Originally Posted by
xybaby
Thanks for the explanation. A very simple example:
Can I use 9 speed rear MTB derailleur on 10 speed cassette?
Can I use 9 speed MTB crankset on a 10 speed drivedrain with 9 speed FD?
First question: Yes. You need 10-speed shifters but a 9-speed era rear derailleur is fine.
Second question: Yes on the crank, probably yes on the front derailleur. I've used a 9-speed Shimano crank on an otherwise 10-speed bike with no problems but I did have a 10-speed fd.