Chain rub front derailleur

[cyclingnolycra]

Hi guys,

I noticed on my new (used) bike that the chain rubs a bit with the front derailleur when in the biggest ring. I tightened the cable tension enough to fix that issue but now in the very smallest two gears (i.e. two largest things on the cassette + the small front ring) I have some slight chain rub against the other side of the front derailleur. It is fairly slight so it's more of an annoyance than dealbreaking.

Limit screws don't do anything it seems - and on closer inspection after loosening them they are loose enough so that they won't affect the derailleur position. This makes me think it's the front derailleur spring not working as expected? It does look a bit rusted.

A possibly related issue is that sometimes I miss shifts from large to small ring after the adjustments (but get them on 2nd go)

Running 105 groupset with compact chainset on Merida Scultura 4000. Any ideas would be appreciated, cheers!

[10speedsemiracer]

The safest way is to start from scratch. Drop the front down to the small ring, remove cable.

(Personally I would take this opportunity to replace the cable and cable housing, but that's up to you).

Check the derailleur is located correctly on the seat tube relative to chain-rings, with cage 2mm above big ring when extended across, and check that cage is parallel to chain travel and rings.

Check that the fully released position of the derailleur results in the inner edge of the cage being 1mm from contact with chain, and that derailleur travel is limited to getting the chain onto big ring without throwing it off.

Before refitting cable, check the bottom bracket cable guide for grit and gunk, then refit cable (taut, but not excessive).

If a new cable is fitted, be aware that a re-tune will be needed in due course to compensate for cable settling and/or stretch.

Park Tool have a good set of videos for things like this, although they are a bit self-serving with the high-profile product placement, but that's to be expected. At least it's not some over the top kid in a Borat accent with bass-heavy dance music in the background.

[Jmuzz]

Big+big and small+small are never going to be right and you just shouldn't be there. So don't set it up to support those combinations because it will tend to cause problems for small+big.

[cyclingnolycra]

Big+big and small+small are never going to be right and you just shouldn't be there. So don't set it up to support those combinations because it will tend to cause problems for small+big.

you misunderstand me or I wasn't clear. I have set it up so in my biggest gear (i.e. big ring, small cassette) it is not rubbing. But now it slightly rubs in my smallest gear (small ring, big cassette),

I don't have the confidence or time to take the cable out and start from scratch (although maybe it will come to that). I have checked that it is parallel to the chain. Height of derailleur could be a bit lower though, looks to be more like 4mm above the big ring rather than 2mm. I will try that, thanks.

[Duck!]

There's your problem. The inner plate of the derailleur cage curves toward the chain more, reducing the range of adjustment as the derailleur can't be dropped in any further.

The 2mm clearance mentioned above is the maximum; aim for more like 1mm above the tips of the tallest teeth.

[10speedsemiracer]

Just be aware that repositioning the derailleur will affect cable tension, and will need resetting, but it does sound like this will alleviate your issue.

[ValleyForge]

I heard an American guy doing TV commentary calling it "chain-chafe". I kinda like the expression.

[cyclingnolycra]

There's your problem. The inner plate of the derailleur cage curves toward the chain more, reducing the range of adjustment as the derailleur can't be dropped in any further.

The 2mm clearance mentioned above is the maximum; aim for more like 1mm above the tips of the tallest teeth.

Thanks that was exactly it. Works perfectly after I dropped it another 2mm. Limit screws work as expected.
Didn't seem like the derailleur height had ever been changed.......... and the prior owner was the first owner. Extremely confused!!