Originally Posted by
watercamper
OK - dumb question time. 1 - change to a 10 spd 12 - 36 cassette ✔ 2 - Install a 9 spd long cage mtn bike derailleur ✔ You say that the 9 spd derailleur will work with a 10 spd cassette. Does that mean I will have index shifting for all 10 speeds or only 9?
There's a general misunderstanding about derailers. The derailer is a robot for the most part. As long as the derailer's cable pull ratio and the shifter's pull ratio match, the derailer will work with any shifter. You could use a rear derailer designed for 6 or 7 speed gear systems with an STI 10 speed system without issue. The one caveat is that Shimano has changed the pull ratio on 10 speed mountain bike derailers with the introduction of the Dynasys group. You can't use Dynasys derailers with any other system so you can't mix and match it into road systems.
The only problem with using a 6, 7, 8 and some 9 speed mountain derailers with the 12-36 is that they may not have the right capacity for handling the 36. The derailer I linked to is a 9 speed derailer with a capacity for the 36 tooth low. Others may work as Shimano is notorious for underestimating their equipment's abilities.