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I recently damaged my front mech, a Shimano Acera, top swing, bottom pull, triple chainring. The rear cassette is 8-speed. So I need to replace this front mech.

The original Shimano Acera is probably circa 2002 so I will likely not get a direct replacement, and in any case it was a bit cheap and nasty and I'd rather go with something a bit more "beefy". I'm thinking of something like Deore XT, but I can't find a model that says it is compatible with a 3x8 drive train.

The options are a bit bewildering. Can someone suggest a part?

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Rear derailleurs don't care whats up in front and vice versa. Rear derailleurs don't care about the number of cogs in the back, but front derailleurs care about the number of chain rings (you can get a triple derailleur to play nice with a double chainring, but you should probably get the right part to begin with).

Speeds are mostly marketing though, as higher speed chains are narrower than lower speed chains (the only times speeds matter are in chain selection (too little speeds and the chain may be too fat), cassette (duh) and if you're using indexed shifters the shifters need to be matched to the speeds (and the appropriate cable pulls of the derailleurs)) - everywhere else, if the chain fits, it should work. For example, if you put an 8 speed chain in a FD designed for a 10 speed derailleur, it will probably rub but if you put a 9 speed chain in an 8 speed system, everything should be fine. However, using a "9 speed" front derailleur with an 8 speed system is fine (a 7 or 8 speed FD would work fine too). The only thing that matters is how much cable the shifters need to pull, and where the derailleur is mounted (top pull, etc.).

Things you need to check:

  • Cable pull (In this case, you need a Shimano mountain front derailleur, not a road FD (else, index shifting won't work). Also, top pull (though your derailleur will probably be dual pull - this is dependent how the frame was designed to pull the cable). )
  • Double or Triple compatibility (In this case, triple - the cages are shaped differently)
  • Size of largest chain ring, and gaps in sizes to other chain rings (The cages are shaped to clear the largest chain ring, so if its too big or small, the cage won't position properly for good shifting. If the gaps are too big, you'll have trouble shifting between them (If you didn't customize your chainrings, chances are you're fine).)
  • Mounting (Diameter of seat tube mounting point or if you have another type of mount, that. Also, top swing. If you can't mount it, you can't use it. )
  • Chain slope (Chainstay angle - If you have the wrong one, the chain might hit the cage no matter how you adjust it.)

All these things are specified in the data sheet for the derailleur models.

Typically, you don't need to be spendy on the front derailleur (especially if you were running acera before) - I doubt you'd gain anything from doing so, other than something slightly prettier and a bit lighter of a wallet. Chances are that getting the FD set up properly (good cable routing, cables in good condition, positioning and adjustments done properly) even if its a cheap one (which I do suggest, the Shimano Tourney) is a better spend of money than just slapping an expensive FD like a Deore one on.

A cheap option is a Shimano Tourney front derailleur. This is what I'd probably go for provided your crankset is within spec, but you can choose any "7 speed", "8 speed" or "9 speed" Shimano mountain bike derailleur which is top swing, dual/bottom pull which fits the appropriate criteria above. You can probably run into your LBS and get them to give you the right Tourney FD for under 15 dollars. They may have a 7, 8 or 9 speed Alivio/Acera one laying around at the same price as well, so you could grab that instead.

Sheldon Brown, as usual, has a better description than I do.

What is the model of the Acera FD you have right now, and what is your crankset?

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I'm not totally sure of the models, I bought this bike used from eBay, it is a Marin Bobcat Trail, and I have radio carbon dated it to about 2002, by looking at BikePedia. As near as I can tell, the FD is an Alivio FD-360 or might be a FD-M191. The crank set is (inexplicably) a Truvativ X-Forge 22/32/42. The bike itself is in pretty good shape and the only thing I've done to it so far is to replace the pedals with some spuds. I think I'm going to have to replace the whole drive train before long though. –  Tim Long Jan 18 at 8:30
    
I have the old FD off the bike now, the only marking on it is "Acera". I'm off into town to my LBS to see what they've got on the shelf, otherwise I'll order one online Monday. Thanks for the very comprehensive reply. –  Tim Long Jan 18 at 8:36
    
Some (many?) of them have the settings engraved on the FD (look for something like 63-66 or 66-69 - that gives you the chainstay angle - the angle between the seat tube and the line connecting the bottom bracket and the rear axle). In any case, if you take the FD (and/or the bike) with you, they can quickly match it. –  Batman Jan 18 at 15:03
    
OK, job done. I eventually bought a Shimano Tourney - cheap as chips and it works well enough. I also replaced the cable as the old one was too short for the new dual-pull mech. The whole drive train is going to need replacing before long, so it will keep me happy until then. Thank you again for your very helpful answer, @Batman –  Tim Long Jan 18 at 17:33

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