Originally Posted by
dsaul
There have been many threads on this subject. I would only consider one of those things if there were no derailleur that would work with the cassette you want to use. For 9 speed Shimano road, any 9 speed Shimano MTB rear derailleur will work and most will go up to a 34t or 36t cassette. I have this one on my 9 speed gravel bike with an 11/34 cassette.
https://www.amazon.com/SHIMANO-Deore.../dp/B00WJGWJUC
Agree but will add that Shimano 8 speed MTB rear will also work with 9 speed cassettes.
I've done several of this sort of thing on road and gravel bikes. The only gotcha I've run into is that some RDs won't easily accommodate a 34t or 36t cassette. Can't tell you which model numbers but some can easily go a few teeth above spec some can't. Sometimes reversing the B screw or using a longer one works.
But just try it. Used MTB RDs can be found inexpensive on eBay.