Proper Installation is Important
The effectiveness of a restraint assembly is also influenced by attachment techniques.
The principal precaution for installing the mounting hardware to the vehicle is to minimize bending stress in the fitting. This is achieved by making sure the belts pull from a straight angle against the hardware. The assembly should be installed so that the straps do not rub against any surface that can cause the webbing to fray. The anchoring mechanisms should also be periodically checked so that they don’t become loose or weakened.
Proper installation of the restraint assembly also means achieving the correct fit to the driver. Belts should be as short as possible to reduce stretching for better control of occupant movement.
The attachment points must provide the optimum geometry to minimize movement of the belts. Lap belts perform best when they act at an angle between 45° and 55° relative to the longitudinal axis of the vehicle as illustrated in part A of Figure 2. This angle permits the lap belt to react to the upward pull of the shoulder harness. A system installed with a shallow belt angle, as shown in part B of Figure 2, permits the shoulder harness to pull the lap belt up off the pelvic area and into the abdominal region with the likelihood of injury to internal organs. The end attachments of the shoulder harness must also be installed at appropriate angles. The ideal position is anywhere between 5° below and 30° above the driver’s shoulder, as seen in part C of Figure 2. If the upper attachment point falls significantly below the driver's shoulder,
then a spinal compression injury is likely to occur. In an accident situation, the shoulder belts pull down and back on the torso as they resist the forward motion of the driver. The resultant restraint force compresses the spinal column and will add to the stresses in the spine already caused by the force of the crash impact. On the other hand, if the trailing ends of the harness are too far above the shoulder (greater than 30°), then two problems can occur. First, tension in the shoulder harness is increased and undue stress is applied to the harness and its structural attachments. Second, excessive angle will cause excessive motion. If the harness belts are too far above the shoulder, they will provide little resistance to forward motion of the driver’s upper torso. The result is impact with the steering wheel and the possibility of neck injury. The shoulder straps should also be 3-6” apart behind the driver's neck to prevent slippage off the shoulders.
