How do you test a seatbelt for effectiveness?

Dear Car Talk

Dear Car Talk | Sep 01, 1990

Dear Tom and Ray:

How can I tell if the seat belts in my 1980 Buick Regal work?

RAY: It's not that easy, Alan. Luckily, my brother free-lances as a crash test dummy in his spare time, so he knows quite a lot about this.

TOM: I assume that you're concerned about the fact that you can move freely when your shoulder harness is attached. You want to know if it will lock up in an accident, right? Well, there are basically two types of seat belt mechanisms, and they require different types of testing..

RAY: The older type is centrifugal. It works by sensing how hard the belt is being pulled, so that if you were to hit something, the force of your body pulling the seatbelt forward would lock the belt and hold you in place. I had one of these that was so sensitive that I had to whisper sweet nothings to it as I coaxed it across my body, or it would lock up every two inches.

TOM: You should be able to get a centrifugal belt to lock by yanking on it. Some belts are less sensitive than others, so you may have to really jerk it quickly in order to make it lock.

RAY: Most modern cars now use a seat belt system based on a pendulum. When the car slows down (do you remember any of Newton's Laws from Physics class?), the pendulum swings forward and locks the seat belt. So you can sit with the car stopped and yank on this kind of belt all day, and it would never lock.

RAY: The only way to test a pendulum belt is to actually simulate an emergency situation. So you'll have to take your Regal out to an empty supermarket parking lot and experiment with it. Drive at 10 mph and them slam on the brakes. If the belts don't lock, in??crease your speed in increments of 5 mph until they do. If you can't get them to lock by, say, 30 mph, start looking for small immovable objects.

TOM: Right. Start with milk crates, then graduate to shopping carts. Start with two or three, and then try a whole line of them. If none of that works, you'll have to crash right into the supermarket itself. Aim for the produce section, Alan...the landing there is a lot softer than canned goods or the freezer section!

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