Fire trucks as well as many other vehicles use Onspot chains.
Onspot chains are a traction control device that can be activated by the vehicle operator with an electrical switch on the dash.
When the dash switch is turned on, air pressure builds up from the vehicles air brake system and an air solenoid that is attached to the rear suspension on each of the rear tires is filled with air.
The two cylinders lower one chainwheel each until they come in contact with the side walls of the tires.
When the tire side wall turns against the chain wheel it causes it to rotate the 6 chain lengths attached to the chain wheel.
The force of the rotation causes the chains to whip out underneath the tires and on top of the ground surface resulting in better friction/traction in foul conditions. (ice, snow, etc.)
When the switch is turned off the hydrolic cylinders retract and pull the chain and wheel back up to the resting position.
