The factors that determine whether a 9-speed derailleur will be compatible with an 11-speed cassette are:
- The derailleur’s rear shift ratio.
- The derailleur’s total capacity.
Understanding Rear Shift Ratios
Modern bicycles have indexed gearing. Every shift represents a click of the shifter that moves or releases a pre-determined amount of gear cable. This property is known as “cable pull“.
Meanwhile, the rear derailleur has a specific travel per 1mm of cable that’s pulled or released by the shifter. This value is known as the rear shift ratio.
The cable pull and the rear shift ratio are pre-determined values that change depending on the drivetrain type (road or MTB) and the number of speeds.
If a derailleur with an incorrect rear shift ratio is installed, the derailleur will not move the chain to the required position for a proper shift to occur.
In order for a 9-speed derailleur to be effectively integrated into an 11-speed drivetrain, it needs to have a rear shift ratio that matches or is extremely close to that of the original 11-speed model.
The rear shift ratio of Shimano 9-speed derailleurs is 1.7.
This means that for every millimeter of cable movement triggered by the shifter, the derailleur moves 1.7mm.
The rear shift ratio of an 11-speed Shimano MTB derailleur is 1.2. This means that the derailleur moves 1.2mm per 1mm of cable movement triggered by the shifter.
If this type of derailleur is replaced with a 9-speed model with a 1.7mm rear shift ratio, the shifting of the bike will suffer tremendously because the derailleur will move more than necessary.
The rear shift ratio of an 11-speed Shimano road derailleur is 1.4. This value doesn’t match the 1.7 rear shift ratio of Shimano 9-speed derailleurs either.
A 9-speed Shimano derailleur cannot be used with an 11-speed cassette when the gearing is indexed.
The rear shift ratio of SRAM’s 9 speed derailleurs is 1.1 whereas that of 11-speed SRAM derailleurs is 1.3. The two ratios are fairly close and thus the chances of getting acceptable shifting are higher. That said, the discrepancy is still large enough to stay away from this combination.
Another requirement for compatibility would be the derailleur’s capacity. For example, many 11-speed MTB cassettes have a massive first gear. In this case, a derailleur with a short cage will be unable to put the chain on the largest cog. The only way to circumvent this issue is to get a derailleur with a longer cage and/or use a derailleur hanger extender.
Friction Shifting = 100% Compatibility
Index shifters should always be paired with their corresponding cassettes and derailleurs because they move the cable a predetermined length with every click. Or in other words, they’re locked in a pattern and cannot operate out of it.
There’s another option, however. It’s called friction shifting.
Friction shifters allow the mix of MTB and road bike parts which is why they’re so popular among touring cyclists. The bar-end version is the most used one.
Unlike indexed gearing, friction shifters are “unlocked” and move the derailleur as much as the rider wants. In consequence, the rear shift ratio of the derailleur isn’t nearly as important because it’s up to the cyclist to move the shifter to the correct position.
The downside of friction shifters is that they’re slower and less beginner-friendly. An average rider who simply likes to play with his bike on the weekend could find friction shifters annoying.
Summary: What You Need To Know
- The rear shift ratios of 11 and 9-speed derailleurs don’t match.
- The dissimilar rear shift ratios will result in less-than-ideal shifting when the rider relies on indexed shifters.
- If the bike uses friction shifters, the combination could work. That said, 11-speed friction shifters are rare.
- Large 11-speed cassettes designed for MTBs require derailleurs with greater capacity.
- It’s recommended to get a dedicated 11-speed derailleur instead of playing with a 9-speed one.