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I learned something new today.

Old 04-09-07, 10:22 PM
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I learned something new today.

Everybody seems to have mixed thoughts about so called "cable stretch". Well, I heard something new today from the bike shop owner/ manager/ mechanic. He said that it is not the cable that stretches, it is actually the housing that stretches. I just thought I would share this since I have never heard this mentioned on this forum.

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Old 04-09-07, 10:30 PM
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I remember there was a discussion about cable stretch not too long ago. What is it with the cables that claim they are "pre-stretched" then?
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Old 04-09-07, 10:33 PM
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I dunno man. When I assemble bikes at the shop, after getting all the shifting setup, I pre stress the shifting cables by pulling on them really hard, and there is noticeable sag in the cable after that, which I then remove by fine tuning the shifting..Now how could there be sag in the cable if the housing stretches? Cable housing for shifting cable is pretty rigid stuff anyway. It doesn't seem to work in my mind, and I'll stick with the theory that the cable stretches.
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Old 04-09-07, 10:38 PM
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Originally Posted by santiago
I remember there was a discussion about cable stretch not too long ago. What is it with the cables that claim they are "pre-stretched" then?
Ummm so they can't stretch any further...duh.

I really don't know. Maybe it's because any further stretching word require more forces then the shifter/RD is capable of. Over time the cable must stretch, but when people come into the shop for their first month tune up, I think it's the housing that has stretch not the cable.
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Old 04-09-07, 10:39 PM
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if the housing flexed and expanded, the brakes/gears would lose adjustment the other way.
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Old 04-09-07, 10:40 PM
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They both stretch... on my mountain bike after I built it up and had about 200mi on it, I pulled the housing and discovered it had stretched about 1/4"... its not too uncommon at all. I just trim it and put it back together.
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Old 04-09-07, 11:10 PM
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I dunno, I just know that it happens and the world is going to end someday.
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Old 04-10-07, 04:41 AM
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It's more that the cable housings "seat" into the frame, cable housing ends, shifters, etc than it is that they "stretch".
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Old 04-10-07, 04:47 AM
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Besides, housings can't stretch in normal opperation as they are under a compressive force, if anything they would shrink.

But I'm goin' +1 on what LowCel said... they seat in to the frame and lever ferrules.
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Old 04-10-07, 02:56 PM
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i believe fagerlin just posted a link (regarding this topic) to another forum a couple weeks ago.
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Old 04-10-07, 05:35 PM
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We need the Myth Busters.
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Old 04-10-07, 05:54 PM
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Housings don't stretch. As mentioned above, they are under compressive forces. That is why compressionless housing is the housing of choice expecially for mech. disc brakes.

Cable can stretch. That is how post-tensioned (as in tensioned afterward, as opposed to pre-stressed) concrete structures are made stronger. Concrete is poured with cable reinforcement. After it sets up, the cable is stretched taut, thereby putting internal compressive tension on the concrete.
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Old 04-10-07, 11:38 PM
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Shift housing does definately stretch... pull any shift housing off a bike after it has about 200mi on it and I promise the inner wire liner extends 1/16th-1/4" past the end of the housing where it was originally cut. I actually just pulled my shift housing tonight on my Haro and found the housing had not stretched since I first trimmed it after about 200mi of riding, which leads me to believe that yes, it is initial stretch as if it were anything else it would probably happen again.
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Old 04-10-07, 11:42 PM
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Originally Posted by seely
Shift housing does definately stretch... pull any shift housing off a bike after it has about 200mi on it and I promise the inner wire liner extends 1/16th-1/4" past the end of the housing where it was originally cut. I actually just pulled my shift housing tonight on my Haro and found the housing had not stretched since I first trimmed it after about 200mi of riding, which leads me to believe that yes, it is initial stretch as if it were anything else it would probably happen again.
not the inners stretching. it's the outer coating shrinking up a bit... getting pulled around as the cables and bike move. the inner wires are "compressionless" but the outer plastic coating is much less compression resistant.
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Old 04-11-07, 07:13 AM
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Originally Posted by seely
Shift housing does definately stretch... pull any shift housing off a bike after it has about 200mi on it and I promise the inner wire liner extends 1/16th-1/4" past the end of the housing where it was originally cut.
Maybe I'm not reading this right, but if the inner wire extends past the housing, wouldn't that indicate the inner wire (aka cable) had stretched and NOT the shift housing??

