Middle chainring and limit screws
 

So, in the spirit of learning how to do some basic bike wrenching I am kinda learning as I go but I have a question concerning middle chainrings ( on a triple of course) and limit screws. When I am in my middle chainring and largest rear sprocket the chain rubs the inside of the front cage. I feel fairly confident that my limit screws are set ok in their inside and outside positions (big chainring/small sprocket, small chainring/big sprocket). Is this just one of those comprises that I have read about when setting front derailler limit screws? I don't think this is an unusual gearing combo it it? Anyway, any enlightenment would be appreciated.

The limit screws have absolutely nothing to do with the center chainring. It's all about cable adjustment.....

this is where cable tension comes into play.

getting it just right is hard because if it's too slack, the chain will rub in the big chainring and small rear cog. too tight and you get what you're getting right now.

is your bike equipped with barrel adjusters for the derailer?

On the middle ring with a triple, you should have some feather adjustment with your front shifter. That is, you should be able to gently push lever inward until it clicks and moves the derailleur inward slightly but not enough to move the derailleur far enough to drop the chain to the samll ring. This, combined with probably slight cable adjustment mentioned by others, should take care of rubbing.

Thank you all. Had not considered cable tension (so much to learn). Yes AEO, my bike does have barrel adjusters for the derailer ( I will try and dink with it as I find this is one of the best ways for me to learn). DOS, your talking about trimming the derailer? My bike has indexed shifters and any movement in that direction with move the chain onto the small ring.

The barrel adjusters move the FD cage in or out by taking up or letting out cable. If you carefully eyeball
the FD cage as you rotate the barrel adjuster you can see the cage move in or out depending on how you
screw the barrel. The barrel should have click stops that are spring loaded. Go back to first principals, put
the chain on the little CW and biggest cog, click the brifter into the completely released condition and
adjust the inside FD cage stop to clear the chain by about 1mm, or a smidge less. Put the barrel adjuster
in the center of its adjusting range before you clamp the cable in the FD and adjust the inside stop. Shift up and down and see how it goes. Some FD cage rub can then be adjusted out by moving the barrel adjuster a counted number of cliks in one or other direction and check for effect. Keep track of direction and click # so you know where you started from and ended up.

What shifters?

What kind of shifters? I was assuming Shimano integrated shifters (aka brifters) -- I have has everything from Tiagra up to Ultegra and they all trim with a triple between the middle and lower ring.

I haven't used Sora shifters in a little while but I'm fairly certain that the 8 speed versions only trim for the inner ring.

The shifters are Shimano flat bar shifters (think Specialized Sirrus comp) not integrated with brake lvrs.

Those definitely do not have trim adjustments. Removing tension from your front derailler cable is the only chance you have at correcting the problem. Hopefully you don't have to remove so much that your shift to the big ring no longer completes itself. In that case, you are stuck dealing with the rub. I was able to get my wife's Sirrus adjusted such that the middle ring worked with the whole cassette without rubbing.

one last resort is to bend the FD cage slightly with a pair of pliers.