, high-tech safety system.
The three-point belt has played a central role for occupant protection in all Volvo cars since its introduction in 1959. The inertia-reel mechanism, which made the belt more comfortable and flexible, became standard in 1969 and is the most notable change since the three-point safety belt's invention.
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.
"The safety belt is unique in that it so effectively catches and restrains the occupant in the seat. Other systems are there to 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, that tensions the belt against the body of the seat occupant when it receives a signal from the crash sensor that a collision is imminent. This reduces the gap between the belt and the body that can be caused 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 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 detect the dynamic mass (the rate at which the passenger is moving forward).
This makes it possible to tailor the force with which the passenger is restrained and to absorb as much of the force as possible. If the passenger is restrained by the belt with excessive force, he/she may suffer injury. If, on the other hand, the force limiter is set too low, the occupant could 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 absorbs some of the energy.
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 involving trucks as well as 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 is relayed via sensors located all over the car while a main processor collates the data and 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 on one of the outside rear seats, the belt protection system is tailored accordingly and will react differently than if an adult were occupying the same seat. 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
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 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, and several solutions have been presented over the years. But there are still many details to iron out.
The four-point belt has obvious advantages - 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); and 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 as the ribcage is the strongest part of the human body and has the best chance of absorbing a collision force. The challenge, therefore, is how 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 development 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 and 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 driver is 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? These are all interesting challenges that the development engineers still have to solve.
Changes in the belts themselves
Volvo and the other car makers today use belts sourced 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 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 a 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 vehicle 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