Offset coils, also called Karr coils, were invented in the early 1900s by Frank Karr, who also started the Spring Air mattress company. They were developed from Bonnell coils, the original support for innerspring mattresses. Since the inception of offset coils, they have been used by many other mattress companies.
The principal distinction of offset coils from Bonnells is the shape of the ends. The top and bottom ends of Bonnell coils are round. Then they are tied into rows by spiral wires. The ends of offset coils are squared at opposite ends. The squared ends of one coil are tied to those of the next coil with a helical wire. This functions as a hinge, which allows each coil to respond individually, making an offset coil innerspring unit more flexible than a Bonnell.
The flexibility of offset coils reduces pressure points on protruding parts of the body, such as shoulders and hips. This also means better back support, especially for side sleepers. Additionally, this reduces motion transfer. Alternating right-hand turning coils with left-handed ones makes the mattress more stable by countering any tendency to lean one direction or the other.
There are two basic forms of offset coils, knotted and un-knotted. Knotted ones are more stable and have a smoother feel. Un-knotted Karr coils are springier and more flexible.
Because they are more complex, offset coils cost more to make and assemble. thus they are usually found in the more expensive higher-end models of mattresses. Those who can afford them value offset coils over Bonnell coils for both their durability and the quality of sleep provided by greater comfort.
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