In earlier SAE papers the authors demonstrated that the ideal crash sensor for use in the crush zone has a constant velocity change response, and various sensors are now in use which perform as acceleration integrators. Further study of sensor performance, however, has shown that crush zone sensors function by being struck by crushed material which is forced rearward during the crash. This observation has led to the design of an inexpensive sensor which measures crush instead of velocity. The crush of the vehicle is used as an accurate indicator of the severity of the crash. This paper presents the theoretical basis for a sensor which initiates air bag deployment when the crush of the vehicle exceeds a pre-selected amount.In a companion SAE paper, the authors have demonstrated that single point sensing in the passenger compartment may result in late air bag deployment on soft crashes, and, therefore, sensing in the crush zone is required. Crush sensors, such as described in this paper, have significant performance advantages over current generation sensors and can be used either alone or in conjunction with electronic passenger compartment sensors.Three types of sensors are currently in wide use to sense and initiate deployment of an air bag passive restraint system. These sensors include air damped ball-in-tube sensors, spring mass sensors and electronic sensors. As first generation sensors, these devices have worked well and have contributed to saving many lives and mitigating many injuries. A great deal has now been learned about the relative advantages and shortcomings of this current technology and how it works, and now this can be used to design the next generation sensors.In this paper the authors will present one new sensor technology which they believe is the least expensive and best performing sensor yet devised. This conclusion is based on the following points: Crashes can only be reliably sensed in the crush zone and therefore crush-zone mounted sensors are required. The amount of crush of the vehicle is the best indicator of accident severity. Current technology crush zone sensors are point sensors and therefore miss crashes where the impact point is remote from the sensor. A simple switch mechanism coupled with an appropriate bracket can be used to sense vehicle crushes regardless of the point of impact.