Deore Derailleur w/ Tiagra STI Shifter Compatibility

Question

I have Shimano Deore RD M591 Rear Derailleur (http://www.jensonusa.com/Shimano-Deore-M591-9-SPD-Rear-Derailleur) that I am having trouble dialing in shifting when used with Tiagra 4700 2x10 STI shifters (http://www.chainreactioncycles.com/us/en/shimano-tiagra-4700-2x10-speed-sti-shifter-set/rp-prod137777) and a SLX 11-36t 10 speed cassette. I was working off of the general hypothesis that a 9 speed mountain derailleur would have the same pull ratio as the 10 speed STI shifters. I can adjust shifting to work as needed for the first 3 highest gears, but then when I shift beyond that towards the "easier" rear cogs it is consistently skipping over a cog (from 3rd to 5th for example). This continues up to the 10th cog, shifting two gears for every one click of the shifter. If I adjust cable tension so shifting is smooth in the middle or low end of the gear range, it is then not accurate at the high end of the gear range.

My goal was to extend my climbing gear range as much as possible so I could take my winter/cyclocross frame and turn it into a gravel bike capable of climbing some steep Vermont dirt roads.

Any thoughts or ideas appreciated. Thanks!

Answer 1

I was working off of the general hypothesis that a 9 speed mountain derailleur would have the same pull ratio as the 10 speed STI shifters.

That's true, except for Tiagra 4700, which rather inexplicably uses the 11 speed pull ratio.

See https://bicycles.stackexchange.com/a/52277/24228

Presently there are two different 10-speed Tiagra generations, 4600 and 4700. (So the STI from one is ST-4600 and the other is ST-4700, etc). 4600 uses the same classic actuation ratio that all pre-11-speed Shimano road groups used other than first generation Dura-Ace. 4700, however, is an oddball group. It's 10-speed but uses the new 11-speed actuation ratio

Answer 2

According to this video despite the claimed 32t cog max, the RD-4700-GS can handle a 10 speed 11-36 without any extra tricks or issues. So you can just get get on of these.

Alternatively, because ST-4700 have the same pull ratio as Shimano 11 speed road groups, you can also use any 11 speed rear derailleur that can handle 11-36, like RD-5800-GS or ST-R8000-GS. Of course none of these are rated as 11-36 compatible, but still work. For example, here's a video of RD-8000-GS working with 11-40 cassette.

If you want, you can even get an 11 speed Shimano MTB rear derailleur and Wolftooth Tanpan which, converts the pull ratio between two systems. JTek Shiftmate can probably do this too.

Answer 3

I have four bicycles with Tiagra 3x10 shifters; two with 4600 shifters and two with 4700 shifters. I ran into the same problem as above and was eventually able to get the 4700 shifters to work with Deore RD M591 rear derailleurs, but only if I used Shimano cassettes and Shimano Tiagra chains. When I first installed the 4700 shifters with the Deore M591 RD I was using SRAM cassettes and SRAM chains and could not get them to shift well over all 10 cogs, but once I changed to Shimano cassettes and Tiagra chains the 4700 shifters work well with the Deore M591 RD.