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I recently bought an old youth mountain bike for my kid and may need to replace the front derailleur. It's 3x7 and the rear seems to be Shimano Tourney. The front is also Shimano but I can't tell what model it is.

What models should I look for? Has it to be specifically 3x7 or any 3x8/3x9/3x10 would work? Would road front derailleurs work?

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    This is definitely a question where a picture is worth a lot because kid mountain bikes exist with all the same multitude of FD compatibility types that adult bikes have had, even though the vast majority are down-pull, down/traditional swing, 28.6-31.8 clamp-on. Sep 23, 2021 at 19:41
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    What's wrong with the front derailleur? It might be a matter of lubrication, or possibly the cables have rust in side the housing and need replacing. Also, you can generally ride a bike with no front derailleur, or lock it in one position with the limit screws and just ride on one chainring.
    – Criggie
    Sep 23, 2021 at 22:43
  • Amended a photo. The derailleur indeed works acceptably, and right now my son just fixes it on the middle ring. However with just $10 or $15 I think I may replace it with one in much better shape and avoid unexpected failures and unnecessary tuning. You may see from the photo that I'll have to change the rusted cable anyway, so why not. Just need to figure out which one to buy.
    – Ning Wang
    Sep 24, 2021 at 3:00
  • If the shifting is bad and not just from bad indexing, I would suspect the problem in the cable/housing, the shifter, the derailleur itself, in this order. I would not hurry with buying a new derailleur. Clean it, lubricate it, but foremost - change the cable and the housing. Sep 24, 2021 at 12:09

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There are a few important parameters and sizing specs one needs to pay attention to when selecting a front derailleur. One would be the way the derailleur attached to the bike. Many are band clamped to the seat tube and the band clamp of the derailleur must be the same size as the seat tube. They also make shims that reduce a 34.9mm band clamp down to the other two common sizes, 31.8 and 28.6mm. Shimano also attaches front ders via a high direct mount bolt where the frame incorporates a spot on the frame's seat tube for the mounting point. In addition a third possibility is an "E-type" derailleur that attaches to a bracket held in place under the lip of the bottom bracket cup. Related to these are the "E2 type" aka: "low direct mount" derailleurs that attach to posts built into the frame.

The next parameter to consider for correct replacement is the size of the outer cage of the derailleur, the shape of which must match that of the large chainring for good shifting. Basically this is determined by the number of teeth in the large chainring. The specs of a proposed derailleur should list a range of what number of teeth the derailleur is spec'd to handle.

There are other not insignificant considerations in selecting a front derailleur. For your application the front derailleur must be a mountain triple, meaning for a mountain bike with a triple front chain set. Also necessary to get right is whether the current derailleur is top-pull or bottom pull which refers to the direction from which the inner cable is drawn to move the derailleur. The design of the bike--essentially where the cable is routed--determines which sort of pull you'll need. If the cable is routed underneath the bottom bracket shell via the down tube, it will come up to the derailleur's pinch bolt from the bottom, when the shift lever is thrown, the pull of the cable is downward, and thus, a derailleur must be a "bottom pull." Conversely the cable can be routed across the top tube and then turn down to a stop on the seat tube where the cable then approaches the derailleur from the top, the cable is pulled upwards and thus this bike will require a top pull derailleur. There are derailleurs that are "dual pull" which can handle either a top or bottom mount cable routing. Shimano's newest front derailleur's are "front pull" derailleurs. These utilize a full length of outer housing from shifter to derailleur and so any design of bike frame can use them, although some newer bikes may have cable guides for front pull derailleurs.

Regarding the "speed" of front derailleurs, this is for the most part a marketing term as an otherwise compatible front derailleur from 6 thru 9 speed will work. It's best practice to stay within +/- 1 speed class of the system since as the number of rear speeds increase the front spacing narrows a bit (as does the chain) and the distance between inner and outer cage plates narrows as well. In practice, for a 7 speed system an otherwise compatible 6 thru 9 speed triple front derailleur will work.

There's a lot of information here so I'll link you to some pertinent articles to get things straight:

Background on types of front derailleurs

Shimano Front derailleur specs. The latest front derailleur offerings for 7/8 speed drivetrains begin with the "Altus" models numbered in the FD-M3000's and down. Shimano stamps the model number of the front derailleur on the inside of the inner cage plate. It will look like, "FD-Mxxx" where x are numbers. The link above is the list of the latest (last 1 to 5 years of production) front der specs so yours may or may not be listed. There is a link within the website to historical documentation within which all models are somewhere revealed.

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  • Thank you for your detailed answer. I amended the post with a photo. So it looks like band-clamped bottom-pull, right? Do you happen to have any suggested replacement model I should grab?
    – Ning Wang
    Sep 24, 2021 at 3:05
  • Yes. That's a band clamp (likely a 31.8mm but a simple measurement will confirm), bottom pull, down swing front derailleur. I didn't get into the types of swing (essentially, top swing and down swing) because the same bike may utilize either one if there are no clearance or mounting issues with one of them. There are many options starting with the Altus line up and continuing down to the Tourney level. Most now are dual pull and handle a large front wheel having 42-48 teeth. Check the specs page I linked to and select from the appropriately spec'd ones.
    – Jeff
    Sep 24, 2021 at 8:40
  • @NingWang if sticking with Shimano a like for like replacement would be a banded high mount dual pull 7-9 speed FD. SRAM used the same cable pull back then so a similar bottom pull FD would work too. Prices have fallen so you likely can get a good FD cheaply. Better FDs tended to stay aligned longer, so Deore, SLX and XT are worth looking at. As Jeff said, above 9 spd chains and derailleur cages are significantly narrower so it's get harder to avoid the chain rubbing, especially with kids who are far more likely to cross chain (high front/ low rear or low front/ high rear gear combinations).
    – DWGKNZ
    Sep 27, 2021 at 13:00
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First get a new cable AND housing. If the FD still doesn't work, take it off, clean it externally and put penetrating oil on the hinges. Or let it soak overnight in light oil. FDs are quite resilient elements and if not damaged or bent very unlikely to fail. So it may still be OK after all.

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