I wanted to be able to climb hills more efficiently, there's lots of steep and really steep hills in my area. I changed out the 30 tooth granny gear for a 26 tooth (26-42-52) on my Catrike Expedition and at the cassette (11-36) I installed a 42 tooth replacement gear, had to remove one gear, I chose the 17t....with this combination I can almost climb trees (haven't actually tried to yet) but slowly...low but slow wins the hill climb on a trike