Shuttle or 18-mile loop. Season March through early December.
It can be done as an 18-mile loop, or as a 6-plus mile shuttled downhill. Top altitude is 5100 feet, lowest 3100. It's a fun and fast downhill route, suitable for either downhill-specific bikes or light cross-country bikes.
The Icehouse Trail itself is 6 miles of almost non-stop downhill, descending 2000 vertical feet. There are occasional very short gentle climbs to keep you honest. Intermittent pumice boulders situated in the trail surface raise the overall technical level to upper-intermediate. The final 1/3-mile plunge off the mesa is advanced technical and requires good skills.
Those who want to earn their vertical -- or who don't have a shuttle -- can ride up Cottonwood Road for an 18-mile loop. The 2000 vertical of climbing occurs over 10 miles (with some up-and-down added) on graded dirt road, so it's basically just a long cruise to the upper trailhead. After the first mile, the Cottonwood Road surface turns to dirt. It becomes more narrow and rough after passing the Broken Mesa trailhead.
Make sure you have loaded the trail gps track on your device.... The start is very difficult to find, and there are no signs above the Broken Mesa/Icehouse trail junction. It is also very easy to lose the trail in the upper sections. Beware that this appears to be a user-created trail, and mostly goes directly down the fall line, which means that it has become a watercourse and is now (in 2016) a rutted rocky experience. This trail would be greatly improved and more sustainable with reroutes off the fall line, which would be a huge project.