Front Derailleur Adjustment
My chain is touching slightly front derailleur when on the biggest back gear (the closest to the wheel)
how do I fix it?
I believe I have Shimano Sora front derailleur.
you have it on the small chainring when you shift to the large gear in back, Right.
A couple of my bikes will rub when in the extreme cross-chaining positions. Not sure whether you are or not, but thought I'd mention that. Sometimes it's hard to dial out that last bit of rub. If it's only in the extreme big/small & small/big combinations, I'll often leave it, as the rubbing noise serves to gently remind me that I should shift gears to improve my chainline.
If you have a clamp on front derailer, check to see if it is parallel to the large chain ring. You can rotate it SLIGHTLY inward at the rear to give a little more room for the chain. Be careful here or there will be interference somewhere else. Another possibility is to remove the screw from the rear of the cage and move the small washer from under the screw head to between the cage plates. This will open up the cage a little, and give the chain more room. I hope this helps. bk
We need more information.
Have you done a full adjustment on the front derailleur?
What front chainwheel/gear are you in when it's rubbing?
Did you try to click the front der lever once to downshift to a "trim" position (not a full shift, justs moves derailleur slightly to eliminate rub)?
Assuming you have it in the small chainring (the front gear "closest to the wheel/bike"), and also assuming that it is rubbing on the inside of the derailleur cage (the side closest to the bike), then there is a limit screw that adjusts the position of the derailleur and it should be set so that in that gear combination (small/large) the chain just avoids rubbing.
Then you can shift it into the big chainring (the front gear "furthest from the wheel/bike") and also shift into the small gear in the back (the one furthest from the wheel) and the other limit screw will let you adjust to avoid rubbing in that combination.
If no amount of fiddling with those two screws fixes the problem you might have to re-adjust the position of the derailleur itself.