The pretension is killing me...

June 23, 2006 8:56 AM

When, why and to what extent have seat belts moved from a mechanical trigger to an electronic one?

My wife was in an accident last week, during which neither the seat belt nor the airbag in our 2005 Honda Civic restrained her. She hit the steering wheel and now has a broken rib and possibly a fractured scaphoid.

We were told that the sensors for the airbags are low in the bumper and weren't hit during the collision because the car she hit has a high back end. This pisses me off, but I chalk it up to shitty design on Honda's part.

The seat belt, however, infuriates me. Honda says it did not lock because its sensors also were not set off during the crash. Sensors? I was under the impression that seat belts had a mechanical trigger instead of an electric one, but this page indicates that our Civic has "pretensioners." Forgive my ignorance of autos here; are pretensioners an additional safety feature of mechanical seat belts or replacements of them?

I'm not looking to sue, as the other driver was at fault (she cut off my wife then slammed on her brakes). I'm asking mostly to find out why a simple mechanism as important as a seat belt would be electronically updated.

posted by Terminal Verbosity to technology (10 comments total)

All seatbelts have mechanical locks. Some newer cars come with sensor-activated belt pretensioners in addition to the mechanical failsafes. A pretensioner basically reels in any slack in the belt in the event of an accident.
posted by pmbuko at 9:14 AM on June 23, 2006

correction: all shoulder belts have mechanical locks. Older lap belts do not.
posted by pmbuko at 9:17 AM on June 23, 2006

Seat belts are stopped by inertia reels. They're mechanical devices that are designed to disallow significant acceleration, such as happens in an impact... or when you move about too swiftly.

Pre-tensioners are an addition, not a replacement. Without them, your body applies a whole bunch of force to the belt, which then locks. With them, the belt slams into you before you start applying the force, thus keeping you quite still and minimizing your injuries.
posted by I Love Tacos at 9:45 AM on June 23, 2006

Yep, what pmbuko says about pretensioners.

NHTSA says:

Some safety belts may be more effective than others. Two technologies for improving the performance or use of belts are widely available as of 2003. Although they are not mandatory for meeting NHTSA standards, the agency regards them with favor and provides consumer information on their availability, by make-model, in Buying a Safer Car. Safety belt pretensioners (installed on 63 percent of MY 2002 light vehicles) retract the safety belt to remove any slack almost instantly in a crash. Load limiters (installed on 84 percent of MY 2002 light vehicles) prevent belt forces from reaching unsafe levels by causing parts of the safety belt to stretch or deform at a predetermined, safe force level. NHTSA's Phase 1 evaluation shows that the combination of pretensioners and load limiters significantly reduces HIC, chest acceleration, and chest deflection scores on 35 mph frontal NCAP tests.

As far as the bumper goes, the federal government mandates bumper heights for cars and trucks. They are not the same.
posted by pmurray63 at 9:50 AM on June 23, 2006

Okay, another question: what's the point of those "interial reels"? I remember seat belts on an older car that were just fixed securely in place. I guess the idea is they want to let you move around without taking off the belt, but that doesn't seem like a big deal. Is that it, or is there some safety reason for going with the more complicated design?

I'd like to know that there's some reason for it, since I find them considerably less comfortable and seemingly less secure than a properly-adjusted manually-adjusted belt.
posted by sfenders at 9:58 AM on June 23, 2006

The advantage is, you don't have to adjust them, they adjust themselves.
posted by kindall at 10:05 AM on June 23, 2006

sfenders writes "what's the point of those 'interial reels'? I remember seat belts on an older car that were just fixed securely in place. I guess the idea is they want to let you move around without taking off the belt, but that doesn't seem like a big deal. Is that it, or is there some safety reason for going with the more complicated design?"

Fixed shoulder belts are a pain if you can't reach the dash while buckled in.
posted by Mitheral at 10:40 AM on June 23, 2006

So, why didn't the seat belt lock?
posted by Terminal Verbosity at 10:55 AM on June 23, 2006

Terminal Verbosity writes "So, why didn't the seat belt lock?"

It probably did, the pendulm mechanism of a seat belt locker is so simple as to be essentially foolproof and sensitive enough that we test them just by hitting the door jamb with a rubber mallet. However the belt being a bit loose (belts are never as tight as engineers would like, that is why we have pretensioners), she probably sitting fairly close to the wheel, and the amazing ability for the human body to stretch allowed human bits to hit the wheel/dash. The belts can also break ribs, especially if they aren't adjusted properly.

I've seen guys strapped into 5 point harness in rally cars who have smashed their faces into the steering wheel during an off¹. Your neck especially will stretch alot because it isn't up to the task of restraining the mass of your head in an accident. In the extreme case you experience a fracture where your neck connects to your skull. This is what killed Dale Earnhardt and is why you see helmet restraint devices (HANS usually though their are other systems) in most modern racing.

[1] This is why you'll often see mismatched helmets in rallying. The driver wears a full face to protect his teeth from the wheel and the co-driver wears an open or partial open face helmet so he can still give directions when the intercoms fail.
posted by Mitheral at 11:43 AM on June 23, 2006

HowStuffWorks has a pretty good article on seatbelts.
posted by jayder at 3:02 PM on June 23, 2006