Explosive Safety

It's no overstatement to say airbags inflate with explosive force: Some have said it's like taking a Mike Tyson punch. You will better survive the force if you make sure your chest is at least 10 inches away from the steering-wheel-mounted airbag. Though it'll be impossible for most to remember, in the instant before the crash make sure your arms are not crossed over the steering wheel. (Another good reason for "three and nine" hand position.) Also, never allow front-seat passengers to rest their feet on the dash or cross their legs guy-style. Either is asking to have their knee ligaments ripped apart and their size 11s stuffed into their mouth.


An airbag aside: Have you ever wondered what happened to knee room in cars? It's been taken away by knee bolsters designed to keep occupants from sliding under the dash in collisions. Knee bolsters serve the same function as the "crotch straps" on six-point racing harnesses. Along with seatbelts, knee bolsters keep occupants in proper position for the airbags to work.

Pretensioners
Simultaneously with airbag deployment, seatbelt pretensioners cinch seatbelts race-driver tight by employing the force of a pyrotechnic charge. Yes, pyrotechnic, just like Fourth of July fireworks. Some pretensioners move the anchor point of the seatbelts, others wind the reel on which the belts are stowed. One way to wind the reel is with a toothed rod geared to the reel: imagine a bottle rocket with a row of gear teeth on its side. The rocket—I mean pyrotechnic charge—moves the rod, which winds the gear. It's the opposite of rack and pinion steering where the pinion gear moves the steering rack. The resulting super-tight belts better position the occupant for airbag deployment and helps keep them from striking interior components, such as the A-pillar and structure around the side window, both of which cause a surprising number of head injuries. That's why the A-pillars are now seriously padded and many cars features side airbags, which are often installed in the A pillar.


Pyrotechnic pretensioners work fast. From the time the sensor says "Fire in the hole!" to when the belts are tight is about 12 milliseconds.


Some manufacturers are moving away from pyrotechnic pretensioners. For use in the S-Class Mercedes, TRW Automotive recently produced a pretensioner that uses an electric motor to wind in belt slack. In addition to the usual crash sensors, the S-Class' computer monitors the anti-lock braking and stability control systems to predict when an accident is likely, such as when a driver jams the brake pedal to the floor and wrenches the wheel hard over. (We know some who'll spend a lot of time with the belts tight.) If the predicted crash doesn't happen, the belts are slackened.