from Rookie’s keyboard
Hello, friends
Today, I am going to share a little secret that you might not be aware of regarding 9spd rear derailleurs. Of course, if you aren’t a rookie, you may already know the info.
The secret?
Well, you can effectively combine a Shimano’s 9spd RD with 7 and 8 cassettes. However, this only works if the derailleur is made by Shimano, not SRAM, and certainly not Campy.
Allow me to clarify.
First, let’s list the compatibility factors:
- Rear Shift Ratio
I’ve explained what that is many times, but I realize that this may be the first post on the site that you’re reading so don’t worry. I got your back.
Rear shift ratio (RSR) a.k.a actuation indicates how much the rear derailleur moves laterally per 1mm of gear cable pulled or released by the shifter.
The RSR of Shimano 7,8 and 9-speed RDs is 1.7. This means that for every millimeter of cable movement coming from the shifter, the derailleur moves 1.7mm.
Therefore, the Shimano derailleurs from above are interchangeable. Don’t forget that derailleurs have no brains. They don’t know whether they are operating on an 8 or a 9spd cassette.
The rear shift ratio of SRAM 8 and 9-speed derailleurs is 1.1 however. In other words, the SRAM models are interchangeable, but you can’t use them with an 8/9spd Shimano cassette due to the dissimilar actuation.
Campy Derailleurs Are Unique
9spd Campagnolo derailleurs are divided into 2 categories – old and new. The old 9-speed Campagnolo derailleurs (before 2001) have a 1.4 RSR that matches that of 8-speed derailleurs. In that case, the models are exchangeable and will work flawlessly.
But the post-2001 9-speed Campagnolo derailleurs have a 1.5 RSR.
Technically, the values are close, and it wouldn’t be a complete mismatch, but for better shifting, it will be wiser to avoid combining a new 9spd Campy mech with an 8spd cassette as the derailleur will move a little too much and will cause mini “over-shifts”.
The Shifters aren’t interchangeable!
Never forget that in the world of index shifters, the shifter has to be designed for the number of cogs on the cassette.
Unlike derailleurs, shifters are smart, and you can’t mix and match models designed for a different number of speeds.
For instance, a 9spd shifter cannot function properly with an 8spd cassette because 8 and 9-speed cassettes have different spacings requiring different cable pulls.
The 9spd cassette has the cogs closer to each other because one extra gear is fitted within the same space. As a result, the cassette spacing is slightly narrower.
The table below shows the cable pull of Shimano, SRAM, and Campagnolo 8 and 9-speed shifters.
Cable Pull
| Brand | 8-speed Shifter | 9-speed Shifter |
| Shimano | 2.8mm | 2.5mm |
| SRAM | 4.3mm | 4mm |
| Campаgnolo | 3.5mm | 3mm (after 2001),3.2mm (before 2001) |
The next table indicates the cog pitch of 8 and 9-speed cassettes. The cog pitch is the center-to-center spacing between two neighboring cogs.
Cog Pitch
| Brand | 8-speed cassette | 9-speed cassette |
| Shimano | 4.76mm | 4.25mm |
| SRAM | 4.73mm | 4.40mm |
| Campаgnolo | 4.9mm | 4.50mm (new), 4.48mm (old) |
As you can see, the 8-speed cassettes have a larger cog pitch which necessitates a longer pull.
The Derailleur Capacity Is Essential Too
The rear derailleur needs a cage long enough to cover the entire cassette. In the vast majority of cases, 9spd RDs have an equal or higher capacity than 8spd models.
Keep in mind, however, that there are also 8-speed cassettes with an unusually large range e.g., 11-42.
That range is usually found on cassettes with more speeds, but some companies (e.g., SunRace) put it on 8 and 9-speed models too.
9spd derailleurs with short cages can’t operate on such a cassette without a derailleur hanger extender.
Friction Shifters = Ballers Once Again
Index shifters put you in a closed system and create incompatibilities when matching different parts.
There’s a way out, though. It’s called friction shifting.
Friction shifters are the ancestors of index shifters and move freely – as much as the rider needs them to trigger a shift.
As a result, they allow you to mix MTB, and road parts easily as the actuation of the derailleur becomes irrelevant.
Ok, friends. It’s time for me to wrap this up. I hope the info is useful to you.
Until next time,
– Rookie
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