Small-statured women and senior citizens can breathe easier knowing Autoliv Inc. is putting into production a seatbelt system designed to modulate during its deployment, thereby reducing chest injury risk.

Autoliv’s “Smart Belt” – which debuts this year on the new Mercedes E-Class and BMW 7-Series sedans – is equipped with load limiters that not only adapt to crash severity levels but deploy in harmony with a vehicle’s airbag system.

“In a crash, our Smart Belt starts – as in most modern seatbelt systems – by tightening the belt up to 15 cm (5.9 ins.), using a pyrotechnic pretensioner,” Gunter Clute, project leader, says. “This eliminates slack and makes it possible to release some webbing at a later state, if the load on the occupant becomes too high.”

And excessive loads have been linked to rib fractures, particularly among motorists 60 years and older, Autoliv notes. That’s because their rib cages can safely absorb only half as much force as someone between 19- and 30-years old.

In addition, a “gearbox” in Autoliv’s Smart Belt system features a torsion bar with two sections, each having different diameters. This enables deployment at two torque settings with the latter complementing an airbag’s force instead of adding to it – again reducing injury risk. This is a significant development in the wake of occupant weight sensing advancements.

So too, with features such as Smart Belt’s capacity to discern between severe crashes and “milder” ones. In the first instance, Smart Belt may shift to a lighter torque – to offset airbag deployment – up to 80 thousandths of a second after a crash occurs. But in less severe events, “first gear” may be in use for only 40 thousandths of a second.

“When the triggering of the load limiter is tuned to the severity of the crash, the same sensor that controls the frontal airbags is used,” Autoliv says.

In vehicles equipped with occupant weight sensors, “it will be possible not only to adjust the seatbelt load to the severity of the crash, but also individually to each occupant.

“This is an important advantage since smaller, lighter weight occupants, such as many women, are more susceptible to high belt loads than the average person, and these individuals do not need the same restraining force as a larger occupant.”