I have always wondered how seat belts work. If you try to move the seat belt at a low speed, there is nothing to prevent the movement, allowing adjusting the seat belt to a correct length. However, movement at a high speed is disallowed. There is some kind of mechanism that is able to detect the speed or the acceleration and prevent movement.

How does the mechanism work? Does it detect speed or acceleration?

What are the chances that the mechanism fails to work in a crash?

  • It also lock up on rapid reduction in speed. Try slamming on the brakes. The Toyota manuals from the 90s use this method to test the seat belts. Oct 5, 2021 at 4:52

1 Answer 1


There are two primary mechanisms that work in combination. The first resembles a pendulum. While the pendulum is vertical the belt works fine. When there is sudden deceleration the pendulum swings forward engaging a cog wheel locking the belt. The weight and length of the pendulum is specifically selected to only engage at or above a predetermined deceleration. The problem with this is that it only responds to a deceleration driving forward.

The second is a set of flyweights. The flyweights are held in by specially calibrated springs and are attached to the main seat belt spool. When the seat belt pulls out suddenly the main spool spins really fast. This spinning causes centripetal forces that overcome the springs and cause the weights to fly out and engage a cog wheel locking the belt. This mechanism takes care of any sudden spooling out of the belt.

All seat belts have some variant of these two mechanisms.

  • In addition to the two types you have written about, there is also a self retracting mechanism which will engage when a crash happens. There is a motor which resides in the retractor base. It is actuated during a crash. Its purpose is to pull the occupant up into the correct position prior to the air bags (I would think during, actually) deploying. This helps keep people from getting hurt by the air bags. Two manufacturers which I'm aware of who use this are Volvo and Hyundai. I'm sure there are others. Dec 26, 2015 at 1:15
  • 1
    @Paulster2 the mechanism you speak of is a seat belt pretensioner. The mechanism is not a motor but a full pyrotechnic one time use device like the actual air bag. I did not mention it because it was outside of the OP question. All major manufacturers use it in current vehicles. Sometimes it is incorporated into the buckle near the center console and other times it is in the B pillar.
    – vini_i
    Dec 26, 2015 at 1:48
  • 2
    So you are correct! I was instructed they were motors, though I've never dealt directly with them ... thanks for the update! Now knowing how these actually work, I'd suggest it is important to know about these since they are a one time use. I would also bet if these were bad, you'd get an error code during SRS POST. Dec 26, 2015 at 2:43

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