This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (December 2007) (Learn how and when to remove this template message)
A coil spring, also known as a helical spring, is a mechanical device which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. They are made of an elastic material formed into the shape of a helix which returns to its natural length when unloaded.
A coil spring may also be used as a torsion spring: in this case the spring as a whole is subjected to torsion about its helical axis. The material of the spring is thereby subjected to a bending moment, either reducing or increasing the helical radius. In this mode, it is the Young's Modulus of the material that determines the spring characteristics.
Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form cylindrical coil springs.
Coil springs for vehicles are typically made of hardened steel. A machine called an auto-coiler takes spring wire that has been heated so it can easily be shaped. It is then fed onto a lathe that has a metal rod with the desired coil spring size. The machine takes the wire and guides it onto the spinning rod as well as pushing it across the rod to form multiple coils. The spring is then ejected from the machine and an operator will put it in oil to cool off. The spring is then tempered to lose the brittleness from being cooled. The coil size and strength can be controlled by the lathe rod size and material used. Different alloys are used to get certain characteristic's out of the spring, such as stiffness, dampening and strength 
Spring rate is the measurement of how much a coil spring can hold until it compresses 1 inch. The spring rate is normally specified by the manufacture. If a Spring has a rate of 100 then the spring would compress 1 inch with 100lbs of load. 
Types of coil spring are:
Coil springs have many applications; notable ones include:
Tension and extension coil springs of a given material, wire diameter and coil diameter exert the same force when fully loaded; increased number of coils merely (linearly) increases free length and compressed/extended length.