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I am trying to solve the neverending issue of my derailleur shifting on its own by replacing the shifter's cable and its sheath to see if it not sliding properly could be the cause of the issue.

Doing so I realized that both derailleurs' cables run beneath the bottom bracket in two rails, without any sheath wrapping them. In this picture, I removed the front cable (that was running in the left rail) while the rear derailleur cable is in place on the right rail.

derailleurs cable rails beneath the bottom bracket

(yes I know the frame is very dirt, I just noticed. Never checked this thoughtfully down there).

I am wondering if this could be the issue of the friction, before installing the new stuff back. I noticed that that piece where the cables run into, can be replaced (though I don't know what to search for. By the way has that component a name?). I am wondering this because the bicycle is quite old (it's a 2001 bike) and perhaps that has been superseded, and, most importantly, because I feel like I cannot lubricate those rails where a lot of friction must happen, because, being those so exposed, the grease down there would catch any kind of dirt and mud and probably make things worse.

Or maybe it's just fine as it is, with no lubrication at all, and I am totally seeking for the issue in the wrong spot. Any clarifications about that piece of gear would be immensely appreciated.

Thank you very much.

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    The scheme you describe is quite common, and in fact is preferred over fully sheathed cable because there is less friction. I've never seen the friction blocks fail, other than when they've been physically damaged (which is rare). – Daniel R Hicks May 16 '15 at 20:17
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    (If the rear derailer is shifting on its own check the frame carefully for cracks.) – Daniel R Hicks May 16 '15 at 20:19
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    I doubt that guide is the problem but that route looks odd to me. I should be freely running down the center. Clean it up and hit with with some WD40. Yes WD40 is not going to last long and will collect dirt - this is just to clean it up and test. – paparazzo May 18 '15 at 15:55
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    Those 2 plastic "rails" are in good shape and they do not play any role in the ghost shifting you describe. You need to look elsewhere. – cherouvim May 21 '15 at 5:16
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A couple of things could be the reason of your derailleur shifting on its own.

I assume that both the derailleurs and the shifters are in good working order, that you washed and lubricated them and correct adjust them with the respective screws. Please refer to Park Tools repair help guide to correctly set all these adjustments. You can find it here and please bookmark it, because their site is nothing short of amazing. And now that we are talking about this, also check for the rear derailleur hanger alignment, as you can see here.

Now that we know this is not a problem, the main reason of poor shifting is your cables being dirty and/or simply old. Replace everything, inner and outer cables. Buy the cheapest ones is usually good enough, a good tip is to put teflon-based oil (cheapest you can find, it's a good buy because you will use it for the derailleurs, chain, etc) inside the outer tube before putting the inner tube in. This will ensure good operation for a long time.

About you bike's cable routing, you can find a new plastic piece for your frame here (make sure this is the correct model). I wouldn't bother if yours look good, just clean it.

The type of routing on your bike is definitely not the best (as you can see lots of debris accumulate there), but that's ok. There is a neat thing you can do: pass the cable for the front derailleur through the standard places (down the inner tube), but for the rear derailleur use a big outer tube that goes all the way from the shifter to the derailleur (check what works best with your bike frame, if down the top tube/seatstays or through the down tube/chainstays, and choose whichever route because since we are using a outer tube debris are not a problem anymore).

This will ensure that cable stays clean and protected and your shifting should be good for a longer time. You can buy cable guides like these (there are more types, just google for it) but I would just use zip ties and do something like this.

Be patience and good luck! :)

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    If you are going for long outer cable, consider not the cheap one. There are 2 types of outer: 1) the metal going spiral way around the teflon (good for brakes and friction gears) 2) the metal is strait cables around the teflon (good for indexed gears). As OP have indexed gears, he need the second type, which is more expensive. I recommend at least shimano outers for it. And ofcourse don't forget about outer ends. – Alexander May 19 '15 at 20:51
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    Long outer cables are for me the best option, mainly for old system like this. A lot of bikes are shipped with long outer cables. The information about the outer cable type is a good hint though. Outer cables I don't think are as important, as the shifter protects it and nothing goes up in the rear derailleur. However the price difference should be minimal so it for sure does not hurt to have outer ends with an o-ring or a nylon ring. – super May 19 '15 at 21:17
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Ghost shifting is generally caused by the frame flexing, not by friction in the cables. Other causes can be a poorly fitting or too short cable housing. Something is causing the length of the cable between the stops to change.

Cleaning the cable run and using a teflon coated shift cable won't hurt and might alleviate the problem if it's a combo of bottom bracket flex and sticky guide. When this is the cause the problem tends to be very intermittent and only occurs when standing on the pedals to climb. However, as Daniel R Hicks suggests, check the frame carefully for cracks. An aluminum alloy frame of that age that has seen a lot of miles is at the end of it's usable life.

If you can't find any cracks, examine the cable housing runs carefully. The cable housing ends should not move at all in the stops when shifting or turning the handlebars. Check the derailleur hanger to make sure it's not loose.

The part you are looking at is called a Bottom Bracket Cable Guide, but your's looks just fine. They rarely wear out, shift cables aren't under much tension, so a plastic slider is plenty slippery enough. On an old enough bike, they are made of metal.

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Before you buy anything,make sure the shifter is adjusted corectly. Put the bike on a stand or whatever it takes to allow the pedals and the rear wheel to move freely. The adjustment should be made from the second highest gear (the second smallest Which is the second from the outside). Tighten the adjuster until the chain makes noise trying to shift into the third gear,then back it off until the noise stops. If that doesn't solve the problem,see if the derailleur or the hanger is bent. There could be many reasons for ghost shifting but if the shifters seem to work smoothly then personally I would not suspect the cables or worry about friction on the cable guide. Hope that helps.

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