Condensed Answer: A front derailleur designed for a triple crankset (3x derailleur) can be used in conjunction with a double crankset and effectively function as a 2x derailleur.
Proper operation requires adjustments to the limit screws controlling the inward and outward movement of the derailleur’s cage.
Derailleurs have a set of screws limiting the inward (towards the bike) and outward (away from the bike) movement of the derailleur. Due to their function, the screws are known as limit screws.
The “L” screw limits the inward movement whereas the “H” screw creates a threshold for the outward motion.
Tightening the screws limits the movement of the cage in the respective direction whereas untightening increases the trajectory.
- Ideally, the “L” screw will allow the derailleur to downshift while preventing a drop of the chain in the bottom bracket area.
- Meanwhile, the “H” screw should permit the chain to move onto the big ring while stopping the derailleur from throwing the chain on the drive-side crank arm.
Proper calibration of the limit screws is essential when using a 3x front derailleur as a 2x.
Transitioning From a 3x To a 2x Drivetrain
There are two ways to transition from a 3x to a 2x drivetrain. Each method affects the settings of the front derailleur differently.
Option 1: Chainring Only Swap
This is the cheapest option and requires the rider to replace the existing small and middle chainring with new ones while dropping the biggest ring.
The replacement is done to keep the gear range fairly wide. Hence why one of the rings is slightly bigger than the previous “granny ring” while the other is slightly larger than the previous mid-ring.
Note: This replacement can be done only when the existing chainrings attach to the cranks via bolts. If the cranks are low-end, then they’re more than likely riveted and do not allow the user to install new chainrings. In that case, the only way to acquire a 2x setup is to get new cranks.
In this case, the “L” screw will not require adjustment because it’s already set to prevent a drop of the chain in the bottom bracket area.
However, the “H” screw will have to be re-set because there’s no longer a large chainring. To do that, tighten the limit screw until the derailleur does not move when trying to shift from the new big ring (old-mid ring) to the previous large ring which no longer exists.
If that adjustment isn’t made, the derailleur will drop the chain on the drive-side crank.
Note: Technically, the transition can be even cheaper by simply dropping the large chainring. However, in that case, the user will lose a notable amount of top speed and may even spin out on descents.
Option 2: Chainring and Crank Swap
Another option is to replace the 3x cranks with dedicated 2x cranks. This is not only the more expensive choice but also the more complicated one.
2x cranksets do not have a small third chainring. Consequently, 2x cranksets are narrower. Or in other words, the larger ring is closer to the frame than the larger ring on 3x drivetrains.
For that reason, it will be necessary to turn in the “H” screw and limit the outward movement of the front derailleur.
The position of the chainrings influences the shifter’s setup too.
The spacing between the small and the large chainring on a 2x setup corresponds to that between the middle and large chainring on a 3x setup.
Therefore, when combining a 3x shifter with a dedicated 2x crankset, it’s necessary to use the second (2 to 3) shift. If the first shift is used, the derailleur will overshift.
The distance between the smallest (granny) ring and the middle ring on a 3x setup is greater than that between the small and large chainring on a 2x setup. Consequently, the first shift on a 3x shifter pulls more cable.
To be precise, the first click/shift pulls 11mm whereas the second pulls 7mm.
There are also Shimano shifters designed to be used with both 3x and 2x setups by turning a screw on the shifter. When switching from a 3x to a 2x mode, the screw eliminates the first shift. Switching from 2x to 3x adds the first shift.
If you decide to use a 3x shifter with a 2x crankset, the “L” screws will have to be adjusted so that the chain doesn’t drop if you accidentally downshift from the small ring on the 2x setup to a non-existing smaller ring.
Using a 3x Shifter To Trim The Derailleur On a 2x Setup
Modern shifters can trim the front derailleur. The so-called trim is essentially a micro-shift that moves the cage of the derailleur slightly so that the chain doesn’t rub against the inner or outer cage of the derailleur in extreme gear combinations. (read more)
The extra shift on a 3x shifter can be used to do the same (to an extent) when combined with a 2x setup.
The secret is to set the “L” screw in a position that makes the shift 1-2/2-1 (the original shift from small to mid-ring) tiny.
When that’s done correctly, the movement of the derailleur is extremely small and the chain remains on the same ring (the smaller ring of the 2x setup) even though the cage of the derailleur is moving slightly.
When the rider downshifts (2-1), the micro-shift will move the derailleur’s cage ever so slightly closer to the bike and increase the space between the inner wall of the derailleur’s cage and the small chainring. This extra space will allow the rider to combine the small ring with the smallest sprockets at the back of the cassette without chain rubbing.
When the rider shifts up (1-2), the derailleur’s cage will move ever so slightly away from the bike and create extra space between the outer wall of the cage and the small chainring. As a result, the user can shift to the smallest cog at the back without experiencing chain rubbing.
Finally, shifting up one more time will move the chain from the small to the large chainring.
The video below illustrates the concept:
The Advantages of Using a 3x Derailleur On a 2x Drivetrain
- Money savings
The main incentive to use a 3x derailleur instead of a 2x is to save money when converting from 3x to 2x.
- Minimal Changes
By keeping the derailleur and the shifter, the user not only saves money but also effort and time since there’s no need to replace the cranks or the shifter.
Therefore, this is the fastest way to switch from a 3x to a 2x drivetrain.
- An Option to Run a Bash Guard
Many MTB riders running 3x drivetrains used to remove the largest chainring and replace it with a bash guard which protects the chainrings and the frame from obstacles on the trail.
- An Option To Revert The Changes
By keeping the existing 3x setup, the rider reserves the option to go back to a triple crankset whenever they want.
A 3x derailleur can be used on a 2x setup, but the opposite isn’t an option because dedicated 2x derailleurs have a shorter cage. This makes them non-reliable for the first shift.
The Disadvantages of Relying On a 3x Derailleur When Running a 2x Drivetrain
The main disadvantage of running a 3x FD on a 2x setup is the need to make additional adjustments to acquire an operational drivetrain.
The techniques described above are not super advanced, but they aren’t beginner-friendly either. In the world of casual cycling, people are often incapable of indexing the gears of their bikes. Thus, the needed changes can seem intimidating to a beginner cyclist.