Date:         Mon, 5 Oct 2009 19:07:17 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Stupid wire question...really... (temp 2 wire)
Comments: To: neil N <musomuso@gmail.com>
In-Reply-To:  <c4e7c5f90910051546r664b26bfq2850f74c875f7110@mail.gmail.co m>
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At 06:46 PM 10/5/2009, neil N wrote:
>So adding shrink wrap would keep the original wire near connector from
>bending as much, thus hastening the stock wire casings' demise? Not
>clear about the "how" behind this,  but of course trust you're right.

There are two issues involved.  The one I mentioned is simply that
the shrink-wrap stiffens the wire so that bends near the connector
prefer the more flexible section right at the connector.  You can see
this effect in your own photo where the wires do a reverse bend right
at the connector.  The short radius of the curve overstresses the
tired insulation and it splits.

The second is the problem that happens any time you have a sharp
junction between two different stiffnesses.  In electronics we call
it an impedance mismatch; mechanical types call it a stress
raiser.  In either case energy is concentrated at the junction
between the two.  It's the reason why the transmission 3-4 slider
fails, the reason why the old Zenith terminal CRTs used to break off
their mountings from the rest of the case in shipping every single
time, the reason why you have to bevel the edge when you're cutting a
tire-tube patch to size.  The closer you get to perfectly sharp
transition, the closer the stress gets to infinite.  It's also the
reason why a windshield crack will keep on extending until you drill
a little hole in front of it.  And related to why you need a great
big hammer to move something, and a small one to deform it at the surface.

In this case the change between terminal and wire is the critical
one; flexing there will lead to the wire itself fatiguing and falling
off the terminal.  The shrink-wrap contributes by forcing more bend
to take place near/at the terminal than otherwise would.  In itself
it's not enough of a difference to cause much of this type problem.

David