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Old 06-05-10, 08:27 AM   #1
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How much does temperature affect cable length?

Got on my bike the other day and it wouldn't shift down to the granny. This hasn't been a problem since last adjustment, some time last fall. Same with GF's bike. AFAICT the only change is the ambient temperature - some 20-30°F warmer.

Metal expands as it heats, so it makes sense that the cable would be a little longer. But this is an issue I've never seen addressed. Anyone thought about it?
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Old 06-05-10, 08:45 AM   #2
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I briefly mentioned the same thing a few months back (or exactly a year back; it was seasonal ) and somebody said that it didn't make any difference.

I know that stringed musical instruments change pitch depending on temperature, and I've tuned my shifter cable tension while plucking the cables along the downtube (not to achieve a certain pitch, but just to confirm that they were taking adjustments). Changing cable tension only enough for a few scalewise pitches also makes a difference in whether or not my RD shifted cleanly.
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Old 06-05-10, 08:46 AM   #3
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.0096" in three feet. No effect.
http://www.engineeringtoolbox.com/li...ents-d_95.html
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Old 06-05-10, 09:15 AM   #4
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Why does it seem seasonal, then? I have to adjust cable tension when it get to 40deg or below, and again once it gets hot. Just coincidence, I guess?

To be fair, I've never kept track of whether I have to tighten or loosen them each time...
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Old 06-05-10, 09:43 AM   #5
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It could be more than just the cable length, think (slight) spring tension, viscosity of lubricants and increased friction do to expanding parts. All of these can be affected by heat, and no one factor alone would make a difference but maybe together they would.

I am in no way certain of this, just a theory
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Old 06-05-10, 10:19 AM   #6
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Ad, dirt - don't forget dirt. A little bit will have a negative effect on how things move, especially if it gets in between two oiled parts....
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Old 06-05-10, 11:27 AM   #7
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If the cable is expanding so is the bike frame, sprocket and chainwheel. What does this mean? I don't know.
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Old 06-05-10, 11:27 AM   #8
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Quote:
Originally Posted by davidad View Post
Given the average length of a RD cable, that would be a variance of about .015" -- a fraction of a millimeter. While I have seen 2mm (stem height, in this example) have an effect on a bike's handling, I don't: a.)believe there will be ANY noticeable difference in shifting performance because of this; and b.) don't know anybody who's so GOOD that they can fine-tune a RD that closely on a continuing basis.

Maintenance/adjustment is something that you just have to do on bikes every now and again -- nature of the beast. It's the price we have to pay for having the most efficient form of transport known to man.
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Old 06-05-10, 11:40 AM   #9
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Given that the cable runs in a housing of similar material, which would expand or contract similarly, the effective change is vastly less than the one thousandth of an inch the wire alone would expand from heat. Nor would any of it's other mechanical properties change enough to make any difference.

If you're seeing any seasonal change in RD trim the most likely suspect is your cable lube which might thicken or thin affecting the action of the wire. It isn't rare for bikes that ran perfectly all summer to start up-shifting sluggishly in the late fall as colder temperatures thicken the cable lube enough that the light RD spring can't pull it all the way back.
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Old 06-05-10, 12:52 PM   #10
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If the cable is expanding so is the bike frame, sprocket and chainwheel. What does this mean? I don't know.
The frame might try to pop apart if it expanded enough to change cable tension. Well, maybe.

Actually, there's some misinformation among some brasswind players (trumpets, tubas, etc) about hot weather and pitch changes. Some of them think that as the instrument gets hotter, it expands, lengthening the overall tubing and lowering the pitch. They don't realize that hot weather raises the pitch (verifiable with an electronic tuner) and that the instrument would break itself apart if it tried to expand that much.

But, that likely has nothing to do with cable tension, which would be more like a stringed instrument -- which I mentioned earlier.

What I'd like to find out next is how far a barrel adjuster moves the cable housing per turn.
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Old 06-05-10, 01:10 PM   #11
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Quote:
Originally Posted by BarracksSi View Post
.....

What I'd like to find out next is how far a barrel adjuster moves the cable housing per turn.
You're not thinking. Look at the adjusting barrel screw, backing it out will add length to the housing, effectively shortening the inner wire by an amount equal to the pitch (not musical here) of the screw. If the adjuster is 5x.8mm then it moves .8mm per full turn, but you knew that. (you just forgot that you knew it)

BTW- derailleur wire tension doesn't (or shouldn't) change as you adjust trim. Your change the length of the housing which has the same effect as changing the inner wire's length in the opposite direction, ie lengthen the housing to shorten the wire.

