April and May can be very windy, winds from the south a lot. Sept, Oct should be great.
Arches NP is doing road improvements during all of 2017 with a lot of traffic delays so it may not be a good place to ride this year.
Check out the Grand Junction area for riding. Lots of maps for road rides and mtn bike rides
here.
A road bike ride on Rimrock Drive over Colorado National Monument should not be missed.