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Old 08-28-12, 06:48 AM   #1
TomD77
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Cable Stretch?

In another thread it was recommended that a poster replace shifter cables due to constant stretch requiring adjustment. How can they stretch? I doubt the cable strands are being stressed to the elastic limit. Are cheap cables of a loose weave?

I've had to replace a cable but it was due to fraying and I paid absolutely no attention to what went back in. I've had no problems than an adjustment every couple to a few months won't deal with.

So is the brand of cable something I need to worry about?
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Old 08-28-12, 07:13 AM   #2
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I restored old VWs as a hobby and over time cables begin to stretch when under constant tension. Once they start to stretch they continue and need to be replaced.

As to bicycles i'll assume the same happens. Now i've owned my current bike for 10 months and 1400 miles and there's been no adjustment needed. But when they do start to stretch, i'll replace them all.

BTW... when i'm done with a ride, I shift into the large chain ring and small cog which reduces the tension on the cables. Will this extend their life... not sure but it sure won't hurt.
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Old 08-28-12, 07:36 AM   #3
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With modern cabling systems, "cable stretch" is actually housing shrinkage caused by ferrules seating after installation. There isn't sufficient tension on the cables to stretch them.

Once a new installation is fully seated, it should provide thousands of miles of trouble-free cycling. When shifting begins to deteriorate, it's because of cables fraying at the shifter, dirt in the housings, or the cables wearing through the lining of the housing.

On this third one, cables wearing through the lining of the housing, the short-term fix is tightening the cable tension. It's not because the cable has stretched, it's because the housing has reached the end of its service life. Another clue that this is happening is when you see rust on the part of the cable that's inside housing. The cable itself is stainless. It won't rust. Once the lining of the housing is worn out, it exposes the plain steel inside the housing. That will rust.
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Old 08-28-12, 07:48 AM   #4
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What tsl said.
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Old 08-28-12, 08:06 AM   #5
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Continuing what tsl said, you (or whoever built the bike) will want to grab a whole bunch of brake and exposed dérailleur cable and give it a good squeeze or tug to make sure everything is seated as much as possible before it goes out the door the first time.

Though I have to say, in the days when steel was real, using Campy cables and housing, bits, seemed to result in the least amount of slop after the above exercise. Must have been tighter tolerances between the interfaces.
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Old 08-28-12, 08:35 AM   #6
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Yep, tsl has hit it well.

And there is no such thing as chain stretch, either.
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Old 08-28-12, 10:34 AM   #7
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Yep, tsl has hit it well.

And there is no such thing as chain stretch, either.
Depends on how you define "stretch." No, the links don't become longer thru some sort of elongation of the metal, but the bushings do wear, making the distance between rivets appear to be further apart (and thereby resulting in wear to the chainring and cog teeth), so it looks like the chain has "stretched." I guess it's one of those things where you use the term "stretch" in an informal way, although you know it's actually "bushing wear." I guess it's more correct to say that the chain has "worn," or that there is "elongation of the chain." But what the heck, I just say the chain has stretched!

And if you leave the derailleurs in their most "relaxed" positions, you're saving the springs more than the cable, but the amount saved is infinitesimal!

As far as cable "stretch," if you suddenly notice the cable appreciably stretching, you've got problems. Some cable strands are broken, and the cable is slowly coming apart. If it's a brake cable, it will actually break at some point (usually under hard braking, just when you don't need it to break!). The old Campag brake levers of the Cobalto era, the ones where you could route the cable either from the top of the lever, or aero along the bars, were quite prone to this. There was one part of the lever, which the cable ran past, that would bite into the cable and fray it within one season. Ask me how I know.

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Old 08-28-12, 10:52 AM   #8
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This is an educational thread.

