What font derailleur works with what frame and gearing combination has become a reasonable question to ask, where fairly recently, it was pretty self-explanatory; you could tell from looking at the frame what type you needed, at most what you needed was calipers, or even a keen eye (to differentiate between a 1 1/4" and a 1 3/8" OD tube, for example).
Where 5-6 years ago there were 2 versions of a typical front derailleur, in the current BTI catalog, there are 37 variants on the Sram X0 front derailleur. Thirty seven... Since only one of those 37 is the proper one for your bike, I thought it would be worth a second to look into identifying compatibility.
Differences
Front derailleurs are categorized for the most part by 3 factors: mount type, cable pull (both of which are determined by the frame), and the cranks/chainrings they will shift (which is a factor of the crank/chainring, oddly enough).
Number of speeds in the rear is also an issue. 8, 9, 10 and 11 speed systems all use different chains - the more speeds, the narrower the chain. The cage plates - the 2 parallel plates that sit on either side of the chain - are closer together on 10 than 9-speed setups, for example. It is possible to run a mis-matched derailleur, but the more mismatched it is, the harder it will be to get it to perform well (11s derailleur with an 8s chain will rub in many of the gear combos, for example).
Examples
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| Front derailleur cable is the one that pierces the chainstay bridge on this one. |
These are much, much simpler than mountain derailleurs, so we'll start here. The only top-pull 'road' front derailleur I know of is the Shimano CX, which is for cyclocross, and not really road. All other road front derailleurs I'm aware of are bottom pull - the cable goes around a guide, usually under the bottom bracket shell, pulling the derailleur from below.
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| Mountain bike, but has a typical road bottom bracket cable guide. Only front derailleur cable present. |
Clamp Mounts
These vary by diameter, which has to either match the diameter of the clamping area of the seat tube, or be larger with a shim. Some derailleurs have a flexible, variable diameter clamp.
"Braze-On"
These have a curved tab with a single, slotted hole, the derailleur bolts to. The reason for this can be cosmetic, for fragile, thin-walled tubes that shouldn't be clamped, or because the frame doesn't have a conventional tube to clamp to.
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| Braze-on derailleur on a titanium frame. The curve of the mount allows rotational adjustment, the slot allows up/down adjustment. |
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| Here is a braze-on derailleur on a carbon frame, with no round tube to clamp. Note that the mount is actually a separate part, bolted onto the frame. This piece was included with the frame. |
The other variable is the Shimano Di2 and Campagnolo EPS electronic variants. These have an electrical connection instead of a cable, mounts are similar to the conventional cable models.
Mountain - Clamp
Now things get a bit more complicated...
This is a high-clamp (in Sram terminology), or down-swing in Shimano, sometimes called "traditional", with a top-pull cable. Most mountain derailleurs these days have arrangements to pull from either direction, but on this frame I had to cut that feature off the derailleur because the rear triangle would hit it when the suspension compressed.
The feature to look for with regard to cable direction on the frame is the housing stop on the seat tube above the derailleur: marked in the picture
This is a high clamp, using the bottom-pull option. Note the cable anchor point, and how the cable wraps around the guide feature. If I were to use this derailleur on a frame that needed a top pull, the cable would skip the reversing feature and simply approach from above.
Note that on both these frames there are suspension components below the derailleur, and a low-clamp (see below) would not work.
This is a low-clamp, or "top-swing" in Shimano. The clamp is below the top of the chainring. It has a cable pull direction-reversing feature similar to the high-clamp derailleur above.
And here is another low-clamp, this one with a top-pull cable. This derailleur has the reversing feature, but it's not being used. Both of these examples are rigid bikes (hardtails), and can use either type of derailleur - though on this example, a water bottle cage mount bolt interfered with a high-clamp. Some suspension frames have components in the area where a high-clamp derailleur would mount, and require a low-clamp.
If your bike uses a 83 or 100mm bottom bracket shell, some front derailleurs will not work, because they don't swing far enough. Frames like this are not common, 83mm shells are primarily found on downhill bikes that don't have front derailleurs, 100mm shells are common on snow or fat bikes, the maker should have a suggestion for derailleur compatibility. Again, unless you went out of your way to get a bike like this, you probably have a 68 or 73mm bottom bracket, and "normal" derailleurs will work.
Confused yet? Don't worry, if your bike uses a clamp-on derailleur, it's pretty simple: is the clamp area above or below the top of the biggest chainring? If it's above, you need a high-clamp, below requires a low clamp. Dual-pull and 34.9mm clamp with shims for smaller sizes mean you really only need to address the high or low clamp question.
