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I am moving my 9 speed Tiagra brifters over to a new frame and need to purchase a new cassette and RD. Can I run, say, a 105 rd-5701 10 speed on the rear with a 10 speed cassette and get all 10 cogs?

I know that Shimano all play together nicely up to 10 speed as between road and MTB and understand that cable pull for 9 and ten speed shifters is the same, but will it move the RD the right amount for 10 speed cassette spacing?

I have read lots on the subject as to what is compatible, but I am not clear what the result might be (at least not to me!!!).

If I have to buy the pieces anyway, can I move to 10 speed to get more cassette options without investing in shifters? It might have to push the capacity on the RD a bit...

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  • Since you're already spending on 105 level parts for the rear, then buying a pair of 105 brifters might make a whole groupset cheaper. Shop around and see what's on offer. If the set has brake calipers or pedals or other parts you don't need, then on-sell them, or upgrade anyway, or simply store them for future use.
    – Criggie
    Sep 25 at 5:23
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No, the number of gears you get is defined by the shifter. Even if the cable pull was compatible, which I don't think it is, between Tiagra 9 speed and 105 10 speed, you wouldn't be able access all 10 gears.

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  • Thanks, was thinking that, From what I read shimano road 9 and 10 speed shifters, with exception of tiagra 4700 10 speed have same cable pull ratios but I didn't get if that could mean the RD could be interchangeable, or whether I'd get an extra click out of the shifter if it could still pull, say to get up onto the largest cog if limits were set.....new to all this, Sep 24 at 18:01
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The problem with your proposal is the spacing of 9 and 10 speed cassettes is different. The 9 speed shifter will pull the 9 speed amount of cable and the derailleur will move the 9 speed space between cassette cogs. If the cassette is a ten speed with 10 speed spacing, the cable pull and derailleur movement (being 9 speed) will be off and shifting will be poor. In addition, the 9 speed shifter will only have 8 detents and can't gain any more. Thus, even if spacing wasn't a problem, one of the 10 speed cogs would be left out.

Now what does work mixing Shimano speed groups is at the derailleur. Any Shimano 7-9 speed plus 10 speed road rear derailleurs will work correctly in any of these speed class systems. In other words, a 7 speed rear derailleur (or an 8 or 10 speed) will work just fine with your 9 speed shifter and 9 speed cassette. This is due to all these speed classes of rear derailleur share the same "actuation ratio." These derailleurs move the same amount per mm of cable pulled by the shifter. The shifter, by it's designed amount of cable pull per lever throw, and the number of "clicks" determines the "speed" of the system, and must be paired with the cassette of that same speed for the system to work correctly. You can certainly use the RD-5701 with your current 9 speed shifter and 9 speed cassette.

To sum up, differing speeds do have different cable pulls and the cassette's have different spacing. The shifter and cassette must match in the speed since a certain, set amount of cable must be pulled by the shifter to move the derailleur the particular intercog spacing of the cassette. Derailleurs (of the above mentioned speeds) can be interchanged between speeds.

One note about derailleurs: there are other parameters that make a derailleur compatible in a given system. While all these speed classes of derailleur share the same actuation ratio and respond (lateral movement wise) the same to shifter input, they aren't necessarily interchangable within every set-up of these speeds. Each derailleur has a maximum low cog tooth count, a minimum high cog tooth count, and total capacity they're spec'd for. Exceeding these specs dramatically will lead to poor or incomplete shifting. For example, one shouldn't place a short cage, 7 speed, road derailleur in a 9 speed mountain drivetrain with a triple front chain set and wide range, 11-34 cassette, and expect perfect shifting throughout the range. The drivetrain it's being thrown into, despite being a "compatible" 9 speed mountain, far exceeds the spec'd limits of the derailleur. It would likely fail to climb the chain on to the 1 or 2 largest cog's of the cassette. The chain would be noticably slack when on the front little ring and midway through the cassette since the short cage has not enough chainwrap capacity.

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  • Quick sequel: On one derailleur, say 9 speed, is the cable pull from gear 1 to gear 2 exactly equal to the cable pull from gear 8 to gear 9?
    – Sam
    Sep 27 at 14:42
  • Yes. Shimano uses identical cable pull throughout the gears of the rear drive train. In front, the amount of cable pull differs between small to medium chainring and medium to large ring.
    – Jeff
    Sep 28 at 1:25
  • To be clear, it is at the shifter where cable is pulled. The rear derailleur moves a certain amount per mm of cable pulled by a shifter. This is termed the, "actuation ratio" of the rear derailleur. Shimano mountain & road 7, 8, and 9 speed plus 10 speed road rear derailleurs all share the same actuation ratio. Thus, a 9 speed rear derailleur will respond to a seven speed shifter's cable pull correctly and move the chain across the 7 speed cassette cog spacing.
    – Jeff
    Sep 28 at 1:37
  • I don't understand "actuation ratio" yet, but "cable pull" seems pretty easy to define: If we mark a point along the exposed part of a rear derailleur cable (bicycles.stackexchange.com/q/67895/48599), then the increments would be equal. That picture, taken from a Park Tool video, would hence inaccurately show one interval larger than the others. Is that right?
    – Sam
    Sep 28 at 7:19
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    I'll have to view your links later but actuation ratio is the amount of movement (in mm) the derailleur cage moves per mm of cable pulled by the shifter. For example: a Shimano 8-speed shifter pulls some 2.8mm of cable with each click and the distance between sprockets of a Shimano 8-speed cassette is 4.8mm. That's 1.7 times as far, so we can say that the Shimano mech has a shift ratio of 1.7. It is this 1.7 shift ratio that is shared among Shimano rear derailleurs (*all 7, 8, 9 speed, plus road 10s), and the reason they can be interchanged between speed classes successfully.
    – Jeff
    Sep 29 at 3:54

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