Frontal Airbags will deploy at at least a 55 mph crash or more.
The force of the collision has to great enough
The force of the collision , with the front of your vehicle , has to be great enough for the airbags to deploy
your car could have been hit by a freight train in the rear and the airbags would not deploy unless, of course, you hit something in the front after the rear end collision had already occurred. the sensors are in the front and are designed to deploy the srs units if the occupants are in danger of going forward not backwards as the case in a rear end collision.
The force that the air bags deploy isn't as great when the collision is less severe
Most air bags will not deploy at less than 10 miles per hour the other posibillity is that there was a fault in the air bag system.
Speed up to avoid the collision.
typically 5 mph +
The front sensors must detect a full frontal crash. If it is a bit off center angle there is a possibility that they may not deploy as the energy required to cause deployment is dissipated .
The definition of 'deploy' is "To position forces in readiness for combat, as along a front or line of defence"
The front driver and passenger air bags are deployed if needed by crash sensors at the front of the Ranger , so unless you crash into something after being rear ended the air bags won't deploy - you should always wear your seatbelt.
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