Mattress Springs and Mattress Coils
Mattress springs are the main support unit of an innerspring mattress...This section will explain each of the four main types of mattress coils                         .
1.Bonnell A mattress spring shaped as most of us would picture in our minds a mattress coil should look like. A rounded at the top and bottom coil. This type of spring is used in only the cheapest of mattresses. The springs are then connected together by a spiral wire called a helix wire (or helical wire) from side to side or head to toe. This type of unit is used only in the low end mattresses,sofa bed mattresses and innerspring futons.
2.Offset This mattress coil unit is the same as the bonnell with the difference being the top and bottom have two or three flat sides to allow more turns of the helical wire to attach to the mattress spring  making the unit stronger and preventing the mattress coil from turning. The helical wire also helps spread weight across more coils.Now that mattresses aren't padded on both sides and can no longer be flipped or turned over (a result of the new federal fire retardant legislation) mattresses with coils that the turns are opposite each other in every two rows of coils will lean less helping reduce the body impression problem.  These are called facing coils or car coil and can be found in king Koil or Spring Air .Sealy uses coils that face the same direction that wasn't a problem when beds were two sided.
3.Continues Wire This is a system where the innerspring is made from one piece of wire runs through the entire innerspring or a least one whole row of the innerspring. The wire is bent to equal a three turn coil. The rows are then connected by a helical wire making the unit a true mesh.
4.Individual Coil unit or individually wrapped mattress coils This unit is built without a helical wire. The coils are wrapped with cloth to keep them from rubbing together making noise. Some use a precompressed coil to give it strength and longer acting spring action. This type of unit works better in the one sided configuration than in the two sided mattress. The advantage of this type of unit is motion transfer, less disturbance when your partner moves. There is also an individually wrapped system called the cable coil system this is where each coil is made up of three strands of wire twisted together to make one piece the coil is four time stronger than a normal coil. Sealy has added a coil that has a dual action by making the top turns a different strength than the rest.
Misc.
Mattress coils come in different gauges, usually between 12.5 and 16.The lower the number the thicker the wire. Many salespeople will try and create the illusion that more coils make a better mattress. Thicker wired coils can't be turned as tightly or the bed would too stiff not allowing any body shaping to occur. Most experts agree that 300 coils full size or 380 queen size are considered adult support. In todays American built mattresses the wire is tempered electro statically and most spring sets will outlast the upholstery layer.
Mattress springs have between three and six turns. Strength of the innerspring is thus determined by the combination of number of mattress springs, thickness of wire, the number of times tempered and the number of turns in each coil. A company called Leggett and Platt makes most of the innerspring units used in bedding today and sells them to the manufacturers who use them to assemble their mattresses.
It is true that there is no one innerspring style better than the other, each will create their own feel and shaping ability to the body. The individual coil unit has the extra advantage of dampening motion and contours more accurately especially with two people or a larger animal in the bed. When a helical wire is used it transfer some of the body shaping from one person to the other making neither as correct as possible.
Offset coil unit
Marshall unit
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