Just realized that the title should say affecting, not effecting.... anyways..

ive recently began an overhaul on my commuter bike, and have since been commuting on my mountain bike.
On my MTB I have some Deore trigger shifters that seem to not want to grab gears in the cold. They shift great on the stand in my living room, but the other night, on my commute home it was just about 38*F and I had trouble getting the shifters to grab gears. Also the return of the thumb lever was a bit slow. Im sure this is caused by the grease in the shifter getting thicker as the weather gets colder, but im not really sure how to remedy the situation. Can I disassemble the shifter and pack it with lighter grease? some oil maybe? Ive never dismantled a trigger shifter before, so I am a little unsure about what needs to be done. If I have to live with the sloppy shifting, I guess I could, but I would really rather not.

Any help from the bike gods?

You might be right about the thicker grease. About the only effective way to get it out (aside from a complete teardown/rebuild) is with a thinner/lighter hydrocarbon-based solvent. WD-40 comes to mind as a quick fix. I'd just shoot some in and see what happens. If you want, you could then follow that up with a lighter lube to keep everything running smooth.

I have experienced temperature-related shifting problems like this as well. I can only guess that it is the caused by the difference in expansion of the cables and frame in response to the cold. I suggest you adjust your shifting cables outside in the cold, where you normally will ride it. Otherwise you might have to adjust them whenever there is significant temperature changes.

Last winter I replaced the shifter cable housing presuming moisture or other foreign substance was freezing up and preventing cable release. (I could force the shift to higher gear but it would not release back to lower gear during sub freezing temperatures) It did not fix the problem --leaving me to presume either the shifter or the internal hub shifting mechanisms were at fault. Either way I have to just deal with it.

That makes sense but standing outside in the cold, adjusting shifters, doesn't sound like a lot of fun.

It is generally a bad idea to dismantle a Shimano shifter. They have no replacement parts available and are difficult to re-assemble. It sounds like the problem is indeed in the shifter. You can try to flush the shifter with generous amounts of some sort of cleaner, some use WD40, then spray in some light lube.

The steel cables contract, but so do all the other metal parts on the bike. I sometimes experience this problem with shooting handguns and rifles when the temperature is below 40 degrees. I've also had the same problem with riding geared bicycles in the winter. Some mechanical devices just have looser tolerances than others, allowing for shrinkage and expansion with the ambient temperature. All of the cables, springes, adjustment screws and the bolts associated with your derailleur and shifters, and even the frame itself, are interacting differently in the cold than at room temperature, and heavy grease does get more viscous, but that alone should not make a noticeable difference.

If you live somewhere that it is cold for half of the year, then the poster who mentioned above that you adjust your derailleurs in the cold probably had the best solution besides turning your ride into a single speed or throwing on parts with looser tolerances.

Do a search on the kinds of keywords in your post- this has been discussed in the fall every year on BF There are lots of threads detailing the flush/relube procedure for shifters that fail to shift properly in cold weather. It's a pretty easy process- good luck!

I'd try the easy stuff first - make sure your cables move freely and that your derailleur is cleaned and lubed. I think trigger shifters rely on the cable pull provided by the derailleur spring to shift in one direction so anything you can do to increase that pulling force should help.

If things are still gummed up, you can try to re-lube your shifter. Usually, you can take the plastic covers off a shifter without harm but I wouldn't disassemble one any more than that or you'll probably never get it back together. Carb cleaner will degunk it pretty well but is a bit toxic for my tastes. Cleaning the shifter in a dish washer is a safer alternative. To re-lube, I'd use moly grease - the kind used for CV joints in cars. You can get a small amount at most car parts stores for $1 or so. It's pretty thin and shouldn't get too stiff when cold, but be careful handling it - it's very messy stuff.

It's the bits inside contracting....after years of use the bits wear normally, then after contraction they don't engage. I ditched mine last winter because of that after 6 years of flawless use. I don't think it's a grease issue, because they would work indoors before I left the house, then after freezing, ca marche pas.

The factory grease in the shifters tends to dry up a bit and get stiff as time passes, and the problem is exacerbated in cold weather. The easiest solution is to remove the plastic cover, squirt some light oil in and around the ratchet and pawls (chain lube is just fine) and then shift up and down a few times to work it in. That'll thin out the grease and usually solve your problem.

With three of my bikes, it's been easily fixed by flushing and regreasing with lighter lube. One was an old Trek with early RSX shifters, one was a mid-90s Fisher with trigger shifters, and one was similar to the Trek's shifters- early 600 parts or something along those lines. My method was to flush the shifters with a good bit of degreaser, then flush that out as well as possible with a light lube (something you can spray or drip in rather than a grease), and then to reapply light lube a day or two later.

The symptoms are what everybody here is describing: it'll shift fine in room temperatures, but the pawls won't catch properly once the temperature drops. Sometimes careful, slow movement of the shift lever will allow it to catch, indicating that the small springs are still trying to push the pawls into place but can't do so as quickly as when the grease is warmer and less viscous.