High-Tech Vehicle Safety Systems

Seat Belt Pretensioners

A propeSeat Belt Pretensionersrly used seat belt fits snugly over the pelvis and across the chest. In a crash, the seat belt retractor locks and the webbing prevents the occupant from moving into contact with hard portions of the vehicle's interior, thus reducing the potential for injury.

In order to obtain optimal protection, the seat belt needs to firmly engage the occupant's body across strong anatomical structures such as the bony pelvis and the rib cage.

This process also needs to occur early in the crash in order to couple the occupant to the decelerating vehicle and provide the greatest amount of ride down. Any slack in the seat belt works against this process and pretensioning systems are used to eliminate small amounts of slack immediately a crash occurs.


The piston is driven along the tube from left to right. Note the compression in the lap belt after activation.

How do they work?

Seat belt pretensioners are typically pyrotechnic devices. They are triggered by the same crash sensors that are used to determine the need to deploy the vehicle's air bags. In minor collisions the seat belt pretensioners may be fired without the air bags being deployed. In more serious crashes both the pretensioners and the air bags will be deployed.

Shoulder belt pretensioners generally feature a turbine device connected to the seat belt retractor spool. The gas generated by the pyrotechnic charge drives the turbine so as to rewind the retractor, thus removing slack in the seat belt. (See animation by Autoliv Inc.)

The pretensioning forces are not so high as to cause any injury to the belted occupant; however, neither will they remove large amounts of slack. It is important, therefore, that occupants ensure that the lap and shoulder belts are properly position and adjusted so as to be snug.

Dummies belted

Dummies belted

What can science tell us?

Three-Point Belt Improvements for Increased Occupant Protection; Mitzkus JE and Eyrainer H; SAE Paper No. 840395; 1984
Designs for a pyrotechnic pretensioner and a webbing clamp, and the results of sled testing of these systems, are described. Both systems were found to reduce forward excursion of the test dummies in simulated crashes, while the force levels produced by the pretensioning system were found to be readily tolerable on tests with human volunteers.

Useful links

Quick Facts

  • Pretensioners 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

Associated systems:

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