Things are about to get a lot more confusing...
Mountain - Direct
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Apologies, I don't own any bikes that use this type of derailleur, nor do I know anyone that does, and to really see what it looks like mounted, you'd need to remove the crank, and I couldn't find any images.
These are Shimano derailleurs, they have "-E" suffix to the part number (i.e., FD-M780-E), Sram does not offer anything that mounts this way.
This is the original direct mount mountain derailleur. The plate with the large hole is pinched between the bottom bracket and the frame, and a single bolt keeps it from rotating.
This design offers no position adjustment at all, it's designed for a specific chainring configuration and that's all it does.
These are dual-pull.
High Direct Mount
Shimano models have a "-D" suffix, Sram calls this "H0" mount. These are dual-pull, and have a cable stop for the top-pull option, which is used in this example.
Here, I removed the derailleur, so we can look at the mount. It has a single threaded hole, and an alignment notch. The derailleur has a matching ridge. No angular adjustment is possible on these - the frame maker had to align the mount with the bottom bracket shell face. Note on this example how no clamp could have worked: there is no tube where a low clamp would be, and no place to clamp around a high mount.
Low Direct Mount
Again, I don't have easy access to any bikes that use these mounts. Similar to how the High Direct mount was essentially a high-clamp or down swing derailleur, with the clamp removed, these are low clamp models, only without the clamp.
Shimano models have an "-E" or "-E2" suffix. Wait, wasn't it E-types that had the "-E"? Yes; you remove the mounting plate, and use the same bolt-holes to mount to the frame. The "-E2" models have no base plate, but the mounts are slotted, so you can adjust for different chainring sizes.
Sram models are for "S3" mount. Sram models are not dual-pull, you need to select a model that has the correct cable pull for your frame. Sram models also do not offer any vertical adjustment, instead they offer different models for different size chainrings. These two facts are a big part of why there are so many models (2 mounts times 2 cable options and up to 5 chainring options for Sram, vs one for Shimano).
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The mount uses 2 bolts, 22.1mm apart (red lines in the picture), with the forward mount farther out from the centerline of the bike than the rear. This is because the forward bolt is what held the derailleur to the mounting plate, while the rear secured alignment to the frame.
Sram "S2" mounts have the same 22.1mm spacing, but with both holes flush. Only a few bikes used this type, and Sram appears to have dropped it from the lineup, at least in aftermarket.
Sram "S1" mounts are also 2 bolts, this time 42.7mm apart, and flush. As far as I can tell, only Specialized bikes use this type. These also are chainring specific and not dual pull.
If your frame uses a direct mount, they should give you more info as to which direct mount to get.
Conclusion
This is getting ridiculous. That's the only way to summarize the state of front derailleurs. Why is this so complicated? One word: suspension. Road derailleurs are all basically the same design with different attachment mechanisms. Mountain derailleurs are all over the place because some frames just won't work with some derailleurs. Thank goodness the component manufacturers have tried to accommodate, otherwise suspension designs would be limited by whatever Shimano felt like making.
The good news is, I think most of this will shake out and disappear, the bad news in that is some people are going to be left with frames that they can't find derailleurs for. I wouldn't even be shocked to hear that clamps go away, though I think that would depend on what the big frame manufacturers do. I don't see any need that wouldn't be met with either a H0/D or S3/E2 mount. From their perspective direct mounts make a lot of sense - they are quicker to install, and don't require as much skill to get shifting well. Trek bikes (for example) live in fear of a loudmouth on the internet saying "Treks don't shift well" because a mechanic at a shop didn't set up the derailleur correctly. They'd like to make that impossible.
But let's think about a bigger question: the future of multiple chainrings. It seems pretty clear that we're asking the chain-derailleur system to do things it was never meant to: accommodating suspension movement and dealing with the shock we're subjecting it to. Downhill bikes these days don't generally have any front derailleur options - you have to run a single chainring. True, it's not really necessary, but if it were easy to provide an option, I think at least some frame makers would. Hammerschmidt was Sram's attempt to get rid of the front derailleur while retaining multiple speeds in front. It was a good idea and a decent first attempt, but hasn't been widely adopted; I think if they improved the weight and drag it might be. The other idea is XX1 - offering the whole range of gearing you'd want in back, with a single ring.
As I see it, there are competing engineering problems, and whichever is easier to solve will win: on one hand is the difficulty of fitting a derailleur to new suspension designs and making it work well, not just at shifting, but at keeping the chain where it belongs. On the other is the difficulty of designing a new system that provides the overall gearing range we need but that doesn't require a front derailleur. Which will be the easier problem to solve? I guess we will see.
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