Or, as others have mentioned that the housing has indeed shrunk a little bit.
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Old 04-11-07, 07:15 AM
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Originally Posted by skiahh
Maybe I'm not reading this right, but if the inner wire extends past the housing, wouldn't that indicate the inner wire (aka cable) had stretched and NOT the shift housing??

Or, as others have mentioned that the housing has indeed shrunk a little bit.
not the inner wire as in the cable, but the inner wire that is the structural part of the housing. the stuff poppingout of the end caps/ferrules is particularly noticeable when the crappy plastic ones are used; i don't have the problem at all when using metal end caps.
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Old 04-11-07, 07:43 AM
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Originally Posted by dminor
After it sets up, the cable is stretched taut, thereby putting internal compressive tension on the concrete.
That's incorrect. The method you describe would put the concrete under tensile stress and make it weaker. The cable is actually tensioned prior to the concrete curing, then released to put the concrete in a compressive state (its strongest).

Further, in practice, the tensile stress placed upon a shifter cable (by the derailleur spring or rider's thumb) is not enough to cause it to stretch enough to have any effect on shifting. "Cable stretch," as scrublover and others have noted, is actually a result of the housing compressing and/or the cable end seating more securely into the ferrule.
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Old 04-11-07, 08:04 AM
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Big fancy words hurt my head. I'm going to get a coffee.
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Old 04-11-07, 09:29 AM
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thank god it all works out though, or else we'd be screwed!!
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Old 04-11-07, 09:54 AM
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Originally Posted by gastro
That's incorrect. The method you describe would put the concrete under tensile stress and make it weaker. The cable is actually tensioned prior to the concrete curing, then released to put the concrete in a compressive state (its strongest).
I was almost going to concede that, once again, you caught me telling more than I know. But I double-checked this. I think the process you've described is actually pre-tensioned. Post-tensioning actually does tension the cables after the pour -- specifically bonded post-tensioned. I refer to this Wikipedia entry (admittedly not the most scientific source to quote, but in this case it lays it out pretty well):

https://en.wikipedia.org/wiki/Prestressed_concrete
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Old 04-11-07, 10:04 AM
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...am i in a construction forum?

**off to home depot**
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Old 04-11-07, 10:06 AM
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Originally Posted by seely
Shift housing does definately stretch...
why stretch?
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Old 04-11-07, 10:48 AM
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Originally Posted by mx_599
...am i in a construction forum?

**off to home depot**
. . . and you thought "post-tension" was getting nervous when riding close to fences, huh?
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Old 04-11-07, 11:04 AM
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Originally Posted by dminor
. . . and you thought "post-tension" was getting nervous when riding close to fences, huh?


i was just surfin' some mx stuff for a few minutes...i miss it.

when does Masters usually start? 40 right? 30 for Vet? it might vary with district...isn't this AMA classification though? are you still an AMA member? i wanted to get my lifetime membership before i run out of years to make it economical...

i think i will make the goal of going to Loretta Lynn for the Masters class in 10 yrs...


**opps, i'm talking FOO now!**
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Old 04-11-07, 11:18 AM
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Originally Posted by mx_599
when does Masters usually start? 40 right? 30 for Vet? it might vary with district...isn't this AMA classification though? are you still an AMA member? i wanted to get my lifetime membership before i run out of years to make it economical...

i think i will make the goal of going to Loretta Lynn for the Masters class in 10 yrs...


**opps, i'm talking FOO now!**
From an official NORBA and UCI standpoint, Master is 30+. With UCI, the current World Cup is actually the Elite class. There are starting to be quite a few retired Elites (former pros like Shaums March) in the 30+ ranks now.

I honestly don't know about AMA. I haven't held a competition license for years now (although for some silly reason I still remember my old membership number). If I was going to dabble in it again, I'd go the AHRMA route - - especially since one of my bikes qualifies under AHRMA rules.
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