Front derailleurs were announced dead a few years ago by so-called bike media, but people still use 2x, even 3x cranksets. I won’t go into detail, but I already wrote what I think about 1x drivetrains.
Most riders have or had problems with front derailleurs, mostly due to problems like chain suck, chain drop, etc. Yes; front derailleurs are tricky, and cause problems when they are not setup correctly. I will go full blast with the front derailleurs in this post, describing types, some compatibility tips, capacity, cable pull types, etc – expect a lot of detail. It’s much more important than a rear derailleur for me, because having a nicely spread, broad range of gearing is only possible with a good balance of cassette and chainring combination. If you know how to match front derailleurs with existing groupsets, you’ll have more “creative” gearing: for example, you can have a touring / randonneur bike with 46-30 crankset and 11-36 cassette combo, which looks weird for both road and MTB, and it’s only possible with matching a road front derailleur and MTB rear derailleur.
How a front derailleur work ?
Front derailleur, as in rear derailleur, “derails” chain from the current chainring to the adjacent one. Simple as that. When it comes to compatibility, things are not that simple, as lots of parameters counts. What is nice about front derailleurs is, they have equal pull rates. Meaning, no matter road, MTB, Shimano or Sram, every front derailleur will work with every shifter, almost. They are not overly complicated. It only has a cage, under spring tension, so that when cable is released, it goes back to “lowest” setting. And 2 bolts to adjust lowest / highest positions, that’s it. No clutch, no jockey wheels, no pulley bearings. They are cheap compared to rear derailleurs, and even cheaper second hand, because people ditched their front derailleurs to go 1x.
Front Derailleur Mount Types
There are basically 3 types of mounts: clamp, braze on, and direct mount. There are also subgroups amongst them.
Clamp mount front derailleurs
This is probably the most common type, at least until the last decade. Front derailleur is mounted to seat tube. There are also low clamp and high clamp versions. In low clamp derailleurs, the top the cage is normally in flush, or above the clamp. This is also a recent type compared the high clamp, looks more streamlined and leaves space for an additional bottle cage on seat tube. On high clamp, top the front derailleur cage sits beneath clamp level.
You can generally use both on a same bike, but sometimes high clamp versions can interfere with bottle cage mounts, if your bike comes with a low clamp front derailleur originally. If you have a soft tail (rear suspension) your frame is probably only low clamp compatible, unless it’s very old.
When choosing a clamp-on, or band front derailleur, you also have to measure your seatpost diameter, as there are various sizes in diameter. Common seat tube sizes are 34.9mm, 31.8mm and 28.6mm. All recent models I know of comes with 34.9mm clamps and shims: if you have a narrower seat tube, you use shims to fit them, so one size fits all.
However, vintage front derailleur have narrower clamps, as vintage steel bikes used to have narrower tubing.
Braze-on
These derailleurs fit on a brazed mount on seat tube. These are sometimes referred as direct mount – yes, they are direct, high mount front derailleurs, as SRAM calls them, to be more precise. They need single bolt to mount, looks more streamlined, and a few grams lighter.
If your frame does not have a brazed mount, you can buy adapters from third party manufacturers. Basically, they are clamps with a braze-on mount on them.
Shimano direct mount derailleurs are denoted by “D” in their model number. Like, FD-M780-D.
Direct Mount
There are 3 direct mount standards that I know of, and one extra, extinct and weird one that fits to bottom bracket directly.
Most known ones are E2 (S3 in SRAM), S1 and S2. E2 derailleurs are universal, I know Shimano and SRAM makes them – but S1 and S2 are SRAM specific only. Specialized bikes use them, probably some Merida’s too. E2 is also used by Specialized and Cannondale, and also a few more smaller but well known manufacturers.
E2 has a backplate which fits to bottom bracket area, with 2 offset bolts across. These derailleurs requires special frames made for them.
S1 and S2 is too much “exotic” to talk about…
Different types of swingers: top swing, bottom swing and side swing
“Swing” tells us how the cage moves along to chainrings. It’s also closely correlated with mount types. Do not mix it with “cable pull”. People tend to mix these two.
“Top swing” front derailleur cages move upwards: hence, these derailleurs come with low clamps. For bottom swing, it’s the opposite.
