Active Head Restraints
In a rear-end collision an active head restraint system uses mechanical linkages to move the head restraint and/or the seat back to cushion the vehicle occupant and reduce the possibility of neck injury.How do they work?
| In a rear-end collision the struck vehicle is driven ahead. The seat back pushes against the occupant's back and accelerates the upper torso forward. If the head is unsupported, usually because of an improperly adjusted head restraint, inertia causes the head to remain in place. Due to resulting relative motion of the upper torso and the head, the neck muscles are stretched. This can lead to a hyper-extension condition - and strain to the soft tissue of the neck muscles - the so-called "whiplash" injury.The head restraint should be adjusted vertically so that the top of the headrest is more or less level with the top of the occupant's head. This ensures that the head restraint will make adequate contact with the back of the head in the event of a rear-end impact. Another important dimension is the horizontal setback - the distance between the back of the head and the front surface of the head restraint - since this affects how soon the head restraint engages the head in a crash. Active head restraints use mechanical linkages to move the head restraint into an advantageous position to protect the occupant from whiplash. Saab's Active Head Restraint (SAHR) has a lumbar pad in the seat back that, when contacted by the occupant in a rear-end impact, causes a lever to move the head restraint forwards and upwards to support the occupant's head and neck. Volvo's WHIPS-seat uses a specially designed hinge to allow the seat back to move rearward and tilt back so cushioning the occupant along the head, neck and back in the event of a rear-end collision. Other manufacturers use cable or electrically operated spring mechanisms to move the head restraint forward.  Saab's Active Head Restraint (SAHR)  Volvo's WHIPS-seat mechanism |
An active head restraint moves forward and upward in a rear-end collision to decrease the space between the restraint and the occupant’s head, reducing the degree to which the head accelerates before making contact. The less acceleration, the lower the chance of injury.
Using a lever-action mechanism built into a seat, the active restraint redirects the force of an occupant’s body as it presses into the backrest to move the head restraint forward. The beauty of this design is that it reacts proportionately to the occupant’s motion. Unlike the type of pyrotechnic charge used in airbags and seat belt pretensioners — the intensity of which may be too high or low — the active head restraints’ motion is dictated by the occupant’s size and weight and the severity of impact.
Active head restraints first appeared on Saabs and are now available from many automakers on all types of vehicles. The devices typically are applied to a vehicle’s front seats. You can find out if a vehicle has active head restraints by visiting the safety section in its Cars.com vehicle summary.
