Are there ever moments when hard braking does not cause the seat belt to lock? (Excluding obvious faults)

Posted 28th Nov 2022 (Posted 18 h, 48 m ago)
Cause I had a near miss, doing 20mph and had to rapidly come to a stop but I didnt feel the seat belt lock. Then again sometimes when I pull the seat belt myself it locks, so just got me pondering.
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    Belts work off the movement of the belt itself, not the car (with some exceptions). Therefore as long as 'you' weren't moving forward faster than the seat (and therefore extending the belt), the seat belt wouldn't lock. However, when you're late for that very important meeting and your frustration level is going through the roof because the belt won't come out of the reel, then it will lock every single time. (edited)
  2. Avatar
    Depends on the exact mechanism used and just how fast you stop.

    Maybe someone can quote better figures, but I saw a figure of 2.5g to engage the mechanism: which would be pretty darn hard braking and you might not have braked that hard.
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    Seat belt wouldnt be expected to lock just because you brake hard.
    Deceleration forces if you hit something are much larger
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    You may have just been distracted and not noticed it, or braced yourself against the wheel and didn't move forward enough to trigger the inertia system.

    The more serious seat belt locks like the pre-tensioner aren't going to go off under hard braking, any more than the airbags or collapsible steering column would activate.
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    If the belt was not snug and had some slack in it, it’ll not lock.
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