This was once a shady, scenic, and challenging singletrack through evergreen forest. The 2002 Bullock Fire changed all that. Some of the forest remains, but parts of the Crystal Spring Trail are fairly open with lots of the undergrowth. It's still scenic, but in a different way.
If you were unchallenged by the out-and-back fun of the Crystal Spring Trail, this loop is a good ride. You'll have pavement at first to warm up, a fast and sometimes loose downhill on the fire control road, 2 miles of challenging singletrack, and a grueling 2-3 mile climb. You can take the loop the other way but the long grind of a climb up the fire control road may make you wonder if the singletrack was worth it.
Ride the Crystal Spring Trail before attempting this loop. Then you'll know if it's worth beating yourself up over.
This trail will either inspire you or drive you into the ground. The last 3 miles is an uphill grind, sometimes through thorny brambles, sometimes unridable. The last mile climbs the Butterfly Trail with almost 700 feet of elevation gain. One rider found the trail so unridable that he wrote to tell me it shouldn't even be in a mountain biking guide.
In other words, you've been warned.
How to get there
If you don't have a Forest Service pass, pick one up at the Mount Lemmon Highway fee station above the Molino Canyon overlook or the self-service station at Molino Basin. Some federal lands passes are also valid, including the America the Beautiful pass. Check a fee station for the current pass requirements.
Take the Mount Lemmon Highway to the Butterfly Trailhead (mile 22.5) and park. The trailhead is labeled with its old name, Soldier Camp Trailhead, on the trail map.
Note: The latter, most important, part of the profile is incorrect. The combined Crystal Spring-Butterfly Trail distance is 5 miles, not the 4.2 miles shown.
Watch out for hikers and yield the trail, smile, and wave. In most cases they will encourage you to pass, possibly thinking that anyone crazy enough to try and ride this trail needs all the help they can get. I have never met anyone on the Crystal Spring Trail, but you never know.
Watch weather conditions. After a heavy rain this trail gets really soft, slick, and seemingly dry roots and rocks will send you sliding down the hill before you know it. This trail is not recommended in rainy weather.
* The loop name reflects the name of the trailhead when this description was first written in the 1990s. The Butterfly Trailhead was called the Soldier Camp Trailhead before all the amenity improvements in the early 21st century.