When I have the symptoms of cable stretch, I tend to assume that I am actually seeing slippage between the cable and the tightened bolt on the derailleur. Are you guys always confident that the bolt is effectively tight and that there is no slippage?
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Old 08-28-12, 12:25 PM   #9
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There have been times when the bolt on the derailleur hasn't been tightened quite tight enough and there has been literal slippage. But on the whole, much has to do with seating of the ferrules in the cable stops on the frame and the derailleurs.

It's also good practice to use either a bench grinder or a bastard file to square off the end of brake cables after they have been cut. On shifter cables, which are built differently, try to bend the cable into the position it will be on the bike before cutting so the internal longitudinal filaments are perpendicular to the cut -- otherwise they will be slightly different lengths, and that can manifest in symptoms like cable stretch.

One thing to note, too, is that on Shimano STIs, threading the cable can be a bit tricky to ensure the end is properly seated. There appears to be a channel now on the shifter that the mushroom end on the cable slips into, and I've found that if I don't get that quite lined up properly, I get some serious issues with shifting.

Another issue that may seem to be caused by slipping or "stretching" cables is if you route the rear derailleur cable to the wrong side of the anchor bolt.
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Old 08-28-12, 12:43 PM   #10
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Originally Posted by tsl View Post
With modern cabling systems, "cable stretch" is actually housing shrinkage caused by ferrules seating after installation. There isn't sufficient tension on the cables to stretch them.

.
Exactly right, and by "pre-stretching" the cables as I was taught to do hands on by John Barnett, you shouldn't have to do much adjusting after initial use. The pre-stretching actually serves to fully seat the housings in the cable stops.
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Old 08-28-12, 04:43 PM   #11
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Tom had the answer in his initial post, the tensile strength of the cable filaments is more than you could pull with a shifter, especially the modern brifter mechanisms. Rowan and tsl added in the cable housing compaction as the segments compress under the pressure of shifts. And the chain metallurgy won't let you stretch the side plates, the pins and bushings wear from the grit that gathers on the parts of the chains. Cleaning a chain and lubricating it properly will slow the pin and bushing wear. I really appreciate Rowan's advice on prepping the cable ends properly and on having the cable on the right side of the bolt, too.

MinnMan, don't over-tighten the cable bolt on the derailleur they can strip the threads, before the bolt will snap, easily. I like the preset allen key wrenches like Park Tool sells for this job. I did strip a Campy NR cable bolt in 1979 and I had to tap the hole larger and get a good grade replacement and drill out the washers with the cable groove. I wanted to cry when I stripped that bolt.

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Old 08-28-12, 04:53 PM   #12
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There's another factor that I've discovered at least a couple of times: Cable housing end failure.

If shift cable housings aren't cut straight across the plastic end caps will hide the sloppy cut - for awhile. Eventually, however the long part of the cable housing will try to exit the hole in the end cap and gradually shorten itself. When that happens, adjusting the derailleur will work for a little while but will have to be frequently repeated. The tip off is 1 or 2 wire strands sticking out of a housing end cap. Remove the end cap, cut the housing straight across, replace the end cap and you're good-to-go.
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Old 08-28-12, 06:21 PM   #13
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Originally Posted by MinnMan View Post
Are you guys always confident that the bolt is effectively tight and that there is no slippage?
Mister Torque Wrench makes certain of that for me. 6-7 Nm and Bob's yer uncle.

Last edited by tsl; 08-28-12 at 06:24 PM.
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Old 08-28-12, 06:40 PM   #14
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I had a problem on a bike with internal cables. I couldn't understand why I couldn't get the brake to adjust. No matter what, it was too loose. Turns out the bushing broke in the internal cable stop and the plastic cable housing was acting as the housing, not the internal metal sheath. So as the tension increased, the plastic would compress. Took a bit of time to find that one.

I've also had the problem described by lhbernheardt. The cable started to break, one strand at a time. I kept having to make adjustments for the FD. Finally it snapped completely. THEN I knew what was causing the stretch...
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Old 08-28-12, 08:16 PM   #15
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To sum up: The truth can be stretched but not cables.
Be sure to mind all the other bits.
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