Then there is side swing, which Shimano loves. This is somewhat more recent. Swing is a misnomer, as the cage just moves across. This is also where things get complicated: side swing front derailleurs come in different forms, including the high clamp and direct mount; and most of them have weird cabling. First side swing front derailleur I know of was SLX FD-M672, which was a low clamp model. It was probably not that, I mean the “first”, because Shimano does make such changes at XTR or Deore XT groupset level first.
Side swing rear derailleurs allow more tire clearance because of their compact size and unusual cable routing and bend. Shimano says they shift better and faster due to dramatic cable friction reduction.
Double vs Triple
MTB double front derailleurs are immediately noticeable by their shorter nose and more profiled inner cage. It’s harder to notice the difference on road bikes.
Are they interchangeable? Can you use triple front derailleur with double crankset, or double derailleur with triple chainring? Well, somewhat. Triple derailleurs generally work better with double, whereas front derailleurs for double chainrings shifts terrible on triple chainring. If that’s OK, yes, they’re compatible. I highly suggest against it. Front derailleurs are very cheap these days. I’d rather go for a Tourney rear derailleur than a such combo.
Cable Pull: where the cable is coming from
In earlier MTB’s, which were basically modified street or utility bikes, cables are generally routed under bottom tube. Then, they decided to route the cables under or over top tube, as the water, mud and dust ingress caused problems in cables. With the hidden cables going popular, they are generally routed under again.
In a top pull front derailleur, cables goes from up, pulls the cable upwards. In bottom, it’s the reverse. Almost all modern MTB front derailleurs, except side pull ones, are dual pull, meaning it doesn’t matter where the cables comes from.
Side Pull front derailleurs
Side pull front derailleurs deserves a special mention: with huge tires becoming the norm and rear suspension bikes (soft tails) getting more common, side pulls are the obvious choice. Normally, cable attaches to front derailleur body at the rear of seat tube, which protrudes to back. This big swing mechanism takes up surprisingly a lot of space, thus restricts tires. Instead of making seat stays longer, this mechanism moved to derailleur body in a more compact fashion.
shimano tourney tz clamp front derailleur top pull adjustment screws
What about speeds: a 3×8 could work with 3×10?
Most people say it’s ok. Cage widths are different. As you know, a 10 speed chain is considerably narrowe than a 8 speed chain, hence the front derailleur cage for a 8s compatible groupset will have a wider cage. They somehow work. But it’s ugly. If you run a lower speed derailleur with a higher speed crankset, you’ll experience chain drop and chain suck problems. This is more common then you think. Problem is not just bad, slow and noisy shifting.
Road vs MTB: MTB derailleur with brifters?
Shifters and front derailleurs are compatible, unless they are on same “discipline”: MTB shifters and road derailleurs are not compatible, or vice versa. Also, if I remember correctly, Campagnolo have a different pull ratio.
Road shifters are different, as they also a trim function (Shimano and Campagnolo), whereas MTB shifters don’t – I think this is because they are designed with different pull rates in mind. Also, some front derailleurs for road bikes are different too: for example, some SRAM models I know have a function called “yaw”, which can deflect the nose of the cage, so you can shift smoother while cross-chaining.
However, there are products like “Shiftmate” which can make different speeds/models compatible.
Does chainline matter for front derailleurs?
I don’t have a specific data and experience on this, but most manufacturers state chainline with their front derailleurs, sometimes with “only!” remarks. I believe chainline is important, as the derailleur can fall short, or long. However, with limit screws, I know that they can compensate quite a lot. I don’t have a Boost frame / crankset to test, so I cannot say anything for sure.
What is front derailleur capacity ?
It’s the difference between two chainrings. But unlike rear derailleurs, it’s not the difference between smallest and the biggest one, rather, adjacent chainrings.
For example, if your front derailleur has 14T capacity, you can use it with a 42-28T crankset, but not 42-26T. If you have a triple chainring, like 48-36-22T, this will work too, as as maximum difference between adjacent chainring does not exceed 14T, although the total difference is 26T.
Obviously, road front derailleurs can handle bigger differences; for example, Tiagra FD-4703-B can handle 20T.