Do cables stretch and/or housings compress?

[lks]

I have replaced cables, without replacing the cable housing, and have not had to adjust the brake or derailleur after it's initial adjustment. I have replaced cable housings, without replacing the cable, and have had to adjust the brake or derailleur after it's initial adjustment, but not by much. I use the best OEM and after market parts, including pre stretched cables, so I don't think that is a factor. My conclusion is, that cable stretch is a myth, at least to the extent that a perfect adjusted rear derailleur requires subsequent adjustment. And that cable housings compress and/or their ends seat in after installation.

 

[bike_racer102]

I have replaced cables, without replacing the cable housing, and have not had to adjust the brake or derailleur after it's initial adjustment. I have replaced cable housings, without replacing the cable, and have had to adjust the brake or derailleur after it's initial adjustment, but not by much. I use the best OEM and after market parts, including pre stretched cables, so I don't think that is a factor. My conclusion is, that cable stretch is a myth, at least to the extent that a perfect adjusted rear derailleur requires subsequent adjustment. And that cable housings compress and/or their ends seat in after installation.

Cables do stretch. And if you replace cables be sure to lube the cable and the housing.

 

[lks]

Cables do stretch. And if you replace cables be sure to lube the cable and the housing.

I don't doubt that some cables stretch, I just never had one that did, before I replaced it. Many expert mechanics recommend Pro Gold's, Pro Link lube. I buy it by the quart for only $50, probably cheaper elsewhere, and they also have a hypodermic type dispenser that allows you to lube your cables/housings, even after they are installed.

 

[daveornee]

I have replaced cables, without replacing the cable housing, and have not had to adjust the brake or derailleur after it's initial adjustment. I have replaced cable housings, without replacing the cable, and have had to adjust the brake or derailleur after it's initial adjustment, but not by much. I use the best OEM and after market parts, including pre stretched cables, so I don't think that is a factor. My conclusion is, that cable stretch is a myth, at least to the extent that a perfect adjusted rear derailleur requires subsequent adjustment. And that cable housings compress and/or their ends seat in after installation.

I agree fully with your conclusion.
Once cable has reached it's adjusted length under tension there is no further stretching.
Housing will move until it is completely seated and then just a little wear on the inside lining makes it's length slightly less over it's life. When you install new housing you should accelerate the seating of the ends such that it has no further seating possible.

 

[fish156]

Once cable has reached it's adjusted length under tension there is no further stretching.

I would say that this is "mostly", but not "completely" true. Metals have all these properties like ductility, hardness, malleability, etc, etc. All metals will bend and stretch to some extent - often almost unmeasureble, but it is happenning continually until it breaks. With metal cables, the stretching property and the resistance to corrosion are a couple of things that make for better, or worse, types. I think a better answer is that most of a cable's stretching occurs in a short time and then it just slows down to the point where it's hard to measure. But, it doesn't just "stop". Sorry to nitpick, but to say "no further stretching" is just not accurate.

I do agree completely that a lot of what's termed "cable stretch" is, in fact, the compression of the cable housing.

 

[otherworld]

Most of what is happening is to do with the cable bedding into the casing. It’s not stretch or compression. If you want a good result, always replace both. A common case is sluggish rear derailleur operation which is often evidenced by increased drag caused by the cable being stuck in the groove its warn for itself inside the casing.

Jay

 

[free_rideman]

I agree with fish156.

Even though cables do stretch just a tiny bit, I think the housing does the most "stretching", in terms of seating itself.

After the first 100 miles or so it should be pretty much in its final place. Depends on what bike it is, how much shifting you do, and how long the cables and everything is.

 

[daveornee]

I would say that this is "mostly", but not "completely" true. Metals have all these properties like ductility, hardness, malleability, etc, etc. All metals will bend and stretch to some extent - often almost unmeasureble, but it is happenning continually until it breaks. With metal cables, the stretching property and the resistance to corrosion are a couple of things that make for better, or worse, types. I think a better answer is that most of a cable's stretching occurs in a short time and then it just slows down to the point where it's hard to measure. But, it doesn't just "stop". Sorry to nitpick, but to say "no further stretching" is just not accurate.

I do agree completely that a lot of what's termed "cable stretch" is, in fact, the compression of the cable housing.

Yes the cables bend and stretch under the oringinal forces, but they don't continue to stretch unless the force is increased. So what is the mechanism for additional stretching of the cable? Is there additional force? Is the cross section area diminished?
Try taking a cable and hanging a known weight on it. It will stretch in response to the weight and then stay that length unless/until the forces acticng on it is changed.
If you are talking about elasticity in a cable with response to increasing force such as applying brakes, yes they stretch in response to the force, but return to their original length in static state.
The exception to this case is plastic deformation of the cable. Rarely are cables specified so thin or poorly alloyed that they would plastically deform in any bicycling applications.

