FATMAP difficulty grade
Unlike PMT, Onza Alley is solidly in the single back diamond range—as long as you don't drop into the uber-gnarly drop on Bailey's Bailout! Onza Alley falls between two blue trails on the mountain, allowing a faster and slightly more challenging way down when compared to nearby Lion Down.
For upper intermediates looking to push the boundary, Onza Alley could be just the ticket. Vail Mountain describes Onza Alley as "a difficult singletrack with narrow, steep sections over roots and rocks." Forget the flow trail—Onza keeps it old school! Source: https://www.vail.com/explore-the-resort/activities-and-events/summer-activities/mountain-biking.aspx