Twistgrip, throttle cable

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Excalibur
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Twistgrip, throttle cable

#1

Post by Excalibur »

Had a throttle cable break today and I had to ride home by working carb throttle arm with right hand. At first I thought it must have pulled the nipple off.
I haven't had a nipple pull off ever since I learnt how to properly solder them way back in the late seventies.

Post mortem study showed that my solder had held up. The inner cable had broken just a few mils from the nipple.

Current thinking: Looks like my later model twistgrip setup is really for a push-pull type setup. Fitting a single pull cable like I did, appears to stress the inner because there's nothing to stop the sleeve rotating too far back.

What are the possible work arounds short of fitting a single pull throttle outboard of switchblock? Pondering how I could engineer an internal throttle tube stop. This would keep it neat and compact. I bet folk have seen this one before? Existing cable lasted 2000 miles.
tg1.jpg
tg2.jpg
Average year about '79 Shovel
Carb: SU Eliminator 2, dual return springs, single pull cable.
Switchblock unit: late 90's?

Thanks for any thoughts.
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RooDog
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Re: Twistgrip, throttle cable

#2

Post by RooDog »

The MoCo has use that style of throttle/switch housing since 1982 and was a great improvement over the earlier FL switches. I see no flaw in the design. I suspect that even though you are experienced in soldering the lug to the cable end the heating of the tempered steel cable led to the failure.
I also believe the throttle twist grip has an internal positive stop at WOT, so the idle cable's absence has no bearing on this situation.
I do have a throttle/switch housing assembly I could open up to confirm, just not this early in the day....
...RooDog....
Excalibur
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Re: Twistgrip, throttle cable

#3

Post by Excalibur »

Roo, thanks for the points and the opportunity to review, study and focus.

I'll elaborate more, my fault for not doing so earlier.

When I talked about no internal stop, I referred to the stop at off throttle or in this case the lack of it. The throttle tube can rotate past that point forcing the inner to bend at what could be as much as 90°. Clearly repeatedly bending inner cable to anything like 90° then back to straight is going to fatigue it to the point of failure.

Had a chance to review the parts and setup. I conclude, the inner is being stressed in a very extreme kind of a way. I have a couple of ideas to retro fit an off throttle stop. Ideal would be at the point where the two housings meet and for safety it needs to be failsafe. I also have a spare throttle tube so between the bits I should find something that works.

Thanks
RooDog
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Re: Twistgrip, throttle cable

#4

Post by RooDog »

Understood.
When installed that cable should smoothly roll with the grip twist and not flex. Surely there is a simple solution, probably an error in assembly. I know I have had my problems assembling such a simple thing as a quick throttle, but then, I am often an idiot, and not as smart as I think I am.
Best wishes, mate....RooDog....
Excalibur
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Re: Twistgrip, throttle cable

#5

Post by Excalibur »

Thanks for the thoughts. Most appreciated. I know what you mean about the simple things tripping you up. I often have to undo a fix or change, sometimes re-doing it 3 or 4 times before finally understanding how it needs to be. :mrgreen:

My off-throttle stop solution yet to be tested..

1/8" thick aluminum insert,
th3.jpg
2pot epoxy resin, bonded in place.
th2.jpg
Other view of insert..
th1.jpg
Probably won't be till Wednesday before I get to fit it. Oh and I should prudently start with a fresh length of inner...
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RooDog
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Re: Twistgrip, throttle cable

#6

Post by RooDog »

Nifty....
Looks to me to be a viable solution. Thanks for sharing....
But, what with the wooden matches?
....RooDog....
Mongrel505558
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Re: Twistgrip, throttle cable

#7

Post by Mongrel505558 »

Wooden matches are probably to give sense of relative size. I like this fix and will probably try it myself. I recently had to replace one of these throttles on the side of the road and thought about how sketchy the whole assembly is. Not exactly a Swiss watch.
Excalibur
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Re: Twistgrip, throttle cable

#8

Post by Excalibur »

Matches there sense of scale. Reporting back later this week. Thanks.
Excalibur
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Posts: 882
Joined: Mon Jul 27, 2015 1:01 pm
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Re: Twistgrip, throttle cable

#9

Post by Excalibur »

Got it all reworked and assembled with a new piece of inner Bowden cable. Reclaimed the existing brass nipple and soldered it with 50/50.
Road tested about 30miles with 2stops. Can't fault it. Happy and I learnt something.
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