 

[lks]

Most of what is happening is to do with the cable bedding into the casing. It’s not stretch or compression. If you want a good result, always replace both. A common case is sluggish rear derailleur operation which is often evidenced by increased drag caused by the cable being stuck in the groove its warn for itself inside the casing.

Jay

You are guessing, unless you are basing your opinion on having cross sectioned worn cable housings and know how to measure the depth of any groove you thought you saw and how to calculate the effective housing radius decrease, that the cable sees. Then you would have to know how to calculate how much it changes the position of the rear derailleur. Try buying a quality Teflon lined housing and a double or triple wound 19 strand stainless steel wire cable that has been drawn thru a die to give it a smooth outer surface and pre- stretched so it won't stretch enough to require you to adjust the rear derailleur more than the first time. Of course, put the correct lube in it and do this periodically, so this super smooth/slick cable won't wear a groove in this hard teflon liner.

 

[lks]

Yes the cables bend and stretch under the oringinal forces, but they don't continue to stretch unless the force is increased. So what is the mechanism for additional stretching of the cable? Is there additional force? Is the cross section area diminished?
Try taking a cable and hanging a known weight on it. It will stretch in response to the weight and then stay that length unless/until the forces acticng on it is changed.
If you are talking about elasticity in a cable with response to increasing force such as applying brakes, yes they stretch in response to the force, but return to their original length in static state.
The exception to this case is plastic deformation of the cable. Rarely are cables specified so thin or poorly alloyed that they would plastically deform in any bicycling applications.

Thank you for the free beer. The next time I replace another friends frayed cable, I will correctly tell him he has over cycled his cable's elasticity. He will then ask me to explain, which I will do for a free beer. If I just told him, it was worn out, he wouldn't ask me to explain.

 

[fish156]

Try taking a cable and hanging a known weight on it. It will stretch in response to the weight and then stay that length unless/until the forces acticng on it is changed.

This is the disconnect in our views. If you do as you say, the cable will continue to stretch until it breaks (eventually). It will not just stretch to a point and stop. In the case of a bicycle cable, every time you shift up you put force on the cable and stretch it. When you downshift you unload that force, but the cable is just that much weaker than before. Every time you cycle up and down you are working with a cable that is getting weaker every time. If this was not the case then cables would never break and would have an infinite life cycle. The reality is that the life cycle is indeed finite and they will eventually stretch to the breaking point. It would be great to have immortal cables. Yes, it takes a long time for this to happen, but it does happen.

 

[daveornee]

This is the disconnect in our views. If you do as you say, the cable will continue to stretch until it breaks (eventually). It will not just stretch to a point and stop. In the case of a bicycle cable, every time you shift up you put force on the cable and stretch it. When you downshift you unload that force, but the cable is just that much weaker than before. Every time you cycle up and down you are working with a cable that is getting weaker every time. If this was not the case then cables would never break and would have an infinite life cycle. The reality is that the life cycle is indeed finite and they will eventually stretch to the breaking point. It would be great to have immortal cables. Yes, it takes a long time for this to happen, but it does happen.

I know we don't now share the same views but my views are schooled from Materials Science text book on the subject of steel. Elasticity and plastic deformation are the particular areas of interest.
Steel cables used in industry for guy wires, winches, etc. do not permanently "stretch" (plastically deform) when used in working range specified.
Bicycle shifting cables loads are friction and pulling against the springs in the derailers. The spring never gets stronger, but the friction does increase the load when grit gets in pivots, housing, guides. However, the cables do not continue to elongate because they are working in the elastic range.
Fatigue breakage at the ends and/or other stress riser points is a distinctly different mechanism.

 

[fish156]

I know we don't now share the same views but my views are schooled from Materials Science text book on the subject of steel. Elasticity and plastic deformation are the particular areas of interest.
Steel cables used in industry for guy wires, winches, etc. do not permanently "stretch" (plastically deform) when used in working range specified.
Bicycle shifting cables loads are friction and pulling against the springs in the derailers. The spring never gets stronger, but the friction does increase the load when grit gets in pivots, housing, guides. However, the cables do not continue to elongate because they are working in the elastic range.
Fatigue breakage at the ends and/or other stress riser points is a distinctly different mechanism.

Ok, I'm not talking about "in the working range specified" or "in the elastic range".

The simple fact IS that all metals do stretch/elongate under load. It might take a billion years for a certain case to fail and the measurements could be in zillionths of microns, but the metal IS stretching under any load, no matter how small. Time and units of measure are relative and you do have to think very small and over very long periods of time see what I am saying. It's kind of like the difference between engineering and basic science. Engineering, or material sciences, if you will, says that a bike cable under tension from a brake spring and cycled by the hand control is highly unlikely to break. And, this is a good thing. Science says that given enough time and enough cycles, the cable will definitely fail, due to the constant load on the cable.