Bike won't downshift when cold

[ketchup]

Anybody have any experience with this problem?

For the first time this winter, earlier this week (outside temp was about 0F) I went out on my bike.  After going less than half a mile, I was unable to shift down, only up on my rear derailleur (Trek FX 7.2, which appears to have a Shimano Alivio rear derailleur from my quick-n-dirty Google-fu).  So I'm stuck in an annoying gear, but get myself back home after playing with it a little.

I bring it back inside, and look at it again a few hours later, and it's fine, shifts as it should.  I write it off as a fluke.

This morning (~14F), I take it outside and the same thing happens.  This time though, I need to get to the bus, so I deal with it for a mile and a half.  And I'm aware of the problem, so I'm able to stay in a relatively sensible gear.

So tonight riding home I'll be dealing with it, but I'll probably check on it first to see if anything is obvious.  I'm hoping this is something simple like something needing to be oiled.  I'm not too mechanically familiar with my bike yet.  It's only a few years old, and when I got it last April it seemed to be rarely used.  So I don't think it's anything failing due to age or wear. 

Thanks.

[the fixer]

I'm not sure where the problem is, but I can tell you that downshifting on the rear increases the cable tension to pull the derailleur in toward the wheel. Shifting up decreases tension. So something is allowing the tension to drop on the cable, but not increase.

The next time it happens, with the bike not in the lowest gear, try pulling on the shift cable somewhere where it runs along the frame with no housing. See if you can get the derailleur to move as you pull on it (for practice, attempt this when you can get it to shift normally so you know what it's supposed to do).

If the derailleur moves when you pull on the cable, you have a problem further away from the derailleur than where you pulled, such as an issue with the shifter (my money's on this) or a piece of cable housing.
If the derailleur does not move, the problem is closer to the derailleur. To rule out a derailleur issue, try pushing the derailleur toward the wheel and see if you can move it directly (there's a built-in spring so it will resist you, but it should move). If you can't, the problem is something freezing up in the derailleur (but I doubt this is it; it moves the other way!). If you can, it's got to be a piece of cable housing in between.

[fodder69]

I think you guys are using different terms for up and down shifting. We should all use the same terms but on a bike people tend to call it downshifting when you go to a harder gear (down the cassette).

Moisture in the cables (or possibly the shifter but much less likely) is freezing and creating enough friction that the spring in the derailleur is not able to pull the cable. You can still shift up (to an easier gear, ie bigger cog on the back) because it is your muscle force that pulls the cable which is enough to overcome the extra friction.

Probable fix is to lube the cables. Do this:

 - Shift your bike so the chain is on the biggest cog on the back
 - Then without pedaling, shift your bike all the way the other direction
 - This should create enough slack in the cables that you can pull them out of the stops on the frame (where the cable housing stops)
 - There is a slot in those stops so the cable comes off.
 - move the housings out and lube the cable.

Easier seen than said, here is a short video that explains it better. He actually just pushes the derailleur in to create the slack in the cable.

http://www.youtube.com/watch?v=ApDg10eJXbg

[NinetyFour]

Nice video.  How do you know it's a he?

[Russ]

you can see his package pretty clearly in this one: http://www.youtube.com/watch?v=g6_l7zIKoZA

although I guess we still don't know his preferred gender pronoun

either way he/she's a dummy for not having a spacer above his/her stem

[Undecided]

you can see his package pretty clearly in this one: http://www.youtube.com/watch?v=g6_l7zIKoZA

although I guess we still don't know his preferred gender pronoun

either way he/she's a dummy for not having a spacer above his/her stem

I don't know the Giant lineup, but does a frame with welds like that come with a fork with a carbon steerer tub?

[Russ]

nah that's metal. pretty sure the bike itself is older than carbon steerer tubes; they haven't made anything in that color for a long time

still doesn't make the spacer situation a great idea

[fodder69]

I am not sure rule 45 http://www.velominati.com/the-rules/#45 applies to mountain bikes.

[Russ]

Using no spacers below the stem is dumb too if it screws up your fit, no matter the bike. but the spacer on top is to allow the stem to use its full clamping area, which is always better than not doing that thing.

[Undecided]

Using no spacers below the stem is dumb too if it screws up your fit, no matter the bike. but the spacer on top is to allow the stem to use its full clamping area, which is always better than not doing that thing.

Perhaps, but on this site, of all places, one has to remember that "better" doesn't matter beyond "good enough"! And for a metal steerer tube, just having the top bolt at the same level as tube is good enough for any duty that bike is likely to see. No need to scare people who aren't bike geeks. 

When the OP does lube the cable, s/he should use something appropriate for that kind of cold, which may be something inappropriate for "good" maintenance.

Edit to add: Depending on shifters on the bike, the issue could be there, too, with the same potential idea to fix---make it dry and use a lube that won't freeze.

[Russ]

My bad, not trying to scare anyone.

Back on topic, after you clean up the cables & housing and lube everything with oil, a little dab of grease on each end of the housing will go a long way to keep water from getting in again.

[Undecided]

My bad, not trying to scare anyone.

Back on topic, after you clean up the cables & housing and lube everything with oil, a little dab of grease on each end of the housing will go a long way to keep water from getting in again.

Yes, and maybe running housing the whole way, if the OP is willing to pull the existing cable off (or at least detach it from the rear derailleur and route it again).

[Thegoblinchief]

I have similar issues when temperature fluctuates above/below freezing like it's done the past week. In my case it was clearly friction on the cables (shifting to an easier gear put obvious slack in the line) fixed by relubing the exposed section nearest the derailleur.