to the OP, you might want to check youtube spindatt channel, he did a comprehensive rockhopper conversion. if you like it, you can either replicate it to your own rockhopper or if you want a faster bike with bigger wheels, apply the same concept to the sphinx
personally, I would look at both bikes original equipment, check which one can get the easiest steerer conversion and check if the rear free hub can take 8,9 or 10 gear cassette and then if all equal, convert the the lightest of both.
Scrap the groupset a put a shimano sora 9x2 with 11-34 (or push to 11-36) 9 speed cassette and select the 2 chainring depending of the terrain you want to ride (compact 50-34 or easy 44-28)
Last edited by Fentuz; 08-12-21 at 01:26 AM.