Bicycle Mechanics - Chain length?

Hello,

How do you tell how long the chain should be? The chain on my
mother-in-law's bike looks like it is too long because the rear
derailer pulley rides on the the low gear. I took a couple of links
out and then it was too short, it wouldn't shift to all the gears. How
do you tell if it has the right derailer?

Thanks.

Top of the forum, sticky link to the Barnett Manuals

http://www.bikeforums.net/barnettes/barnetts_ch26.pdf

Don't mess with your mother-in-law's bike chain... you're only going to dig yourself in deeper lol.

Wrap the chain around the biggest chainring and the biggest cog in back while bypassing the rear derailleur. Add one inch of chain. That'll get you back to the place where you started.

Now look at the back of the derailleur. If it has a screw that points directly backward (not all derailleurs do), that's the "B" tension adjustment. Screw that in until the upper pulley doesn't rub on the big sprocket anymore.

Actually the best sizing method I've used is "big big + 2", so you wrap the chain around your biggest front gear, your biggest rear gear, find where the ends meet and add two links, then cut the chain. Pretty simple. Your issue isn't chain length though so much as the B-tension screw as was mentioned previously.

I just installed a new chain this weekend. The guy at my lbs said to route the chain from the biggest front ring to the smallest rear and through the derrailleurs. Pull the chain's ends snug and when the rr derrailleur is perpendicular to the ground, then you've found the right length. I made a chain tool from a clothes hanger to hold the chain together and give me enough slack to remove links without having to remove and re-route it. It's a 6-7" length of wire with hooks bent into either end that fit through the chain-works really well.

Found the problem. The derailleur is 20 years old and the spring is worn out so it doesn't hold the position it's supposed to. I told my mother in law that we can replace it and it will be smoother but I think the bike is so much better than it was that she's happy with it the way it is.

Oh, just get the spring and install it sometime when she's not lookin'. She'll be even further pleasantly surprised. Otherwise, you and she will be fiddlin' with it trying to keep it as smooth as it is right now, which, if the spring is that worn, is inevitable. Just make it right before one or both of you gets frustrated with it. She'll love you for it, to boot.