El Gato27

What affects the positioning of the front derailleur when in the middle crank?
When the derailleur is in the middle crank I experience chain rub and would like to shift the position of the derailleur.
Do I do this with cable tension or with the limit screws?
I'm am under the believe that the limit screws are to position the derailleur to prevent chain drop.
Thank you.

Andrew R Stewart 

Limit screws only effect the ends of the cage's travel. Typically when the chain is on the middle ring it's also between the two limits so the screws usually don't deal with that middle ground.

Ft der positioning has a few dimensions/axis of note. Height as in how close to the large ring's teeth tops is the cages outer plate. Rotation as in how the cage's plates or their centerline is relating to the ring's plane. Generally the cage centerline is fairly parallel to the rings planes. Limits of cage travel in and out is next. How much travel per cable movement (actuation ratio) is critical to indexing working as intended. And of course compatibility among the various parts is needed for indexing to work well.

But since we don't know which parts are at play in this thread we can't offer more specific help, and know we are on the right path. Andy

JohnDThompson 

I assume that "middle crank" refers to the middle chainring on a triple crank?

Traditionally, chain rub with the front derailleur is dealt with by moving the shift lever slightly until the rubbing stops. But this assumes a friction-shifting set-up. If you have indexed front shifting, eliminating rub can be a problem. Sometimes cable tension can be tweaked to fix it, or changing the angle of the derailleur or height of the derailleur over the rings slightly. But sometimes you just have to put up with it.

Andrew R Stewart 

This! Shimano once had a small printed doc to be included with a new bike's manual. Well they have a lot of these but this one was about chain rub. It has a pictorial description of a triple crankset with the cog/ring combos that were "acceptable" if they had chain rub. It was as though god (small g intended) had spoken and decades of expectations were moot. One part of me was happy that Shimano had finally acknowledged some chain rub, given indexing's inability to really fine tune cage position at all price levels. The other was saying that now Shimano had little reason to do more to allow for fine trimming.

I have mentioned this small printed "instruction" over the years and few remember it. Tossing out the fine print is so easy... Andy

fietsbob

Shimano's index front shifting re designed the.. cage shape, so it will stay out of the way , on the middle chainring

until you go to the chain angles to the rear on the extreme ends of the cog stack 1 st & last..

on a triple the gear ratios overlap there..

so rather than cross chain middle to biggest rear cog ,

go to granny & a middle cog * , then down shift on the rear from there..

* being about the same gear ratio ..

Likewise on the front , middle cog & big chainring,

rather than middle chainring and small cog & chain dragging on the FD cage..










...

hokiefyd 

I have a few modern bikes with modern front derailleurs (both with triples in the front), and the cages are designed so that there is pretty much no chain drag in any combination. On both bikes, I can use any of the nine sprockets in the rear in the middle ring in front without drag. I think one of the bikes will drag very slightly in the small/small combination. But really, modern cages have made front index shifting very nice.

To the OP, I would start at the basics. Search YouTube for Park Tool's front derailleur adjustment video. They show you how to go from the very beginning -- unclamp the cable, get your limit screws set, and then clamp the cable and get cable tension set. It could be that your cable is too slack when the derailleur is in the small ring. If this is the case, then it may not be moving the derailleur far enough to the right in the middle ring for the chain to clear the cage in all combinations. It can be difficult to tune, so it's often best to start at the beginning and reset the whole system (following Park Tool's nice instructional video).

El Gato27

Thank you all for the input, I have gone through the Park tutorial. I'll just live with it.
I normally prefer to leave the derailleur in the middle front chainring and just shift across the rear, but sometimes I'll get chain rub on hills.
I know I can shift to the smaller chainring but involves a small amount of mental anguish (JOKE).

hokiefyd 

I take this to mean you have chain drag on the inner cage plate, when the chain is in the middle ring in the front and the largest one or two sprockets in the back? If this is true, then it could mean that the derailleur is slightly farther to the right than it might ideally be. You could turn the barrel adjuster on the shifter in (tightening it) a full turn or so, then re-check. In a perfect world, this would slacken the cable just enough to let the derailleur fall just slightly left, while still providing good shifting performance. If you go too far left, you'll get strong downshifts but upshifts might be less crisp. There may be a nice balance where you like the shifting and get no drag.

Of course, you certainly can leave it alone if it's slight or not bothersome to you. But this is BikeForums, where it's law to strive for perfection, right? :-)

mitchmellow62

At least some of the shimano brifters have a "half step" adjustment for front derailleur cage position to eliminate chain rub on the side plates when over the middle ring of a triple. My experience is with early 2000's 9 speed/triple Tiagra and Dura Ace. In both cases the derailleur could be pulled to a partial "half step", eliminating the rub.

Andrew R Stewart 

But we still don't know what kind of der, shifter or even bike the OP is referencing. So the Op might have friction shifters for all we know. Andy

mitchmellow62

I didn't mean to imply that the OP has brifters. The OP seems inexperienced. I can remember being very inexperienced. I didn't realize the left Tiagra shifter had the ability to trim the front derailleur until I had owned the bike for some time. I was only attempting to point out that some shifters have a feature to "trim" the front derailleur and my experience happens to have been with two 9 speed brifters. Wouldn't it be terrible for the OP to play with cable length if his shifters have that? Friction? Do you really think the OP would be asking how to change the position of the front derailleur while over the middle chain ring if he has a friction shifter? It also seems a little presumptuous to tell him to live with it without knowing more.

Cycle Tourist

I usually try to set the middle chain ring with the adjuster nut so it doesn't rub over the whole motion through all the back gears. Provided you've set the hight and angle of the FD correctly that should be doable. Then set the stops for the big and small chain rings so they quickly shift but no further than necessary to get a clean shift.

If that doesn't work I've got a nice friction shifter for your triple that I'll sell you.😁

El Gato27

The bike is a 2008 Fuji Absolute 3.0 with Alivio index trigger shifters and derailleurs.No trim, only one of my bikes has the trim feature. I understand the concept of chain rub and dropping to the small chainring to avoid chain rub as that would have improved parallelism. I have experienced this on other bikes as well.

I'm willing to accept chain rub from the small and large chainrings on the extreme opposite cogs. I just think a derailleur should be able to shift to all the cogs without chain rub from the middle chainring.

Maybe that is optimistic thinking.

hokiefyd 

In many cases, it can. I think yours is just an issue of very fine tuning one way or the other. I think most all of my triples will shift the entire rear cassette from the middle ring.

Else, you can adjust your derailleur so you get your chain rub in the middle/small combo rather than the middle/large combo, if you can't quite get it perfect on both ends. You're probably more likely to want to run middle/larger than middle/small.