Originally Posted by legalize
What I think is happening is this: The middle chain ring is "picking up" the chain when the chain is on the smallest chainring and the smallest cog.
I thought it was mainly the derailler at first because the derailler does infact rub against the side of the chain when the chain is on the smallest chainring and the smallest cog, and the 2d and 3rd smallest cogs as well.
But when I push the derailler out with my finger a tad bit (is this harmful, by the way?) so the chain does not rub, I then spin the wheel. If I have someone apply a resistance to the wheel when I am peddling, the middle chain ring will pick up the chain just a bit and then drop it back down. Usually it doesnt completely send the chain off the ring but this did nearly happen when I was riding the bike.
All of my 'testing' is done with the bike upside down since I don't have a stand. Is this ok to do?
Before this whole mishap, and after the bike shop had adjusted my front derailler, my gears were working like a charm. Although the front derailler did sometimes rub the chain on the side, the front gears changed awesomly and the rear gears changed even better.
...
I'm not familiar with a 12 inch point and measuring a chain. Can I just do this with a normal ruler? And, what do I do?
No, it's not harmful to move your front derailleur with your finger, unless you're moving it in a direction that it's not meant to go. Which isn't what you're doing. You're just moving it in the same direction that the cable normally pulls it, that's okay. It's a good way to check how an adjustment would affect the position. Because it can be adjusted to stay where you're pushing it. And it sounds like maybe that is needed here, because it should not be rubbing the derailleur cage (the open structure that it passes through) when it's in *three* of the smallest cogs in back. That's excessive. The shop that installed the chainrings should have readjusted the front derailleur after they were done. I'd take it back and tell them (and show them) that it's rubbing in three different positions on the back. Better yet, you can adjust it yourself, or at least study up on it so you know what they should do. Go here to read about front derailleur adjustment:
http://www.parktool.com/repair/readhowto.asp?id=75
This will seem like a lot to digest right now, but read it over a few times, and refer to your derailleur, and you'll begin to see what they're talking about. It's really much easier than it seems! But a workstand would be a very good thing to have, so you can turn the chain with the bike upright. Otherwise, you won't get a good idea of how the chain will travel through the front derailleur under use.
About having the bike upside down, just try to avoid having it rest its weight on the shifters or cable bends or anything else fragile. The main problem with having it upside down is that it's easy to get disoriented about the cause and effect of what you are adjusting, because everything is, well, upside down.
Right, you can measure your chain with a regular old 12-inch ruler. It's not the most precise tool for the job, but it'll give you a good idea. The little pins that connect each link are what you're using as the start and end point of your 12-inch measurement. Line up the beginning of the ruler with the edge of one of these pins, and then at the 12-inch mark, see if it lines up exactly the same with a pin. If the pin nearest the 12-inch mark is 1/16th of an inch or more beyond the 12-inch mark, then your chain has lengthened due to wear and should be replaced. Depending on the conditions that you ride in and how you maintain your chain, this can happen in 1000 miles or thereabouts. It may take some practice to get that measurement, you have to hold the ruler steady and the chain straight.