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Old 01-01-13, 06:45 AM   #1
goldfinch 
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rear derailleur cable tension

Here I am again, stopping by to mine you folks for information. I bow to your superior knowledge.

Anyway, I have a Cannondale Quick hybrid which I ride a lot. Yesterday on a ride the cable to the rear derailleur snapped. It had rusted through. I bought a replacement cable and strung it through the old housing. I am on a trip and working on bikes on a picnic table is a bit awkward.

For the life of me I can not seem to get the cable tension correct. I have strung derailleur cable twice before and set up the tension using Park Tool's instructions. I tried that here but cannot get it right.

According the Sheldon Brown:

Shifting to larger sprockets is accomplished by tightening the cable;
if such shifts are slow, the cable is not tight enough--
turn the barrel counterclockwise to tighten it.

Shifting to smaller sprockets is accomplished by loosening the cable;
if such shifts are too slow, the cable is not loose enough--
turn the barrel clockwise to loosen it.


Of course, first of all you have to be in the ballpark of correct tension just by how tight you pull the derailleur cable when you string it through and tighten the nut.

Here is what I did and the symptoms:

At first, I could not shift onto the smallest sprocket. So, I turned the barrel adjust clockwise. Not enough room to turn enough so I started over and let out a little cable at the nut. OK, now it shifted to the smallest sprocket and up to the second sprocket. After a lot of fussing.

But, I would not shift well up from there. And, it wouldn't go onto the two largest cogs. So, I turned the adjuster counterclockwise. It could finally get it up to the second to the largest cog but not the largest. And now it won't do on to the smallest two cogs.

I rode my bike five miles so I could meet my year end goal. The shifting will slip from larger to smaller cogs.

So how come I can't get this right? Suggestions? Before the cable broke the shifting was fine.
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Old 01-01-13, 07:16 AM   #2
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If the shifting was working before the cable snapped, you shouldn't have needed to play with the barrel adjustor's, it should have just been fit 'n' ride.

You say the inner was rusted, would be a reasonable assumption that the outer is sufficiently corroded as well to be wrecked, and this could be your problem.

Also you don't say what position the shifters were in for the dérailleur when you put the cable back on, on the basis that your bike has a Shimano Alivio RD (the Quick 4 has this as stock), follow their instructions and with new outers, you should be back to normal shifting http://techdocs.shimano.com/media/te...9830705210.pdf
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Old 01-01-13, 07:45 AM   #3
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You say the inner was rusted, would be a reasonable assumption that the outer is sufficiently corroded as well to be wrecked, and this could be your problem.
That's what I think too. That short section of cable housing immediately adjacent to the rear derailleur is particularly subject to causing trouble.
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Old 01-01-13, 07:46 AM   #4
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The cable would only go in if it was in a certain gear so I believe I did that correctly.

Ugh. So you think I might need new housing?
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Old 01-01-13, 08:44 AM   #5
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Ugh. So you think I might need new housing?
Almost certainly. The symptoms that you described are exactly what worn cable housing does.
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Old 01-01-13, 10:11 AM   #6
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I agree with the others that you probably need a new rear housing loop. This wears very quickly owing to the sharp curve, and the rust is a pretty good indicator that it's shot.

You may want to also to replace the HB to frame housing section while you're at it and start fresh. However this section usually lasts 3 times as long as the rear section.

If you're on the road and don't have spare housing, go to a gas station, pull the cable apart and ask them to spot you some grease. even a badly worn housing should be OK for a while with fresh grease.

Before stringing the cable doublecheck the RD limits. They should be OK, but this is a good time to confirm. Shift through the range by pushing the lower body in and letting go. It should shift the entire range without over shifting at either end. Return the derailleur to high and string the cable, shift the lever to high, set the adjuster all the way down and pull out all the slack.

Now shift the lever one click to second, and while pedaling, back up the adjuster until the RD makes the shift. This is fairly close, and you'll fine tune until it shifts equally well in both directions.
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Old 01-01-13, 10:52 AM   #7
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I found a hole in the wall bike shop that was actually open on New Years day (the owner was doing the books). He let me stop by and pick up housing. Recabled and all is well. The owner was nice enough to cut the cable for me to match what was on my bike as I am without a cutter here.

I didn't check the derailleur limits, kind of a pita when working the bike upside down. But I haven't had any symptoms of an issue.

Thanks everyone.
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Old 01-01-13, 05:44 PM   #8
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Bike needs to be upright when working with drive train components. Find something to hang it on.
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Old 01-01-13, 05:55 PM   #9
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Bike needs to be upright when working with drive train components. Find something to hang it on.
Agreed that it might affect trim slightly, but a limit adjustment of the RD isn't affected by gravity.
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Old 01-01-13, 07:56 PM   #10
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If all you are doing is changing the cable, absolutely do not mess with the limit screws. And you may well need to use the barrel adjusters, unless you are very lucky and hit it just right the first time (never happened to me!).
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Old 01-02-13, 01:00 AM   #11
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Chain should be on the smallest sprocket and shifter should be in the corresponding position when you snug up the cable and tighten the holding bolt. Agree there should be no reason to mess around with the limit screws.
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Old 01-02-13, 05:54 AM   #12
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Bike needs to be upright when working with drive train components. Find something to hang it on.
Hard to do in an RV resort. Next year my stand is coming with me for the winter.
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Old 01-02-13, 05:59 AM   #13
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Chain should be on the smallest sprocket and shifter should be in the corresponding position when you snug up the cable and tighten the holding bolt. Agree there should be no reason to mess around with the limit screws.
I knew this and I got everything working fine, but I have a related question. Does it matter what ring the chain is on in the front? I would think not, but thought I would ask.
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Old 01-02-13, 07:39 AM   #14
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the best way is to not cross chain when making adjustments.
it probably wouldn't make a difference but why ask for more troubles.
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Old 01-02-13, 09:57 AM   #15
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Dis-connect and so isolate cable in the housing drag , from your derailleurs, and the lever, frictions..
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