RandomTroll

For a year or so I've had trouble shifting from the middle chainring to the small one. It shifts properly when there's no tension on the chain (: it shifts well when I'm not riding; it shifts properly if I remember to shift before I'm climbing.) The front derailleur ends up against the seat tube (: it travels as far in as it can; after shifting I have to pull the lever back to keep the chain from hitting the FD).

Shimano Deore crank; 24, 35, 52 tooth chainrings; downtube shift lever, bare shift cable (the cable guide under the bottom bracket looks and feels okay.) I bought the FD about 18 years ago; it's some kind of Shimano (105? I can't find an ID on it. It had a Deore XT when I bought the bicycle; when I replaced the frame with a larger-diameter model I bought a new FD; I seem to have discarded the invoice.) I've used a few different chains: Taya (unknown model), Shimano HG70, SRAM PC 830

Retro Grouch 

Without seeing the bike my guess is that it has to do with the position of the derailleur on the seat tube. Since you say it shifts inward all the way to the seat tube, I'd try changing the angle of the tail of the cage in about 1 degree.

Another trick you could try would be to spacer your BB outward a millimeter or two.

The nice thing about a problem like this is that, whenever you eventually solve it, it'll be really rewarding. The more frustrating the problem the better it feels once you figure it out. Please do me a favor and let me know how it comes out. Good luck!

fietsbob

Mine dont because I dont leave contact with the chain Dragging, against the cage ..
shift then re center to not touch each other ..

But I have seen It as the case in beater bike repairs for customers that don't or didnt care.

We're not talking cyclists who care about their Bikes ... Much.

Andrew R Stewart 

Yes, a ft der can and will wear out in various ways. But the need to soft pedal is greater with SIS controlled shifters then friction controlled ones, partly form what FB said. As the sharp edges wear smoother on the rings and the chain wears more flexible side to side it takes more der movement to get the chain to ride up and off the ring it was on. SIS levers don't compensate for this, they only more the same amount of cable as when all was new. Also as there is a tendency for the ft der to down shift so quickly one can overshift and drop the chain onto the shell. This is why the inner limit is suggested to be set so there is ALMOST no gap between the chain and the der inner plate. Any gap here and the chain can go further and derail. Andy.

Miele Man

Interesting forum handle/name.

Is the front derailler the proper one for a triple crankset?

If your shifters are friction you might be able to bend the front of the outer cage inwards a bit to give the chain that extra nudge it needs. I've also had luck by moving the drailler slightly so that the back of the ouetr cage touches the chain and causes it to shift easier. Be careful that the derailler cage doesn't hit the tire. As another poster mentioned; you might be able to put a thin spacer between the fixed cup and the bottom bracket shell. You then ned to move the left cup inboard the same qamount.

Cheers

Aubergine 

I have seen front derailleurs that have been used so much the metal on the sides had almost worn through. Does your derailleur have that sort of wear?

Another thing I wonder about is whether you have made any changes recently to your chainrings or chain.

Brian25

If your chain ring sizes stated are accurate, then your problem is that your mid chain ring is too small. Just imagine for a minute what your crank set would look like with the large chain ring removed. You will notice that the derailleur is mounted extremely high over the mid ring. The cage of the derailleur is unable to push the chain down to the small ring. The old standard is to have no more than 10 teeth jump from mid to big ring, so I would recommend using a 42 mid ring. Now Shimano has come up with a crankset that has something like 30, 39, 50 chain rings and you have to use one of their matching derailleurs.

Brian25

A old stock Deore crank usually comes with 24, 36, 46 chain rings I believe previous owner swiched the big ring to a 52, giving you the shifting problem that you describe. instead of putting on a 42 mid ring, it might be better to put on a 46 large ring.

oldbobcat

In response to the original question, sure. The bushings get loose, the outer plate takes an outward bow, the downshift spring fatigues, and the knockdown ridges on the inner plate of triple units wear off. The last one is a problem with upshifting, though.

A lot of times, the general sloppiness of a slightly worn derailleur can simply exacerbate small discrepancies in the initial installation. Be sure the outer plate is parallel to the chain rings. Get the unit as low on the seat tube as possible. I aim for 1.5 mm above the tallest teeth on the big ring, as long as the inner plate doesn't hit the middle ring on upshifts. If that's the case, raise it to where it clears the middle ring.

Then, if the chain rubs on the inner plate on the small ring and you can't get the derailleur closer to the frame, toe the tail in a little. If the cage clears the chain all right, but still won't knock the chain off, bend the lower edge of the outer plate in a bit. I like using a disk brake rotor straightening tool, but channel lock pliers or a small adjustable wrench work fine, too.

Don't overlook cables either. A slightly sticky cable can dampen the vigor of that spring knocking the chain down. Replace if that's the case.

And if all this fails, it just might be time for a new one.

Looigi

Could it be something as simple as the low limit screw needing to be turned in a bit?

hatrack71

Or even a bent cable where it would be going through the guide under the BB or near the ends of cable stops/housing? I've had that before. Also, as mentioned above, lube all those points the cable comes in physical contact with (housing inside, ends, BB shell guide, etc.). Drag could be present somewhere. Check the ferrules to make sure they are not crushed slightly and pinching the cable. Is your shifter stiff or does it move easily?

habilis

Do you really have a problem, or are you asking the der to accommodate bad shifting procedure? The crank must be rotating for a shift to occur, but you shouldn't be trying to accelerate or climb while shifting. Your old derailleur may have been more tolerant, but shifting under load is still bad for all components. A chain under tension must be forced off the chain ring with lots of pressure.

The tendency of the der to touch the frame can be fixed by adjusting the lower limit screw, but it sounds like you only have this problem when you shift improperly (while under load). The extra cable tension needed to force a bad shift moves the der past the small ring.

ultraman6970

That fd might be gonzo, get a new one.

RandomTroll

I bought a new front derailleur, a Shimano Sora FD-3503-B (the cheapest I could find). It shifts properly. The spring feels stronger.