Using a clutch rear derailleur with 9 speed (or even 8 speed!)

What is a clutch-type rear derailleur?

A clutch-type rear derailleur has a mechanism at the top of the cage that adds some resistance to the cage moving. This acts to keep better tension on the chain over rough terrain or when suspension movement results in chain growth or shrinkage. The clutch is engaged using a lever at the bottom knuckle. With the lever you can deactivate the clutch mechanism to make wheel removal easier.

Why use a clutch derailleur?

Clutch derailleurs are the way to go if you are running a 1x / single front ring setup and don't want the extra weight, expensive or hassle of a chain guide. A clutch-type derailleur combined with a narrow-wide front chain ring will provide a chain-drop free setup with serious weight and complexity savings over a more traditional front chain guide.

What clutch derailleurs are available?

Shimano was the first to come out with a clutch-type derailleur. They now have no less than 5 different models of clutch-type derailleurs (with the "shadow plus" designation):

• Shimano XTR Shadow Plus RD-M986 Rear Derailleur
• Shimano XT Shadow Plus RD-M786 Rear Derailleur
• Shimano SLX Shadow Plus RD-M675-GS Rear Derailleur
• Shimano RD-M820 Saint Shadow Plus 10 Speed Rear Derailleur
• Shimano RD-M640 Zee Shadow Plus 10 Speed Rear Derailleur

SRAM now also make clutch-type rear derailleurs with the "Type 2" designation.

• SRAM X0 Type-2 10 Speed Rear Derailleur
• SRAM X9 Type-2 10 Speed Rear Derailleur
• SRAM X.7 Type 2 Rear Derailleur, Grey, 10-Speed Medium Cage

What's the problem?

There's only one problem, Shimano only started making clutch-type derailleurs in their 10 speed group sets. These derailleurs use a different activation ratio to the earlier 2:1 9 speed (and 7/8 speed) derailleurs.

However, all is not lost.  There are two options to getting a newer 10 speed Shimano clutch-type derailleur working with a 9-speed cassette and shifters.

Option 1 - SRAM 9 speed shifter

SRAM rear derailleurs have had a 1:1 activation ratio for a long time.  As a result, you can use any SRAM 9 speed shifter with a Shimano 10 speed rear derailleur and it will work perfectly with a 9 speed rear cassette.

Option 2 - Shimano 9 speed shifter, modified Shimano 10 speed rear derailleur

The other approach is to modify the cable attachment point on your Shimano 10 speed rear derailleur so the activation ratio becomes 2:1. This then means you can use 9 speed Shimano shifters with the modified 10 speed derailleur.  There's a thread on Hucking Kitty that sets out all you need to know about this mod.

Option 3 - Hybrid Shimano 9 speed / 10 speed clutch derailleur

Take the clutch mechanism from an XT Shadow+ rear derailleur and add it to a 9 speed XTR Shadow rear derailleur.