Let me try and make it simple: The front derailleur isn't designed to hit against the chain. It's designed for the chain to run through it smoothly but the derailleur must shift the chain on and off the big chainring when you use the shifter.
If the chain rubs, here is what you must do.
(1) Mark the height of the derailleur with some tipex, biro or paint as a precaution so you know how high it is.
(2) Change gear to the point the rattling is worst. Find the worst rattling gear - this is usually the smallest gear or first gear while you're on your small chainring.
(3) Dismount and slacken off as much cable tension as you can on the front derailleur. Unscrew the allen key behind the derailleur which will allow you to move it a little up or down and to the side. Now, keep the height the same as it was (using the mark as a guide) but shift the device away from the chain so the chain only just skims the inside as you move the pedals round by hand. Tighten the allen key just tight enough to hold the mechanism in place. Pedal by hand again and test for rubbing. If it still rubs, slacken the nut a touch more, move and retighten. Finally, if all is O.K. tighten the screw fully. The chain should only barely skim the cage in first gear and maybe make a tiny noise in the process. As you change gear there should be no rattle in second or third gear.
(4) Now comes the really hard part. Get on the bike and try to shift to the big chainring but in second gear and be careful as two things may happen. (A) Either the chain won't go on or (B) it may fly right off the ring, so pedal very cautiously and be ready to brake. If the shift fails to happen, then you need to adjust the screws on the derailleur. One screw will bring the derailleur so far inwards and another screw so far outwards. These screws basically make the thing work or you use them to stop the chain totally flying off or maybe flying into the pedals. They make it work but barely affect the rattle.
When I last fitted a brand new Shimano derailleur, I found the screws were fine as they were and the shift worked soon as I mounted the cage on the frame.
If the chain doesn't make it onto the big chainring, adjust a screw till it does.
(5) O.K., so now you have the no rattling on your small ring and you can shift to the big ring but, more often than not, you'll be getting terrible rattling on the big chainring after you shifted (maybe more so in first gear). Usually this can be adjusted by slackening or maybe tightening the cable tension between the shifter and front derailleur. So, slacken the cable. If it's worse, tighten it instead. The rattle should get less, especially as the chain moves its way down the gears.
To simplify things even more: Remember that cable tension alters the position of the derailleur so if the cable is very tight it may actually pull the cage into friction with the chain. So tension is a factor.
Plus, where the chain rattles, you need to move the cage manually away from the chain till it runs through without chatter in the most extreme gears.
Finally, it really does take a bit of practice, cussing and frustration till you get more skilled in the task.
I could do it for you in maybe 35 minutes but I spent hours in the past trying to get rid of chain chatter.
keydates said:
Here's the story: I brought my bike over from the US to France in a cardboard box (with some padding, of course). Before leaving I had my bike tuned up, so it was running basically like new. Unfortunately, due most likely to being banged up during the flight, the chain now rubs against the front derailleur in almost every gear that I choose (very annoying, of course), and the shifting is a little off.
My question is, for someone with a limited knowledge in bike repair (me), would it be better to try and fix these problems myself, or bring it to a bike shop? If the answer is the former, how exactly would I go about repairing it?