I have different setups for different courses. Big Finn Hill Park was dry, flat, and had lots of long straight-aways so I put on a 42/18. LD/CX had a tight course with a very technical spiral, and the Rapha/Focus GP was wet (but sadly not as muddy as I anticipated) so I was using a 38/18 for both of those. Quicker power out of the technical corners on the tight course, and more oomph through the mud on the sloppy course. I could have gone with the 42/18 for Starcrossed (dry, flat, lots of straights) and a 40/18 for the Rapha/Focus GP (same course, just a little on the damp side, no deep mud to contend with.)