Innerspring mattresses have wire coils as the support structure. An innerspring mattress has a strong border wire attached to the perimeter of the coils. The border wire supports the coils, helping to retain shape and reduce sagging. The edge is reinforced with specially shaped springs or foam to support the sleep surface right to the edge, to help prevent ‘rollout’ and edge breakdown. The upholstery is separated from the innerspring system by an insulator, a tough padding, wire or netting layer that prevents the upholstery from sinking into the coils.
Innerspring Support System
Number of Coils
Not so long ago, experts were saying that the more coils a mattress had, the more support it would give. Today, we have realized that this is not necessarily. The way that the coils are constructed is the main factor in determining the amount of support the user gets.
This all depends on the mattress size, but mattresses generally come with 300 to 800 coils. Based on a queen-size mattress, 500 to 800 coils is considered firm. However, some of the best and most comfortable mattresses only have around 400 coils, and even fewer in a full mattress will provide decent support. This can impacted by the use of a heavier gauge wire, therefore adding more steel to the mattress.
There are a number of misconceptions with regards to coil count and firmness. Doctors once said that firmer mattresses give better support. This was actually only true with a small percentage of people who sleep on their backs and/or stomachs. Research shows that most people sleep on their sides. The side of the human body is not flat (like a back,) and a mattress that conforms better to the body and gives better support putting the spine into proper alignment or it’s natural resting position has proven to be better. Individually wrapped coils have proven to be the best factor in support for side sleepers.
It may seem obvious to most that heavier gauge (thicker wire) coils would offer a greater deal of support than lighter gauge coils. It has been proven that this is not true and some mattress manufacturers compensate for thinner gauge steel wire by increasing the coil count and specifying the type of coil.
Low coil counts and heavy gauge wires can seem firm in the store, but it is likely that such a mattress will not hold up long because it simply does not have enough coils.
Many premium mattresses feature 14-gauge (1.63 mm) coils. Coils are measured in quarter increments. The lower the number, the larger the diameter of the spring. With coils in the 14 to 15.5 (1.63 to 1.37 mm) gauge range, it is important that the total coil count be high the coils would otherwise give easily under pressure.
A 12.5 gauge (1.94 mm) innerspring coil, the thickest typically available, may feel rock hard in a double mattress even with a coil count of 400 or less.
How are the Coils Connected?
We have found that the number and quality of these interconnecting wires, or helicals, is not published by the manufacturers. If there are too few of these interconnecting wires a mattress can lose its shape more quickly than one that has an adequate amount. Mattresses that sag quickly usually have few helicals. Some manufacturers make individually pocketed coils (see types below,), which disperse movement from different sides of the bed because they are not connected. However, in this case, the pocket or independent coils provide the support required by the individual coil instead of the interconnection between coils.
Types of Coils
Most manufacturers stake claim to having the best coil-to-gauge ratio. However, there are only a few innerspring manufacturers and four general types. One thing to remember is that while there are many different types of coils and many manufacturers, most manufacturers have their coils made by one company.
There are a number of different types of coils:
Open Coil or Bonnell Coil: These are the oldest and most commonly used. They were adapted from a design used in buggy seats in the 19th Century. This hourglass-shaped wire coil is joined to adjacent coils by small wire spirals called helicals. The open coil design provides good initial support but these coils fatigue more quickly than more technically advanced coil constructions. You’ll know this mattress has reached the end of its life when you and your bed partner are experiencing the ‘roll together’ or you suddenly realize that your mattress feels more like a hammock.
Offset Coil: This coil construction is similar to the open coil but it has a squared head, which in theory enables the mattress to contour better. It is a descendant of the Bonnell or open coil. An offset coil is slightly more cylindrical in shape than an open coil, which may make it more durable. It is also laced to its neighbors by helicals. These are usually the most expensive coils.
Pocket Coil/Independent Coil or Marshal Coils: A pocket or independent coil is a cylindrical spring wrapped in its own fabric pocket. This construction is designed to give the most contour of any support system. Each coil works independently which means that you will feel your partner move much less. Each pocket coil absorbs the weight placed on it without distributing the load to other coils. Because of this, it is possible that the coils will provide less support over time, particularly for heavier people. This type of coil is a good choice for people who toss and turn and for their bed partners because the motion separation may prevent waking from a deep sleep which is very important for well being. Simmons uses this type of coil.
Continuous Coil: The continuous coil is exactly what it says – each row of coils is made of a single wire attached to the next row by helicals. This type of coil is often made into a lattice of coils rather than rows, allowing more coils per mattress than other constructions. This design allows each coil to deflect weight to a great number of springs resulting in less stress on each coil. Mattresses with continuous coils keep their shape longer. Serta and Kingsdown use these types of coils.