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I'm trying to fix my wife's bicycle which sat out all winter. I can't get the rear derailleur to downshift from the twist grip shifter. I have inspected and cleaned and lubricated the cables, nothing is frayed or stuck. I can move the derailleur with my thumb quite easily, and if I hold it I can move the shifter. I can also upshift, but I can't get the derailleur to downshift from the shifter. Any ideas? It's a Shimano system.

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    Buy a new shifter (SRAM Attack! or Shimano Revoshift or a trigger shifter from Shimano for the same number of speeds or a friction shifter). Also, modern cables are generally not intended to be lubricated. – Batman Jun 11 '15 at 3:50
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    @Batman Pls stop advising against cable lubrication because you are not correct! I see you doing this on many answers.. When the system is not sealed it's always good to lube it. It prevents corrosion and grants easier opperation (with light teflon oils). It's true modern cables are plastic but that doesn't mean not to lube it. Old bikes used heavy oil or grease for this which actually slowed the cable, this is not true anymore. Don't believe everything you read on sheldonbrown.com/ – Jerryno Jun 11 '15 at 14:54
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    @Jerryno - Most modern cable housing has a polymer sheath and most cables are now stainless steel. Lubing will attract dirt and actually degrade performance. I find that once the cable housing has worn out a bit of lube can temporarily bring back to life, but it is a stop-gap measure. – Rider_X Jun 11 '15 at 19:37
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    @Rider_X Wet lubing will attract dirt but teflon based not so much. The polymer-stainless cable-housing will work nicely without lubing for some time but then like you said lubing will improve stuff and not do any damage or negative thing. – Jerryno Jun 11 '15 at 20:00
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    Generally one of three problems: 1) The cable has seized up. 2) The cable has stretched and needs to be tightened. 3) The crappy twist shifter has bit the dust. – Daniel R Hicks Jun 11 '15 at 22:57
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I solved a similar problem by dousing the inside of the shifter liberally with WD40 and leaving it overnight, I mean really soak it until it's dripping. I had to do this twice but the shifting was perfect afterwards.

  • Since the WD40 will wash out any existing lubrication, you might want to add some other lube for when the WD40 evaporates. – stib Aug 22 '17 at 2:46
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When downshifting you are increasing the tension of the cable and moving the chain to the gears with more teeth. You've said there is no friction along the cable path and that the derailleur can move freely, so if you can't increase tension, I would say it's the shifter.

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    By downshift I mean I can't seem to overcome the spring pressure in the derailer with the shifter. There's too much friction somewhere, just not sure if it's in the shfter, the cable or the derailer. If I release the tension I can easily move the shifter, the cable and I can move the derailer, but when I put it all together, I can't move it at all. – Oliver Harris Jun 12 '15 at 0:53
  • @OliverHarris Then it's definitely the shifter, – Cory Roy Jun 12 '15 at 16:25
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The derailleur has a spring pulling it against the cable because the cable can only pull, not push. Is that spring intact? If not, it's probably best to replace the derailleur.

  • The spring seems to be in tact. I can move the derailer with my thumb easily enough, but I can't move it with the shifter. – Oliver Harris Jun 12 '15 at 0:54
  • Meaning the cable is stuck in the housing. Unhook the cable from the derailleur and see if it moves at all (try to push it back into the housing / pull back out) If it doesn't move very(!) easily replace housing and cable. – Carel Jun 12 '15 at 13:53
  • @OliverHarris In most cases with grip shifters, when it's not pulling the derailleur, it means the shifter is broken. – Alexander Aug 13 '15 at 12:06
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Sounds like your shifter is jammed. You might be able to repair it, but I'd just get a new one. To confirm this diagnosis, remove the cable. If you still can't move the shifter, then you know it's bad.

And next time, don't leave your bike out all winter.

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The rubber-hose-like cable housings used to be made of lower quality materials such as plastic. This made the amount of friction within the lines to be high enough that lubrication was needed, usually a heavy grease or oil.

On modern lines with the flexible, soft rubber-like material, lubrication has an adverse effect on your cables, making them feel kind of gunky, slow, etc.

As for the gearing problem, I would first try to maneuver the barrel adjuster to see if that solves the problem.

If not:

  • loosen the gear cable just a tad, (maybe a quarter inch)
  • re-index your gears

From what you say your problem is, I would think that one of these methods should work. If not, however, I would look into replacing the cable altogether. If nothing else, you could also just take it to your LBS and have them take a peek.

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Since you mentioned the cables and derailleur are both freely moving, it's most likely the shifter. Now I had a jammed shifter before but it wasn't due to corrosion. Cables need to be very freely moving, otherwise what can happen is the head of the cable inside the shifter can become dislodged and jam, preventing the shifter from moving. I had this issue personally on a 24 mile ride, causing me to be stuck in one gear for a majority of the ride.

Considering the bike was left outside, corrosion can be the culprit, especially for cables. Sometimes a cable can look and feel fine but in fact it is too restrictive for proper shifting. The easiest fix would be to replace the shifter and cable. It's not a difficult nor expensive repair, just make sure you get a replacement shifter with the correct ratio (1:1 for SRAM derailleurs, 2:1 for Shimano derailleurs). Most shifters require you to snip the cable anyways before taking it off. If you must leave the bike outside, consider going with a full length housing.

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