ok, fouind this; here it is;
'92-96 Frame Crumple Zones
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This is all my opinion based on what I've read, learned, & seen. Anyone who disagrees is welcome to post his own opinion (& any supporting documentation OR none) to this thread.
Why were the crumple zones added to F150, Bronco, & certain F250 frames in '92? Some people think it's to protect people in smaller vehicles. So why didn't the big trucks get them? They'd do more damage... Some people think it's to protect the rest of the frame. Same question - why not on the larger trucks, whose heavy cargo & larger cabs would do more to their own frames? Being more expensive vehicles, it would make MORE sense to protect them IF that was the purpose of the crumple zones.
So why, then? What else happened to these trucks & the laws governing their design around that time? There was a major body redesign, so it makes sense that any redesigns would be done together to reduce retooling costs at the factories, and to ensure that impending future mods are compatible with current designs. It was about that time that SRS airbags were becoming mandatory, and since the law specifically required them in vehicles under 8500lbs GVWR, it's reasonable to think that the crumple zone is specifically related to the airbag.
As fast as electricity & explosives are, they still take time to work. Chemical reactions take time, and the materials containing them have inertia, so an "instant" airbag isn't physically possible. A few milliseconds is pushing it, since there has to be some logic involved to prevent false deployment, so a little bit of time is essential for SRS to work right.
But too much time is obviously no good since it might allow the occupant to hit the dash/steering wheel. And if the occupant is too close to the airbag when it goes off, he could be burned, or the explosion could cause more injury than hitting the wheel/dash would have. So timing is absolutely critical.
That's why the crumple zone. It's carefully designed to absorb a specific amount of energy which would otherwise STOP the rest of the frame/cab, causing the moving occupants to hit the dash/wheel. This allows the truck's frame/cab to continue moving forward AFTER the bumper has stopped (against whatever the truck is hitting) as the zone crumples. During that time, at least one of the airbag sensors on the core support triggers, and the falsing sensor on the base of the B-pillar also triggers, completing the circuit from the battery, thru the ig.sw., thru the airbag module, thru the clock spring, to the airbag, which detonates & inflates the bag in front of the occupant.
By then, the crumple zone has done its job, and the engine/trans (the densest parts of the truck) are now pressing the frame & radiator against the obstacle, slowing down or stopping the cab/steering column/seat/seat belt. These work against the occupant, slowing down/stopping him. But since the steering wheel can't be designed to be usable AND safe to slam into at 60mph, the airbag is there to keep the occupant away from it JUST long enough for him to stop, along with the rest of the truck. The pendulum in the seat belt reel locks it momentarily, at this time, as well. The inertia switch also opens at this point, cutting power to the fuel pump(s).
After everything stops, the airbag sensors return to their normal states, and the pendulums in the seat belt reels release them (assuming the vehicle is still upright) so the occupants can get out. The airbag deflates (it's not airtight like a balloon), but the inertia switch remains off until manually reset.
So what would happen without the crumple zones? As strong as truck frames were being designed in the 70s (when the '80-96 chassis was in design), it's strong enough to stop the whole truck almost instantly from 60mph, given a solid enough obstacle. That would mean that the steering wheel would be doing 0mph while the occupant was still close to 60mph, and tension was just rising in the seat belts.
That's what happened before airbags & crumple zones. People who wore belts were still killed or seriously injured because vehicles were designed & built SO strong. A 200-lb man falling at 60mph onto a 2" wide web strap could be bruised, cut, crack ribs, or rupture internal organs - and that's what it's like when the seat belt jerks him back from 60-0 in a millisecond.
But if he's stretched the belt to its limit, and is now a few inches away from a steering wheel filled with explosives, he's in serious danger of having his head pushed over his back when the airbag deploys. That's why the crumple zone is an INTEGRAL PART of the airbag system on 1st- & some 2nd-generation airbag-equipped vehicles. They're too rigid for the airbag to get inflated before the occupant is dangerously close to it. So if the crumple zone is modified or disabled in any way (aftermarket winch bumpers, front hitch receivers, special fabrication, etc.), then the airbags can be dangerous. That's why those parts must be SPECIFICALLY designed & certified before they can be legally marketed for airbag-equipped vehicles. It's also why vehicle mfrs. specifically exclude front-end modifications from airbag warranties & occupant injury liability. Insurance companies sometimes do the same, because it's inherently dangerous to modify the system in any way, unless the design has been reviewed & certified compatible with airbags.
On most modern vehicles, while the crumple zone is still there, it's in the design from the beginning, so it's spread out & hidden more, and not usually noticeable. Just a few small carefully-distributed notches, holes, & bends in an otherwise rigid-looking frame/cab/unibody structure. But since the airbag law went into effect before Ford could redesign its entire truck line (which at that time shared 1 frame design), they had to make this obvious modification to remain in compliance on the lighter trucks. Now, trucks over 8500GVWR (the SuperDuty line) are totally different from those originally required to have airbags, and it's a direct result of airbag laws. Manufacturers who had already been working on new vehicle designs were able to integrate airbags more seamlessly than Ford trucks, so they may not be as obvious on the same year models as ours.
by Steve83
