This is a switch from 9 speed to 10 speed.... I have a 10 speed FSA carbon crankset 52-39-30 I wish to run cassette 11-40 10 speed... I will size chain length to shift 39-40 and 52-30 no interest in running big big. Suggestions for rear derailleur and shifter. I have read that Shimano SGS long cage with B screw all the way in will shift to 40 cog. Thoughts?
For this setup you would need a derailleur with a capacity of (52-30)+(40-11) = 51 teeth. This page states that the SGS has a total capacity of 45 teeth. That's seems quite a bit below what you are trying to do, even if you don't plan on running big/big.
Why change to 10spd? Why not just get an 11-40,9spd cluster, they are fairly cheap.
Go ahead and try it, you can always change back.
I got 3 sets of 11-40 suntour 8spd cluster, and changed them on families bikes using existing derailleurs.
2 of them work flawlessly, they are just ordinary shimano mtb type derailleur for 34T sprockets
1 is a bit marginal, and I might swap back to the 34T, if I can be arsed. So it isn't bad. That derailleur is noticably smaller than the other two, so not suprised.
I had to change the B screw for a longer one.
I am really happy with 11-40, basically I just ride in middle front sprocket now.