Date: Mon, 5 Oct 2009 17:44:01 -0700
Reply-To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject: Re: Stupid wire question...really... (temp 2 wire)
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re this flexing issue in the last paragraph in David's post. .
it's important that wire flexing takes place in the wire, and not at the end
of the wire.
If anyone has ever seen the cables and wires that go to the starter solenoid
as the van comes from the factory,
the wires and cables are tie-wrapped to the side of the starter solenoid.
This way, flexing of the wire takes place in the wire, a couple of inches
from the end, and not at the very ends of the wires where the connectors
are, where it would be more likely to fail from flexing.
this is similar to why ALL transitions from metal pipe to flexible hose in
brake systems have a bracket right there.
If there is no bracket there, the movement and flexing of the hose is
transferred to the metal pipe, which can fatigue and break, resulting in
loss of braking power. On the front of vanagons, at the caliper, there is a
short pipe, then a bracket on the caliper, then the flex hose going to the
body of the van, where there is another bracket at that end of the hose.
People are turning out disc brake conversions where there is no bracket to
support the short pipe on the caliper. Eventually, flopping and flexing of
the hose with suspension movement could cause one of those short caliper
brake pipes to fatigue, crack, and leak.
you can bet your bottom dollar that vehicle manufacturing standards specify
that there is never any opportunity for the movement of a hose to exert
flexing force on a brake pipe. Metal does fatigue after all, and it's only a
matter of time and bumps in the road, until one of those pipes starts to
leak.
I could say 'Safety Warning !'
----- Original Message -----
From: "David Beierl" <dbeierl@ATTGLOBAL.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, October 05, 2009 4:07 PM
Subject: Re: Stupid wire question...really... (temp 2 wire)
> 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
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