Ordinary mattresses are quite thick. They have room for high quality springs to float before they compress fully. High quality springs have more coils. (See Image at Left).

Sofa bed mattresses are less than half as thick as a good standard mattress. There isn’t a lot of room for springs to float before they fully compress. Top quality springs are out of the question. (See Image at Right).

All mattresses are not created equal

Foam mattresses compress as needed . The foam acts like an infinite number of springs. It gives much better support, and molds to the body. We use only very high quality foam, which always snaps back to shape.

An inner spring mattress needs lots of springs. The more springs there are, the better the mattress molds to your body.To make a really excellent spring mattress, huge numbers of springs would be required

Spring Quality

The most important consideration in mattress springs is Permanent Set: A condition that comes from deflecting a spring beyond the elastic limits of the material; the spring does not return to its original condition when the load is released. The more Active coils, (coils that are free to deflect under load) you have for any given length, the less likely the spring will be to deteriorate.

As discussed above, the spring has:

– Free Length: Overall length of a spring when no load is applied.

– Solid height: Length of a compression spring when it has been deflected enough to allow each adjacent coil to touch each other.

– Working length: (The difference between the free length and the solid height).

Active Coils: The total number of coils minus the number of closed coils forming the ends is a measure of the spring quality. Unfortunately, the more active coils there are, the less working length we have. This means that the thicker the spring mattress, the better.

A disadvantage of springs is Hydrogen Embrittlement: Hydrogen absorbed in electroplating of carbon steels causing the spring material to become brittle, leading to cracking and spring failure.  Plated springs are thus a problem, and un-plated springs are more likely to rust. Plastic coating is an alternative, but that’s extremely expensive.

Springs are also subject to internal stresses created during the winding process. The best springs are Stress Relieved:  The heat treating of springs at a temperature so as to relieve stresses. Many mattress springs are not stress relieved because it is expensive. It is a potential factor in mattress failure.

Springs can make a satisfactory bed, but have the severe limitations expressed above, compared to a foam replacement mattress.