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I bought a 1x10 speed Ridley cyclocross bike with 105 road shifters and a 105 rear mech. The bike shifts fine but I would prefer a rear mech with a clutch. I bought a Shimano M5120 rear mech that said it is suitable to use in a 1x10 system, but I’m unsure whether it will work with the 105 road shifters.
Any advice greatly appreciated. Lee
No it won't work.
The RD-M5120 uses the "MTB" 10/11 speed cable pull ratio.
Your 105 uses the traditional pull ratio, the one that was used for "MTB" 6-9 speeds and for "road" 6-10 speeds (exluding Tiagra 4700 and GRX).
The problem here is that it might be tricky to find a rear derailleur with clutch and traditional pull ratio. If you can find some old "MTB" 9-speed part then it will most likely work assuming the other parameters like big sprocket support and capacity are fine. If not, then you are probably limited to using GRX derailleur, but it uses the "road" 11-speed ratio even if used in 10-speed systems. So you have to replace your levers with GRX or Tiagra 4700 levers in that case.
What might help you is some dongle to translate the cable pull ratio. However, they are usually nearly as expensive as the rear derailleur itself!
No, MTB and road rear mechs and their shifters use different cable pull ratios (at 10 speeds and higher). A road 10-speed shifter will not be able to control an MTB derailleur properly. There are certain convertors available, although they are not very cheap (for how simple devices they are / e.g. J-tek et al.).
