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?
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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.
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Saab's Active Head Restraint (SAHR)
Volvo's WHIPS-seat mechanism
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What can science tell us?
Saab Active Head Restraint (SAHR) - Seat Design to Reduce the Risk of Neck Injuries in Rear Impacts; Wiklund K and Larsson H; SAE Paper No. 980297; February, 1998
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In 1997, SAAB introduced the SAAB Active Head Restraint (SAHR) in the all-new 9-5 vehicle as a first application of crash-activated systems to mitigate whiplash injuries. In addition to the active head restraint, the SAHR system comprises design features in the seat back to control and distribute those loads on the occupant that are generated in rear impacts or during rebound from the restraint system in frontal impacts.
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WHIPS - Volvo's Whiplash Protection Study; Jakobsson L, Lundell B, Norin H, and Isaksson-Hellman I; Accid Anal Prev; Vol. 32, No. 2; pp. 307-319; March, 2000
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This paper is a review of Volvo's Whiplash Protection Study (WHIPS), which is the result of more than ten years of concentrated research efforts in the area of neck injuries in car collisions, with the focus on rear end car impacts. The study follows the whole chain from accident research to the development of a seat for increased protection against whiplash associated disorders - the WHIPS-seat.
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Effects of Head Restraint and Seat Redesign on Neck Injury Risk in Rear-End Crashes; Charles M. Farmer, Joann K. Wells and Adrian K. Lund; Traffic Injury Prevention; Vol. 4, No. 2; pp. 83-90; June, 2003
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This paper reports on an examination of automobile insurance claims to determine the rates of neck injuries in rear-end crashes for vehicles with and without redesigned head restraints, redesigned seats, or both. The improved geometric fit of the head restraints in many newer vehicle designs is seen reducing the risk of whiplash injury substantially among female drivers, but has very little effect among male drivers. An additional benefit, producing about a 43% reduction in the whiplash injury claims, is provided by new seat designs, such as active head restraints that move upward and closer to the driver's head during a rear impact.
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Useful links
The Proper Adjustment of Your Headrest Could Save Your Neck (IBC)
Head restraints (IIHS)
How To Adjust Your Head Restraint (RoSPA)
Neck injuries in rear-end crashes (IIHS)
New Head Restraints Can Help Prevent Whiplash (Edmunds.com)
Volvo Cars' whiplash system WHIPS celebrates 10 years (The Car Guide)
Mercedes-Benz Introduces Neck-Pro (CarPages)
riACT Rear-Impact Active Head Restraint (Johnson Controls)
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Quick Facts
- Whiplash occurs due to relative motion between the head and the upper torso in a rear-end collision
- A properly adjusted head restraint can minimize head and neck motion
- Some active systems move the head restraint forward and/or upward to protect the occupant
- Some systems allow the seat back to move rearward and recline to cushion the occupant in a rear-end crash
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