Condensed Answer: In general, it’s possible to combine a Shimano front derailleur with a SRAM shifter because both brands use extremely close cable pull ratios for the front mech.
That said, in some cases, the user may experience some chain rubbing when riding in the biggest or lowest gear due to cable pull dissimilarities and the narrower cages of Shimano derailleurs.
To remove all chances of incompatibility, it’s recommended to rely on a dedicated Shimano shifter.
What is “cable pull”?
Modern shifters are indexed. This means that the movement of the shifter lever is segmented. When the lever passes into a new section, it makes a click to inform the rider of the change. The rider can switch between the different sections but cannot leave the shifter in between.
Each shift/click pulls or releases a certain amount of gear cable. The cable is pulled to shift up to a larger cog or chainring and released to downshift onto a smaller one.
What Is Derailleur Ratio?
The shift ratio is a term that describes how much a derailleur moves per 1mm of cable pulled or released by the shifter. For example, if a derailleur has a 1:1 shift ratio, then the derailleur moves 1mm per 1mm of cable pulled or released by the shifter.
If a derailleur is combined with a shifter that pulls more or less cable than necessary, the derailleur will either under-shift (fail to reach the desired cog/chainring) or over-shift (reach beyond the desired cog/chainring).
Hence why it’s not always possible to mix derailleurs and shifters from different brands.
Front Derailleur Compatibility
Front derailleurs are more tolerant to dissimilar shift ratios because the jumps between the gears are fewer and thus ultra-precision is not necessary for acceptable performance. The limit screws on the derailleur help a lot too.
In most cases, the cable pull of front Shimano and SRAM shifters and the shift ratios of the respective front derailleurs “agree” enough for decent “cooperation”. Some modifications may be required, but the system is expected to be workable.
However, this is the case only when the shifters and the derailleur are designed for the same bike type. Or in other words, MTB derailleurs should be coupled with MTB shifters whereas Road derailleurs should be combined with Road shifters.
This is necessary because Shimano’s front MTB and road derailleurs have different shift ratios. For that reason, you can’t use MTB shifters with a front road derailleur or road shifters with a front MTB derailleur.
One example of a Shimano Front Derailleur (FD) and a SRAM shifter that is known to work is Shimano CX70 FD and SRAM Apex 10 shifter.
Technically, Shimano CX70 is a cyclocross front derailleur (hence why it has a shorter cage), but it is compatible with brake-shifters because cyclocross derailleurs are built for drop bar tech (learn more).
SRAM Red Shifters With Shimano 7900 Front Derailleur
SRAM Red shifters are known to have trouble pulling enough cable for shifting to the large chainring when combined with a Shimano 7900 FD. This happens because the derailleur is designed for a shifter with a longer pull. Thus, by default, the combination is known to fail.
However, it can work if the gear cable routing is changed.
Normally, the cable is routed through a channel facing the frame. (image below)
However, if the gear cable is inserted on the opposite side of the pinch bolt (the side facing away from the bike), the lever of the front derailleur that the cable pulls will be shortened.
The new pulling anchor decreases the mechanical advantage that the shifter has. As a result, the shifter pulls more cable and thus displaces the front derailleur further and thus closer to the chainring. The extra movement makes it possible for the derailleur cage to move the chain onto the larger chainring.
To understand how this works, one has to look at the operation of a lever system.
In the image above, the B part of the lever is longer and thus has greater leverage. If the B side is used to create input/displacement, the greater mechanical advantage will result in shorter movement of the A side.
This may seem counterintuitive because one may associate additional leverage with greater displacement, but this isn’t the case. Extra leverage amplifies the applied effort and thus makes it easier to move an object, but it actually decreases the output displacement created by the lever.
In the next image, the B side of the lever has a poorer mechanical advantage because it’s shorter. As a result, it’s harder to lift the A side. However, the output displacement (movement of the A side) is greater.
By moving the gear cable to the opposite side, we’re decreasing the leverage/shortening the lever. As a result, the input of the shifter causes greater movement of the derailleur.
The downside of this approach is that there’s a greater chance for the gear cable to come out. It’s also more difficult to shift due to the reduced leverage.
Nonetheless, the system works sufficiently well as riders have reported over the years.
SRAM’s Attack Shifters
SRAM Attack shifters come in two forms – as trigger and twist shifters. Those are old-school SRAM shifters specifically designed to be compatible with Shimano derailleurs. Hence why they’re found on older low-tier MTBs and hybrids.
FAQ: Can I use a SRAM shifter with a Shimano rear derailleur?
To ensure smooth shifting across the rear cassette, the movement of the rear derailleur has to be very precise. Therefore, an indexed system does not tolerate irregularities.
Apart from the aforementioned Attack shifters which are deliberately designed to be compatible with Shimano mech, SRAM shifters are incompatible with Shimano rear derailleurs due to a dissimilar cable pull.
The table below contains the cable pulls of SRAM and Shimano rear shifters.
|Number of Speeds||SRAM||Shimano|
|10||3.1||2.3 (road) / 3.4 (MTB)|
|11||3.1 (road) / 3.5 (MTB)||2.7 (road) / 3.6 (MTB)|
Since the cable pull of the shifters is different, the derailleur will not move as much as needed for a proper shift when using a SRAM shifter.
For example, an 8-speed Shimano derailleur has a 1.7 rear shift ratio. (The derailleur moves 1.7mm per 1mm of cable pulled or released by the shifter.) When combined with a Shimano shifter, the derailleur will move 1.7 x 2.8 = 4.76mm per shift.
If it’s combined with a SRAM shifter, the derailleur will move 1.7 x 4.3 = 7.31mm and will therefore “over-shift”.
Summary: What You Need To Know
1. In many cases, a SRAM shifter can be combined with a Shimano front derailleur. In extreme gear combinations, one can expect some chain rubbing.
2. In rare situations, a SRAM shifter may fail to pull a sufficient amount of cable to shift onto the big ring. In that case, the user can re-route the cable to decrease the shifter’s leverage and increase the cable pull. That said, this practice is not in the official manual, and the user is doing it at their own risk.
3. The shifter and the derailleur type should match. (MTB shifters for MTB derailleurs and road shifters for road derailleurs)
3. Shimano rear derailleurs cannot operate properly with indexed SRAM shifters due to the dissimilar cable pulls.