Re: Drop-in coil springs
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A drop-in spring grid is inferior as it has no webbing for base support, its self supporting or rides on a few metal cross strap bands The unit is wholly self-contained and is simply screwed to the frame of the furniture. It 'floats' in the frame and has lots of metal to metal contact (this can cause squeaking down the road as the oils evaporate off the metal and surface rust tends to form). They are fast to install, and relatively inexpensive. This photo off the Smith Bros of Indiana web site:

An 8-way Spring Set up is more costly to build and takes more time to make. Here each spring rides independent of the other, but are tied in with the strings to work in unison and is supported by drum-type webbing at the base. Look at the time it takes to do a simple old chair correctly. The Hancock and Moore procedure is even at a higher level than shown here:
https://youtu.be/jFWjBx4Zc8Q
Duane Collie
Straight answers from thirty-six years in the business.
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