High-Tech Vehicle Safety Systems

Seat Belt Load Limiters

A properly used seat belt fits snugly over the pelvis and across the chest. In the event of a frontal collision, the lap and shoulder belts restrict the occupant's forward motion to prevent potentially injury-producing contacts with the dashboard, steering wheel, and windshield. The restraining forces applied to the occupant by the seat belt are spread across the strong, bony structures of the body.

The bones of the pelvis are exceptionally strong and can withstand considerable force without fracture. The rib cage is quite flexible but, at a certain point, the ribs will no longer continue to bend, and may break. Fractures to the ribs may result in serious injuries to underlying soft tissue such as the heart and lungs. This is a particular concern for the elderly who have more brittle bones and less tolerance to high collision forces.

A load limiter is designed to allow the seat belt force applied the the chest to rise only to a point where serious injury is unlikely. The seat belt is then allowed to extend in a controlled manner, maintaining a constant restraining force to absorb energy. The occupant's forward motion is finally arrested by the front air bag that deploys in the crash.

Load Limiter

Mechanical load limiting device

How do they work?

A simple form of load limiter is a fold sewn into the seat belt webbing. The stitching holding the fold in place is designed to pull apart when a certain amount of force is applied to the belt. If this force is exceeded, the stitches are ripped out, and the webbing unfolds, allowing the occupant greater forward motion.

A mechanical device that uses the same principle takes the form of a "ladder" with a set of open, metal teeth (see photograph at right). The seat belt retractor is held in place at the bottom end of the ladder and its motion restricted by the presence of the teeth. As the seat belt force increases, the teeth begin to deform, allowing the seat belt anchor to move along the length of the ladder device.

An alternative type of load limiter uses a torsion bar built into the seat belt retractor. The torsion bar is a metal rod that will twist when sufficient force is applied. In minor collisions, the torsion bar will hold its shape, and the seat belt retractor will lock normally. But, when the force applied by the webbing, reaches the design limit, the torsion bar twists and allows the webbing to extend. (See animation by Autoliv Inc.)

Dummies belted

Dummies belted

What can science tell us?

Comparison of Thoracic Injury Risk in Frontal Car Crashes for Occupant Restrained without Belt Load Limiters and Those Restrained with 6 kN and 4 kN Belt Load Limiters; Foret-Bruno JY, Trosseille X, Page Y, Huère JF, Le Coz JY, Bendjellal F, Diboine A, Phalempin T, Villeforceix D, Baudrit P, Guillemot H and Coltat JC; SAE Paper No. 2001-22-0009; November, 2001
This paper reports on real-world frontal crashes in which the vehicles were equipped with load limiters and pyrotechnic pretensioners. A 4 kN load limitation was shown to provide a very important reduction of thoracic injury risk.

Effects of Shoulder Belt Limit Forces on Adult Thoracic Protection in Frontal Collisions; Mertz HJ and Dalmotas DJ; SAE Paper No. 2007-22-0015; October, 2007
An analysis of crash data from the U.S. National Automotive Sampling System (NASS) was done to estimate the effectiveness of various levels of shoulder belt limit loads in reducing chest injuries to older occupants. The result indicated that a 2.5 kN shoulder belt limit load would substantially reduce shoulder belt-induced AIS ≥ 3 chest injuries in 99 percent of frontal collisions to all adult, front outboard seated occupants whose normalized bone strengths are greater than 0.4.

Useful links

Quick Facts

  • Load limiters require no action on your part - other than seat belt use!
  • Always wear your seat belt
  • Make sure that the lap belt is positioned low down across the pelvis
  • Always have the torso belt over the shoulder; never place it under your arm
  • Make sure that both belts are snug; don't allow any slack
  • Young children should be placed in appropriate child restraint systems
  • Check the owner's manual for specific information about the seat belts in your vehicle

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