Author Topic: Cabel physics- simplistic dynamics, but perplexing analysis  (Read 3291 times)

steven mandell

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Cabel physics- simplistic dynamics, but perplexing analysis
« on: December 24, 2013, 06:15:40 PM »
Out of sheer frustration, I have decided to share my incomplete attempt at analyzing why when I push on my AC Petite's choke knob, it springs back out w/o enabling and effecting a full opening of the choke butterfly.
I am hopeful of the possibility of creating a new cable may solve the physical problem, but am not content to remain ignorant of the dynamics of the involved dysfunction. 
Straightening the solid wire core cable for a length of about 6 inches from the carb end, and a full foot from the knob end, as well as the 3" stem on the knob end did provide less resistive actuation, however despite the apparent free motion of the cable in its housing the knob still bounced back out about 1/2" when reconnected to the choke lever.  The cable was apparently preferring to flex in its housing, and create a steeper arch in the housing, which also pulled the non restrained choke end of the cable's ferrule off of its carb mount, as the greater circumference thus created would require.

As a temporary partial solution, I have taped the carb end of the housing to the carb, and installed 2 interwoven compression springs concentric to the exposed cable that have the effect of creating an adjustable pushing of the choke valve open. However this doesn't reliably open the choke the last 15 degrees or so, and of course should not be necessary as it is reasonable to presume that things worked correctly when new.  This is especially perplexing because the clothes pin style spring concentric to the choke shaft did seem to provide a reasonable amount of choke butterfly opening force, and the cable felt fairly free in its housing.  The relatively short length of the choke lever does put some considerable angularity into the choke cable against the inside of its housing as it prescribes its arc of actuation.  But then again, this is entirely a stock setup, so it must have been sufficient to work properly as designed.
The only thing that I can come up with is that the inner lining of the cable housing provides too much resistance to the cable sliding through it very near the choke end of the cable where its stock length and required 90 degree change in orientation creates the aforementioned heightening of the housing arc instead of pushing on the choke lever arm.

Anyone else's insights or experiences will be appreciated.
Photos forthcoming in next post.

steven mandell

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Re: Cabel physics- simplistic dynamics, but perplexing analysis
« Reply #1 on: December 24, 2013, 06:40:48 PM »
Before and after compensatory (bodge) engineering to keep cable end from retracting, and assist choke butterfly opening. :-\

Bob Purton

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Re: Cabel physics- simplistic dynamics, but perplexing analysis
« Reply #2 on: December 24, 2013, 09:26:24 PM »
I have had the same problem before with old choke cables. Once they get old and bent around a little sometimes different angles and slight twisting actions that pushing a ridged cable through them causes can close up the ID and grab onto the inner cable. A new one will solve it. I'm just about to fit a new one that I got from the MOC for my Nobel.
« Last Edit: December 30, 2013, 09:59:09 AM by Bob Purton »

steven mandell

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Re: Cabel physics- simplistic dynamics, but perplexing analysis
« Reply #3 on: December 24, 2013, 10:16:20 PM »
Upon closer inspection, I see that their is no inner housing lining.
Its just a very long and very tightly wound spring that would seem naturally resistant to deformation.
Springs to tend  to spring back into their original shape (that is why they are called "springs").  So that leaves me still in a bit of mystery about one the most simple devices known to man.

Where can I get a new choke and clutch cables for my AC?
The problems that I am having  trying to effect successful clutch disengagement is a whole other story.
As a test of how well you paid attention to my previous diatribe, ..points will be awarded to the first person who correctly identifies the mechanism of effect that explains how the accompanying pictured clutch cable housing's outer vinyl covering and ferrule managed to get peeled back and distorted.


« Last Edit: December 24, 2013, 10:22:35 PM by steven mandell »

Bob Purton

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Re: Cabel physics- simplistic dynamics, but perplexing analysis
« Reply #4 on: December 24, 2013, 11:23:50 PM »
I can only speak for whats the norm in the uk but bowden cables having linings is a fairly recent thing. A sixty year old one certainly would not have one. Thats why old cables have grease nipples built in or oiling devices.
Yes they are long springs that would spring back when new but not when they are sixty years old and knackered!
If you have the original cables and in the UK you would send them to Speedy cables or Johnsons cables and have new ones made, there must be similar companies over there that do the same thing. Personally I make all my own with the exception of choke cables. I turn the ends on the lathe and solder them on using the flared end method. I have never had one come adrift so far.