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29 ing effect on the small driver's ability to control the vehicle The provision of the shoulder strap height adjuster and the or, at the very least, on the small driver's ability to maintain shoulder strap tension adjuster features would seem to be continuous visual contact with the road in front of the vehi- "must have" design requirements to provide an ergonomically cle. The seat lap belt might lock up or cinch tight; and the sound design of current safety belts installed with air-ride shoulder strap was likely to slide or rub against the driver's seats in Class 8 trucks. neck and upper chest area in ways that most small drivers found uncomfortable. On the test ride in the truck, the research team ergonomist estimated that while riding over 7.3.6 Variation in Truck Safety Belt Installation and Other Factors bumpy roads and over railroad tracks, the air-ride seat verti- cal stroke easily rose and fell through a range of anywhere This ergonomics assessment focused on safety belts. It did from 3 to 6 in. in height. Although the description of the not exhaustively examine the combination of all major brands "bounce" of the air-ride seat pertains to larger drivers as well, and models of Class 8 trucks and seats used. The following the problem seemed more acute to smaller-statured drivers. design, installation, and use variables pertinent to driver safety belt use were not evaluated: 7.3.5 Seat, Safety Belt, and B-Pillar Placement in the Truck Cab Different truck models and configurations. Different seat manufacturers' designs. Did the use of the An ergonomic evaluation of safety belts must include sig- safety belt and shoulder adjuster mechanism differ or did nificant consideration of the choice of seats installed in the its performance differ as a function of truck seat design? truck. There are several different seat vendors that supply air- Safety belt design differences, old seats versus new ride and other seats installed in trucks, and there is no stan- belts. dard combination included in all trucks supplied by the various OEMs. While installing various truck seats and safety belt combi- 7.4 ADDITIONAL ERGONOMICS AND nations, there will be slight differences in functionality and the HUMAN FACTORS CONSIDERATIONS comfort aspects of the seats and of the safety belts (lap belts 7.4.1 Ride Quality and Comfort Factors and shoulder harness strap). In part, these differ in each truck cab configuration as a function of where specifically the seat In all motor vehicles, including Class 8 trucks, there are mountings are positioned in the cab, and with the structural many seat design features that interact with the issues relat- design of the cab, in particular with reference to the B-pillar ing to use or non-use of safety belts. Insofar as seat design behind the driver. As these characteristics relate to a driver's affects drivers' perceptions of ride quality, the seat design comfort in the use of the safety belts they can be attributed to features also impact the likelihood of whether or not a driver these key variables, both of which affect the functionality of will wear his/her safety belt, and if so, perceive that he or she the safety belt and its relative comfort characteristics: is wearing it comfortably. Truck drivers seated in their cabs are exposed to WBV and Precise positioning of the shoulder loop web-guide rel- are affected by seating dynamics (suspension, springs, verti- ative to the seat back and cal stroke, lateral movement, etc). The dynamic response of Additional features the OEM placed on the seat safety the seat can be the factor most easily used to control human belt (e.g., shoulder strap height adjuster and shoulder exposure to WBV. Seats can increase as well as decrease strap tension adjuster), whether these were sold as stan- vibration experienced in the driver's position. dard inclusions on particular truck models or installed as additional options at the special order request of the client (fleet or independent owner operator) purchasing 7.4.2 Interaction of the Air-ride Seats the vehicle. and Safety Belt Use When the B-pillar is farther back from the driver's seat, the Three-point truck safety belts are anchored to the truck cab "angle of attack" of the shoulder strap to the seat must be B-pillar wall behind the driver's left shoulder as he/she sits fac- modified through use of a strap height adjustment, or else the ing the front of the truck. Consequently, when the air-ride seat shoulder strap will cause discomfort to the driver during bounces up and down on its suspension to dampen or attenu- upper torso body movements to operate the truck. Addition- ate the WBV exposure to the driver, the shoulder belt tends to ally, the lap belt end brackets and the seat lower tether mount- chafe the neck and shoulder as the strap slides across the ing brackets must be positioned accordingly for a B-pillar driver's upper torso. Additionally, the lap belt may expand or which is farther back from the driver. These positionings contract, even tighten around the driver's waist as the air-ride become a factor for the driver when he/she makes fore and aft seat bounces up and down. This interaction of the safety belt and higher or lower adjustments in the seat. (lap belt and shoulder harness strap) with the air-ride features