Folks call it a tension adjustment, but in reality it's a length adjustment which effects the position of the RD. The tension is determined by the strength of the RD return spring and remains (almost) constant during the process unless the cable is binding.
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Old 06-05-10, 01:24 PM   #12
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If you've adjusted it particularly fine and close to one of the limits then that amount COULD make just the needed difference to foul things up. But if the adjustments are made to be in the middle of the sweet spot this amount shouldn't be a big deal.

Keep in mind that the housing and frames are moving by some amount with the temperature as well. In some cases these things may add up and in others they would cancel each other. But either way it's not normally an issue for most folks. But again if some bikes are set up just on the edge of making the shift then the slight change in lengths could be the difference between shifting and not.

Personally I'd say that wear in all the items would be afar more likely culprit. But the seasonal average temperature could be the last trigger to produce the effect that the wear had already started.
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Old 06-05-10, 01:26 PM   #13
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Quote:
Originally Posted by FBinNY View Post
You're not thinking. Look at the screw, it will add length to the housing, effectively shortening the inner wire by an amount equal to the pitch (not musical here) of the screw. If the adjuster is 5x.8mm then it moves .8mm per full turn, but you knew that. (you just forgot that you knew it)
Nope, I just didn't know the thread pitch of the adjuster.

Unless I'm screwing up my math (which is possible), the variance listed a few posts back is between a half- and quarter-turn of a barrel adjuster (.015" = .38 mm). Not enough to totally throw shifting out of whack, but enough to make it act funny now and then.

I'm not discounting all the other parts on the bike, though.
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Old 06-05-10, 01:40 PM   #14
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To get back to the original post, though --

Quote:
Originally Posted by DMF View Post
Got on my bike the other day and it wouldn't shift down to the granny. This hasn't been a problem since last adjustment, some time last fall. Same with GF's bike. AFAICT the only change is the ambient temperature - some 20-30°F warmer.

Metal expands as it heats, so it makes sense that the cable would be a little longer. But this is an issue I've never seen addressed. Anyone thought about it?
Most FDs that I know of require the cable to be pulled to shift to the bigger ring, and lengthening the cable would actually make it harder to make that shift. It should drop right down to the granny if there is zero cable tension and the limit screw is adjusted correctly.

That's how my bikes work, though. Aren't there reverse-pull front derailleurs out there that will shift to the big ring with zero cable tension (and, if there isn't enough, they'd have trouble getting to the granny ring)?

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Old 06-06-10, 12:26 PM   #15
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Okay, discount the cable (BarracksSi is right). But I needed >1/3 turn to get it shifting again, so something was different.

Possibilities:
  • Limit screw and stop <- these are leveraged so any change is magnified.
  • Decreased chi from the rider <- I can't just will it to happen any more.
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Old 06-06-10, 12:37 PM   #16
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Given that the cable runs in a housing of similar material, which would expand or contract similarly, the effective change is vastly less than the one thousandth of an inch the wire alone would expand from heat.
To elucidate a bit, the cable is fixed at both ends, so increasing length tends to produce more slack.

The length of the housing affects the path the cable must traverse. A longer housing makes the path longer, tending to produce less slack.

Do the two offset equally? Doubtful. Different materials and different construction. But I agree that they tend to cancel.

Quote:
Originally Posted by various
.. lube ... dirt ...
Nope. We're not talking about sluggish operation here, but a hard mechanical limit that allows the FD to move to X position at one time, yet does not at another time. At low limit the cable is slightly slack - it's not hung up.

Quote:
Originally Posted by BCRider View Post
Personally I'd say that wear in all the items would be afar more likely culprit.
Can't wear if it's not used.

I used the hell out of it yesterday. It's shifting as well as it has in a long time, so maybe it was right on the edge. I was; I was barely able to climb the stairs 2 hours later. That effect might be ascribed to wear...
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Old 06-06-10, 05:58 PM   #17
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The coefficient of expansion of steel is .0000065 per degree F change. So the change in length of a 36" cable for a 70 degree temperature change would be 0.0000065 x 36" x 70 degrees = 0.016". This is about 1/64". At the same time though the cable housing is undergoing about the same change in length. At least the cable housings that I use have a steel core.
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