A pretensioner is a device that actively withdraws a portionofbelt (about 10-15cm) duringthe impact, in order to keep the car occupant adherent to his seat and prevent contacts with the steering wheel, or with dashboard components. In its general form, a pretensioner is composed by a front-installed sensor (with an intervention threshold among g-15 g) and by an effector connected to the belt system itself.

A research has been performed on a mechanical pretensioner, on a pyrotechnical pretensioner and on standard automatic seat belts, with and without slack. Each device has been tested by simulation in a frontal impact at 35 mph with a GM Hybrid II dummy.

The laboratory results show a significant reduction in HIC vs standard belts, especially when poor wearing conditions are present (slack). The reduction in forward displacement of head is particularly important for the interaction with the steering wheel (or dashboard components for the passenger) in small car interiors, in order to offer a better protection to occupants of economical, large diffusion cars.

One can individuate some differences in the functioning of the pyrotechnical and the mechanical system, mainly in the direction of the retraction force, that is door-oriented in the first on, inside-oriented in the other one.

Other characteristics of the pretensioner, not evaluable in this study, can present relevant effects in preventing dangerous side effects of seat belt action. It will be very interesting to investigate the performance of pretensioners -both pyrotechnical and mechanical- in real car accidents.