On Fri, 07 Dec 2012 15:18:29 -0800, recyclebinned wrote:

Adding a second spring significantly reduces ...
the severity of the force with which the garage door comes down

Agreed!

That reduced-severity action actually works both ways.

1. When the door is going down (as you noted).
And,
2. When the door is coming up:
When a single broken spring, especially when it breaks near the
stationary cone, can cause the torsion bar to spin violently such that
there can be damage the top part of the door and to the cables.

In addition, two-spring systems balance the forces on the two cable
drums, which is important for systems (like mine) with weak cable drum
support to start with.

Two springs also enable the advantage that MORE SPRING SIZES (i.e.,
thicknesses) are instantly available to the homeowner, who then has more
options to choose springs of the desired life cycles.

A minor advantage of two-spring systems is that winding each one is half
the work of winding a single-spring system; but that's a one-time bonus.

Of course, two spring systems typically being longer (combined) than
single-spring systems have disadvantages too (e.g., they take up more
room so that you may have to move the spring anchor plate, they can cost
more, there are more things to break and replace, they can be heavier,
etc.).

All in all, very few professional installers told me by phone that they
would change out my single-spring system for a two-spring system, and,
Dan Musick advised me against it (even though his web site explains all
that I've said above).

Nonetheless, if you wish to convert from one spring to two, this handy
calculator tells you all that you need to know!
http://ddmgaragedoors.com/springs/st...s.php#database

For example, here is my current spring:
$51.56 36,000 cycles 0.250"x36" 14.14 pounds Lift=123.3# SPB-250-36-00R

If I converted that to two springs, Dan's site recommends:
$34.56 77,000 cycles 0.207"x28.5" 9.09 pounds Lift=61.9# SPB-207-28-50R
$34.56 77,000 cycles 0.207"x28.5" 9.09 pounds Lift=61.9# SPB-207-28-50L

So, the two-spring system would only cost about $15 more and would weigh
only about 4 pounds more, yet it would lift the same 123 pound door and
each spring would last more than twice as long as my single spring system.