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Breakage of cable threads

Old 03-24-22, 12:48 PM
  #1  
travelinhobo
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Breakage of cable threads

I was told in the past year by a bike mechanic that tightening the allen bolt down on a cable too tight can cause the threads to break (thus resulting in having to buy a new cable when too many of the threads have broken ). True or not? A biker friend said no and a self-professed ego

mechanic claimed there are only 2 ways cable threads break, one of them being when the cable gets bent. Which he then proceeded to do as he finished my installation of the pictured brake cable. (When I asked him why he'd done that if it can cause the threads to break, he gave me a laughable response.)
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Old 03-24-22, 01:05 PM
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bboy314
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Overtightening the pinch bolt can indeed mash the threads to the point of eventually breaking.
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Old 03-24-22, 01:22 PM
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When talking about ropes and cables they are called strands.

This is another case where it depends. As well it might be more a thing of vintage days when cables were not as good a steel as they are now.

Most cables today will probably survive any tightening and you'd either break the pinch bolt or strip the corners off the hex that your wrench fits in or on. Though I can see where some cheap'o designs might smash the cable strands and then let them break from getting work hardened over time as they flex. Sort of like when you bend a coat hanger wire multiple times to break it at a certain point.

Last edited by Iride01; 03-24-22 at 01:27 PM.
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Old 03-24-22, 01:40 PM
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I think overtightening the pinch bolt enough to sever the cable strands would probably break the bolt or strip the threads in the brake arm first. As to bending the cable, yes, repetitive bending and straightening can eventually cause it to break from fatigue but one bend like you picture is harmless.
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Old 03-24-22, 02:35 PM
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Also, the details of the clamp matter a lot. Are the edges rounded? Are there sharp edges to hole in the bolt the cable goes through? Does that hole line up nicely with the flat or groove the cable sits can mean that too tight a nut will damage strands. Flip side - more than tight enough? Why? Yes, for brakes, especially front brakes, tight enough is critical but still, we don't need more. And compromised strands means we've just eaten away at the safety factor we were tightening that nut to achieve.
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Old 03-24-22, 02:38 PM
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I find that I don't need to mash the cable with the pinch bolt to the point of strand separation. The design of the cable pinch system is the deciding factor in how tight.
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Old 03-24-22, 09:22 PM
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tFUnK
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Can happen, especially if you reclamp the cable at a bend or where the strands have already flattened out. More of a function of repeated unclamping/reclamping and less of a function of overtightening just once.
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Old 03-25-22, 06:53 AM
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Originally Posted by travelinhobo
a self-professed ego mechanic claimed there are only 2 ways cable threads break, one of them being when the cable gets bent. Which he then proceeded to do as he finished my installation of the pictured brake cable. (When I asked him why he'd done that if it can cause the threads to break, he gave me a laughable response.)
In this case, any damaged strands are "benign" as the cable past the bolt isn't doing anything. Bending it keeps it from sticking straight out and catching you in the leg. He could have clipped it off right at the bolt to do the same thing. Re-adjusting the cable later would be a problem...
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Old 03-25-22, 11:56 AM
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Those bolts don't need to be that tight on decent groupset. 7Nm should be about as much as you need.
torque wisdom here
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Old 03-25-22, 12:43 PM
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SurferRosa
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I'd be more worried about stripping the bolt threads.

I don't use a wrench, only a socket driver or allen key (with my hand close to the bend).
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Old 03-25-22, 02:01 PM
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travelinhobo
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Ok. Thanks for all the responses.
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Old 03-27-22, 11:26 AM
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Interesting question. I've definitely broken cables by overtightening. But that was years ago, I was a kid and an idiot, but most importantly the clamps would have been steel the same as the cable. Most clamps now are aluminium and you often see a small impression of the cable into the soft aluminium. So it may be that it can't happen any more. I still don't overtighten those things though (no reason to).
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