There are classes of drop bar bikes that some riders prefer with a wider range of gearing than a standard road or mountain bike group offers. Road tandems, touring/trekking bikes, gravel/adventure bikes, to name a few, can be ridden in a wide enough range of conditions that it can be good to have a big gear nearly as big as a roadie 53/11 and a small gear nearly as small as a mountain bike granny. That is a big ask, to be sure. Not all bike frames can handle that, let alone their components.
Before going any farther, I admit to being a Shimano biased amateur bike mechanic. I will touch on SRAM a little. Campy does not make an MTB group so they don’t really play in this arena. So I may miss some options, but I am pretty sure this covers most of the bases. If not, please feel free to let me know.
In the distant past there were various ways to make a wide range drop bar group. A triple road group, i.e. with a 3 chainring crankset, was one way. Mixing and matching road and mountain bike components could also be part of the solution. These options worked great up through 9 speed Shimano. Starting with 10 speed, Shimano began to break this compatibility between its road and mountain groups. Road bike 10 speed shifters did not work with 10 speed mountain bike derailleurs anymore. They could work with a 9 speed MTB rear derailleur, but that meant using older components in the mix. When Shimano went 11 speed, compatibility was nearly gone. No road shifters worked with any MTB derailleurs, and high end triple road groups were gone too. Admittedly the loss of road triple was not a big blow: it was possible to get the same range with new 2×11 speed groups. This was all happening at a time when gravel/adventure bikes were rising in popularity. It seemed that component makers were caught off guard .
In order to understand compatibility between shifters, derailleurs and gears (cogs/chainrings), it is helpful to break down compatibility into three categories: cable pull, gear capacity and max gearing.
Cable Pull This is the amount of cable movement needed move a derailleur enough to shift one gear. The amount of cable pull that a derailleur needs and the amount that the shifter delivers must be the same. If not, derailleurs will position the chain centered on the gears.
Gear Capacity This is the sum of the range of gears of the cassette and the crankset. For example, a compact 50/34T crankset has a range of 16T. An 11/28T cassette has a range of 17T. Together they have a range of 33T. A wide range drivetrain will have a greater capacity. The chain must be long enough to wrap around the big/big gear combo. When you shift that chain into small/small, there is chain slack to take up. The wider the gear range, the more slack must be taken up. It is the job of the rear derailleur to handle that. The cage (the plates that connect the derailleur pulleys), must be longer to take up more chain slack. Long cages slow down shifting, add weight, and stick out to interfere with the world, so you don’t want a longer cage than needed. Shimano makes 3 cage lengths: SS or short, GS or medium, and SGS or long. In general, SS is for tight ratio road race bikes. GS is for normal 2x gearing, and SGS is for 2x wide range or 3x gear ranges. In the current lineup Shimano offers SS and GS for road and GS and SGS for mountain. They have never offered an SGS rear derailleur for a current 11 speed road group, and that limits the gear range of drop bar groups.
Max Gear Size and Range Rear derailleurs have limits on the max size cog they can handle. SRAM’s commonplace 42T big cog will not work with older derailleurs, for example. The mech simply cannot pivot out far enough to reach the bigger cogs. Sometimes you can make move them out farther with adapters or by using long “b tension” bolts. But that can mean the derailleur will not be able to get close to the small cogs and compromise shifting in that range. Front derailleurs are a little different. The curvature of the shifting plates is designed for different sizes of chainrings. Shimano’s road derailleurs accommodate this. For example with the newest Ultegra R8000 band mount front derailleur is offered in L (large) and M/S (Medium/Small) options.
Other Compatibility I am going to focus on shifter/derailleur/gear compatibility, but that is not the whole picture. As if it were not enough to get the components all working together on paper, they must all work together when bolted onto a frame and under power. Putting a modern road bike crank a mountain bike may not go well. Integrated spindles may not be wide enough. That can be solved sometimes be using older BBs / cranksets where the spindle was part of the BB, like good ol’ square taper or Octalink. Putting a road bike crankset on a mountain bike may work at the bottom bracket, but the frame may not have enough clearance for the rings or a good place to mount the front derailleur. Road front derailleurs are bottom pull only, so keep that in mind. Chainline is another matter. The width of the spacing of the bike at the rear may move the cassette inboard or outboard far enough to make cross chaining even worse than normal and even affect 2 or 3 gears at the extremes. If you start to consider plus side tires or fatbikes, you may have tire clearance between the chain and frame to consider.
So for the options below, let’s presume you have the other compatibility issues sorted, i.e. just look at shifter/derailleur/gear compatibility. I have built bikes using options 1-4 below, so I know they work. The drivetrain I make is a SRAM XD 10/42T cassette paired with a 16T spread crankset: 24/40T or 26/42T, so a gear capacity of 48T.
