Condensed Answer:
It’s possible to integrate an 11-speed derailleur in an indexed drivetrain with an 8-speed cassette and 8-speed shifter only in the following cases:
Option 1:
- 11-speed Shimano or SRAM MTB Derailleur
- 8-speed MTB SRAM Shifter
- 8-speed SRAM or Shimano cassette
Option 2:
- 11-speed Shimano Road Derailleur
- 8-speed Campagnolo shifter
- 8-speed Campagnolo cassette
If the rider wants to use another derailleur, it will only be possible if the rear shifter works in friction mode.
Derailleur and Cassette Compatibility In an Indexed Drivetrain
Indexed drivetrains have the following characteristics:
- The movement of the shifter is segregated into clicks.
- Each click represents a shift up or down the cassette.
- With each click/shift the shifter pulls or releases a pre-determined amount of gear cable known as cable pull.
- The movement of rear derailleurs is programmed via the rear shift ratio.
- The rear shift ratio indicates how much the derailleur moves per 1mm of cable pulled or released by the shifter. (E.g., If the rear shift ratio is 1.1, the derailleur will move 1.1mm per 1mm of cable movement.)
- The pre-determined cable pull of the shifters and the rear shift ratio result in controlled movement of the rear derailleur that matches the cassette spacing. As a result, in a properly indexed drivetrain, the shifting is thought-free and very fast.
- The values of the rear shift ratio and the cable pull are the key to index shifting, it’s not possible to mix all kinds of shifters and derailleurs. If the rear shift ratio and/or the cable pull aren’t correct for the cassette in question, the derailleur will not move to the right places, and shifting will fail.
- The distance between the cassette cogs is crucial too. Each derailleur and shifter combo is designed for specific cog spacing. If the cassette in question doesn’t meet the criteria, then the indexing will fail.
The formula for success is: Shifter Cable Pull x Rear Shift Ratio = Cog Pitch
(The cog pitch is the center-to-center distance between two adjacent cogs on a cassette.)
Compatibility Between 11-speed Derailleurs and 8-speed Cassettes and Shifters
To determine the compatibility between 11-speed derailleurs and 8-speed cassettes and shifters, we can use the formula above.
The analysis will be classified by brands because each of them has separate values for the cable pull, cog pitch, and rear shift ratio.
Shimano
The data for Shimano 8-speed cassettes and shifters is:
- Cable pull = 2.8mm
- Cog pitch = 4.76mm
(There are no separate MTB and road shifters in the 8-speed eco-system.)
When applying the data in the formula, we get the following equation:
(Shifter Cable Pull x Rear Shift Ratio = Cog Pitch)
2.8mm x Rear Shift Ratio = 4.76mm
Rear Shift Ratio = 4.76/2.8=1.7
If we can find an 11-speed derailleur that has a 1.7 rear shift ratio, the derailleur will be compatible with an 8-speed Shimano shifter and cassette.
However, there isn’t an 11-speed derailleur that has a 1.7 rear shift ratio.
Conclusion: 11-speed derailleurs are not compatible with Shimano 8-speed cassette and shifters.
SRAM
The data for SRAM’s 8-speed cassette and shifters is:
- Cable pull = 4.3mm
- Cog pitch = 4.73mm
After applying the data in the formula, we get the following equation:
4.3mm x Rear Shift Ratio = 4.73mm
Rear Shift Ratio = 4.73/4.3=1.1
If we can find an 11-speed derailleur that has a 1.1 rear shift ratio, the derailleur will be compatible with an 8-speed SRAM shifter and cassette.
Shimano’s 11-speed MTB derailleurs have a 1.1 rear shift ratio and can therefore be used.
SRAM’s 11-speed X-Actuation (MTB) derailleurs come close with a 1.12 rear shift ratio. The percentage difference between the original and SRAM’s 11-speed rear shift ratio is only 1.8%. Thus, there’s a very good chance that SRAM’s 11-speed MTB derailleur will deliver a satisfactory experience.
Conclusion: SRAM’s X-Actuation 11-speed derailleurs and Shimano’s 11-speed MTB derailleurs can be integrated into an 8-speed SRAM drivetrain.
Campagnolo
The data for Campagnolo’s 8-speed cassettes and shifters is:
- Cable pull = 3.5mm
- Cog pitch = 4.90mm
After applying the data in the formula, we get the following equation:
3.5mm x Rear Shift Ratio = 4.90mm
Rear Shift Ratio = 4.9/3.5=1.4
If we can find an 11-speed derailleur that has a 1.4 rear shift ratio, the derailleur will be compatible with an 8-speed Campagnolo shifter and cassette.
The rear shift ratio of 11-speed Campagnolo derailleurs is fairly close at 1.5, but it’s not ideal.
However, Shimano’s 11-speed road derailleurs have a rear 1.4 rear shift ratio and therefore meet the criteria.
Conclusion: 11-speed Shimano road derailleurs are compatible with Compagnolo’s 8-speed cassette and shifters.
Friction Shifters = An Always Viable Option
Friction shifters are old-school. It’s up to the rider to determine the position of the shifter lever for a shift to occur. There are no clicks. The shifter moves freely over its amplitude and allows the user to move from one end of the cassette to the other very quickly.
Consequently, the rear shift ratio of a derailleur becomes irrelevant. If the derailleur can cover the entire cassette, it fits into the drivetrain. For that reason, people who want to mix road and MTB parts of different speeds often rely on friction shifters.
The downside?
It’s a manual operation with a fairly steep learning curve. No matter how good you get at friction shifting, it’s just not possible to match the speed of indexed shifters in the hands of an experienced user.
Additional Questions and Answers
I understand that some 11-speed derailleurs cannot be integrated into an 8-speed drivetrain. But I have those parts already. What can I do to get a working drivetrain fast?
So, you have the following parts:
- 11-speed derailleur
- 8-speed shifter (indexed)
- 8-speed cassette
The quickest way to a working drivetrain is to get a derailleur that can be integrated into that drivetrain. The options are:
Shimano
If you have a Shimano 8-speed cassette and shifter, you can use Shimano 8/9 derailleurs as well as Shimano’s 10-speed Road models since those have the same rear shift ratio – 1.7.
SRAM
If you have a SRAM setup, the options are SRAM’s 8/9 derailleurs and 11-speed Shimano or SRAM MTB derialleurs.
Campagnolo
If you have a Campagnolo setup, the options are:
- Campagnolo 8-speed derailleur
- Campagnolo 9-speed derailleur (old versions)
- Shimano 11-speed Road Derailleur
