Hi Steve,
I wanted to “catch up” a bit with your journey and also introduce an idea that may help explain why some people are more comfortable with an innerspring mattress than with an all latex mattress.
A “warning” for those who read this first that this is more technical than most people would want to know (unless like you they have an engineering background or inclination and are interested in the more “technical” aspects of mattress construction) and in some cases it may tend to do more to confuse with its technicality than help with understanding of what any person’s “best” mattress may be. Bear in mind too that this comes from a lay person with a more “intuitive” understanding of the forces and mathematics involved rather than a more technical or scientific ability to express all of this through mathematical expressions like Hooke’s law or Young’s modulus and various torsional, compressive, and tensile deformations.
It’s also something that I’ve touched on with the concept of “dominating” layers in the other forum (and a bit here) but never really explored in either forum I’ve posted in. It is perhaps (at least IMO and experience) the most difficult part of mattress layering and design to completely understand and incorporate because it has so many variables in how it affects a particular mattress/person combination. It is also connected to zoning and the difference between “point” response, … “area” response … and “overall” response of different layering patterns and components.
The first step of this has to do with the nature of innersprings themselves and how they respond, deform, and compress under pressure. In general … innersprings store more energy and are more resilient than foam. This means they have more “pushback” because more energy is stored under the general area of compression. Foam has a greater hysteresis in other words (disperses energy throughout the material) than innersprings.
Innersprings have two common components to their basic design.
The first of these is the combination of an initial softer spring rate followed by a secondary firmer spring rate. This is accomplished through a combination of torsion and compression in different designs. Spring rate itself is the equivalent of a combination of ILD (or the newer term IFD) and compression modulus (how quickly a foam becomes firmer than it’s 25% ILD) in a foam. Spring rates are usually expressed in “weight per unit of length” of compression. Because most innersprings have design components where two different spring rates are combined … they would be most similar to two different layers of foam … each with different ILD’s (or IFD’s).
The second of these is how much the spring rate of individual coils is affected by the attachment or connection to the coils beside it or the degree to which other springs act in parallel with them (and to a degree with the insulators used over them) or the degree each coil can act individually. In other words … to differing degrees determined by their type and design … they all have one spring rate determined by the “softer” part of their design and how much each individual coil is initially affected by their “neighbor’s” compression and then a different spring rate determined by the “firmer” part of their design (which is also connected to differing degrees by the adjoining coils).
First … lets take a look at the “individual” springs used in each type of coil and how they act by themselves.
Bonnell coils have an hourglass shape and can be either knotted or unknotted. The larger diameter coils in the top and bottom are softer than the narrower diameter coils in the middle so the softer compression of the wider upper turns (which is affected by whether it is knotted or not, the shape and design of the upper turn, and the point outside or inside the lower coil diameter in the upper turns “end”) is the “softer” part which compresses first and once this has compressed enough then the compression of the narrower diameter turns of the spring is the firmer part.
Offset coils can also be knotted or unknotted (LFK coils) and also use various different designs to create a softer extended “hinged” part (torsion) and then a firmer part when the softer “hinging action” has become firmer and the “unhinged firmer part” (compression) comes into play. They also can use a slightly smaller diameter in the middle (although not as much of an hourglass shape) or can be more cylindrical from top to bottom (with the exception of the offset upper turn which extends beyond the underlying coil diameter). The offset part of the spring which uses the more pronounced hinging action creates most of the softer part and then the body of the coil creates most of the firmer part. In other words … the part of an offset coil that extends beyond the diameter of the turn below it uses more torsion while the part of a spring that is compressed within the diameter of the turns below it uses compression.
Pocket coils (or Marshall coils) also use various methods to create a softer firmer part and a firmer secondary part although they generally have less of a “double spring rate” than they do a “single spring rate” which more gradually firms up. Some of them however use different shapes, combinations of springs, or methods of pocket attachment to create different spring rates for different degrees of compression.
Continuous coils (or Miracoils) also use a combination of torsion and compression but because of their design (one continuous coil) generally have less of an ability to act alone than other coil designs.
So the “point” of this post is really the recognition that most innersprings have a combination of a softer response or spring rate and a firmer response or spring rate built into the coil and also have differing degrees of ability to act either individually or in groups (less conforming but more supportive) because of their connection to other parts of the spring assembly.
Both of these come into play whether the foam above them is softer than one or both spring rates or firmer than one or both spring rates. If it is softer … it would result in more “progressive” compression while if the foam above is firmer … it would tend to dominate one or both spring rates and would tend to “bend into” the softer part of the springs compression under it. When they once again “matched” in resistance, they would both continue to compress in parallel. Firmer foam that “bends” into the springs would create a wider “area” response with compression while softer foam that compressed before the spring would create a more point specific response of the foam above it.
Phoenix