#1: Make a Franken-derailleur. Hack a Shimano 105 5800 rear derailleur and a Shimano LX T670-SGS rear derailleur. This is documented well here. Remove the long cage from the LX and install it on the 105 and voila, an SGS 11 speed road derailleur! That solves the cable pull and capacity issue. To solve the max gear issue you may have to use the upside down b-tension screw trick, or use a derailleur hanger extension like the Wolf Tooth Road Link. I have pored over Shimano’s exploded diagrams of various derailleurs and I cannot find any other combo that would allow an SGS cage on an 11 speed road derailleur.
Pros: Relatively simple, no adapters needed.
Cons: You may have to tear apart a derailleur in ways you never have before. You are limited to just those two derailleurs as far as I know, and they are old generation derailleurs. You get no shadow low profile or adjustable clutch, so for off road use you might get a lot of old school banging around of the derailleur. Small cog shifting feels numb and slow. And the derailleurs are a little on the heavy side, if that matters to you.
#2 SRAM To The Rescue! SRAM offers a clean solution: Their 11 speed drop bar shifters are compatible with their 10 speed mountain bike rear derailleurs and 11 speed mountain bike front derailleurs. I had SRAM Rival shifters, an X-9 10 speed clutch rear derailleur (a rare beast), and a GX 11 speed front derailleur and it worked a charm. Other combos will work. If you are not trying to make existing Shimano components work and you can go all SRAM, this is a good option.
Pros: Lots of options for components, easy compatibility, no hacking or adapters needed.
Cons: Limited to 10 speed rear derailleurs, not the latest and greatest.
#3 Cable Pull Adapters Mix Road shifters and an MTB derailleur with a cable pull adapter. I know of 2: the JTEK Shiftmate 8 / 8A, and the Wolf Tooth Tanpan. This allows you to pair a long cage SGS MTB derailleur with your road shifters.
Pros: You can use any 11 speed MTB derailleur you want which means current current models.
Cons: Adapters do add complexity to the cable routing. Surprisingly, if there are no other problems in the cable run, they do not affect shifting. But any problem, like kinks in cables, gunk or corrosion in the cables and housing, etc. will all be magnified by an adapter. I like the Tanpan better. The set screw and internal routing keeps the cable from unwinding in the adapter. The adapters are not cheap, but most of these drivetrains means some added cost.
#4: Go Electronic. Shimano allows mixing road shifters and mtb derailleurs with their Di2 groups. SRAM may allow the same, but, well, I am a Shimano guy, remember? 2nd Gen Di2 did not initially allow the mixing and matching of components, but a later firmware update added that ability. You are limited to making both derailleurs mountain bike components, i.e. you cannot use and Ultegra front derailleur and an XTR rear derailleur. I am not sure about first gen Di2 either, if you want to consider that you have some homework to do, sorry.
Pros: Di2 shifting. If you have not experienced it and you don’t have the money for it, do NOT ride Di2. Electronic shifting is not a gimmick or small evolution over mechanical shifting. It is a true game changer.
Cons: The same con as always for electronic shifting: the price. Now that we have 2nd and 3rd generation electronic shifting systems, some deals can be had on older parts or bikes if you are patient. That is how I did it.
#5 Lose the Brifter Presumed in all of these options is the use of “brifters” (combined brake shifter) and indexed shifting. If you consider bar end shifters that offer a friction mode, you open a whole new world of options. You need to keep them in friction mode all the time. That means losing the clicks in your shifting and learning to shift by feel: a skill that has been lost in the history of cycling for most. But considering the way you might want to use a bike like this, it does have its appeal. Almost any shifting problems you encounter with a friction shift bike can be overcome by your own skill, on the fly, anywhere in the world, without special tools or parts. Sounds like an adventure bike spec to me.
Pros: Eliminates the cable pull issue completely and with a simpler drivetrain.
#6 (ish) Hope for other options. Shimano introduced the RX800 components which bring shadow profiles and adjustable clutches to 11 speed Shimano drop bar shifting. Now if they only made an SGS version of the rear derailleur. We can hope. Shimano has also introduced 12 speed mountain bike groups, but not 12 speed for road. Yet. Perhaps when they do, the cable pull of the road shifters will be the same as the MTB derailleurs and we can have the mechanical mix and match nirvana of 9 speed. Farther out on the horizon, maybe a 1x group one day will deliver this kind of range. But we are talking about maybe a 9/60 cassette with 13 or 14 speeds (in order to get decent gear spacing). A new player on the component scene, Rotor, has introduced a 13 speed group. And Shimano has patents on 14 speed components.
So one thing is a constant going forward: change. Fortunately, amidst all of the turmoil of the ever changing bike drivetrain technology, there is usually a way for us outliers to make a group that gives us all of the features we want.