from Rookie’s keyboard
Hello, friends
I got to spend an afternoon at a local bike shop where I met an old acquaintance of mine from a local skate park.
He offered me to stay in the shop and just observe how they work. Of course, I ended up doing some work on a few bikes there myself just for fun.
I changed 2 flat tires and equipped a bike with a new derailleur. What you might find interesting is that I installed an old 10spd road derailleur on a bike with 9 speeds. I ended up saving the client a good chunk of cash because the old derailleur cost him just 5 bucks – it was scratched and whatnot but working perfectly.
Some rookies might be surprised that you can do this, namely integrate a 10spd RD in a 9-speed transmission, but the explanation is not that complicated.
Rear Shift Ratio
The rear shift ratio (RSR) describes how much the derailleur moves per 1mm of cable pulled or released by the shifter during up and downshifts.
- 10-speed Shimano MTB derailleurs have a 1.2 RSR. So, they move laterally 1.2mm per 1mm of cable pull or release.
- 9-speed Shimano MTB derailleurs have a 1.7 RSR and move 1.7mm per 1mm of cable movement.
When you combine a 10spd Shimano MTB derailleur with a 9spd shifter, the derailleur won’t move sufficiently for ideal shifting. The shift either won’t take place or will be incomplete, and the chain will keep rubbing against the cogs.
So, avoid this combo.
A Road Derailleur Saved The Day(my case)
10-speed Shimano road derailleurs have а 1.7 RSR (the same as 9spd MTB/road) and are therefore compatible with 9-speed drivetrains.
Note: 10-speed Campagnolo derailleurs have а 1.5 rear shift ratio matching that of 9-speed Campagnolo derailleurs too. (But as always Campy is a closed system).
You can also use friction shifters…
Indexed shifters move а predetermined amount of cable with each click. This makes them open to incompatibilities.
If you combine an indexed shifter with a derailleur that has a different RSR than what’s required for the cassette in question, you will experience non-satisfactory shifting performance.
For example, 9-speed Shimano MTB shifters have a cable pull of about 2.5mm. The shifter moves 2.5mm of cable with each click, and the derailleur moves 2.5mm x 1.7 = 4.25mm.
A 10-speed Shimano MTB derailleur has a 1.2mm RSR. When combined with a 9-speed indexed shifter, it moves 3mm (2.5mm x 1.2) – 1.25mm less than needed for a proper shift.
1.25mm may not sound like much but makes a difference, especially on cassettes with many cogs and a narrow cog pitch.
Friction shifters, on the other hand, are“free”.They move as much as the cyclist wants them to.
Ergo, they can compensate for a mismatched rear shift ratio and allow the mixture of otherwise non-compatible bike parts.
The downside of friction shifters is that they’re slow and more difficult to work with, especially if you’re a beginner cyclist.
The table below contains the RSR of 9 and 10-speed derailleurs.
| Number of Speeds | MTB Rear Shift Ratio | Road Rear Shift Ratio | ||||
|---|---|---|---|---|---|---|
| 8 | Shimano – 1.7 | SRAM – 1.1 | Shimano – 1.7 | Campagnolo – 1.4 | ||
| 9 | Shimano – 1.7 | SRAM – 1.1 | Shimano – 1.7 | Campagnolo – 1.5 | ||
| 10 | Shimano – 1.2 | SRAM – 1.3 | Shimano – 1.7 | Campagnolo – 1.5 | SRAM – 1.3 | |
| 11 | Shimano – 1.1 | SRAM – 1.12 | Shimano – 1.4 | Campagnolo – 1.5 | SRAM – 1.3 |
Until next time, friends.
– Rookie
Leave a Reply