Such components comprise, in particular, a seat proper which is adjustable in height relative to a base, a seat back which is adjustable in slope relative to the seat proper, and also a headrest which is likewise adjustable in slope relative to the back.
The position-adjusting mechanisms of the components in question are generally designed to withstand the forces normally exerted on the components by a person sitting on the seat, and in particular the weight of the person, thrust from the person's back.
They are not always strong enough to withstand the sudden shocks to which vehicles are subjected during accidents, and in addition they may be left unlocked while being adjusted.
When such mechanisms are not strong enough or when they are not locked, the consequences for passengers in the event of a shock may be severe, for example as follows:
a seat proper that is lifted relative to its base may drop suddenly, thereby causing the "anti-diving" bulge on the front edge of the seat proper to drop as well: since the seatbelt associated with the seat is generally secured to the body of the vehicle, its vertical position does not change, thereby causing the gap left for the user between the seat proper and the seatbelt to increase suddenly, thus making the belt completely ineffective since the body of the user is no longer held to the seat and is free to slide forwards; and PA1 the hinged back of the seat may tilt suddenly forwards, either because of its own mass or else because of the mass of the user when using a belt secured to the seat. PA1 the spring is rated so that the flyweight remains stationary so long as the deceleration applied to the seat remains below a predetermined threshold, preferably equal to 100 m/s.sup.2 ; PA1 the position-adjustable component of the vehicle seat is the seat proper thereof, the support of said component being the base of said seat; PA1 the position-adjustable component of the vehicle seat is the back thereof, the support of said component being the base of said seat; and PA1 the seat is fitted with two locking devices of the kind defined above, placed on respective sides thereof.
To remedy these drawbacks in the context of locking the angle of a vehicle seat back, proposals have already been made to suspend a pendulum mass or "flyweight" from a shaft linked to the frame of the seat proper, which mass has catches for co-operating with notches linked to the frame of the back in the event of sudden deceleration of sufficient force being applied to the vehicle (Document DE-A-2 708 461).
The angular locking obtained in that way is effective, but it can be achieved for a small number only of preadjusted angular positions of the back.