Abstract: In an anti-skid braking system, an improved sensor responsive to forces aciated with decleration of the vehicle road wheel. The sensor comprises a housing driven at a speed related to road wheel speed; an annular inertia mass is connected to the rotary housing via a multi-convolution hair spring wound to bias the inertia mass against the direction of housing rotation. Deceleration of the housing causes the inertia mass to angularly advance relative to the housing. A cam-follower mechanism translates the angular advance into linear motion of an output stem member located on the housing rotational axis. Stem member movement can be used to effect pulse control of the anti-skid braking system.