A snaking single track descending into a shaded backside canyon of the San Gabriel Mountains. Trails varies from fast and flowy to exposed and technical. Scenery begins with soon leading through trees and along rock outcroppings till reaching the end of the ridge spur, where it begins to switchback and drop into what is known as the "Grand Canyon" (At this point there are beautiful vistas of the canyon and surrounding mountain-sides.) At the bottom of the canyon there are groves of trees, a creek and meadows strewn with rocks. The trail here meanders a short way along the canon floor, and then begins a sharp (smooth but steep...with slippery leaf scatter on the trail at certain times of the year, and numerous switch-backs) 800' climb back up to the adjacent ridge, followed by a short 200' descent to the connector trail/ire-road (Mt Wilson Toll Road) which can be followed back down to Altadena.
The Idle Hour trail see's few hikers. it is mostly used as a connector route/loop for Mt Wilson trail runners. Rarely do I encounter more than a 2-3 people on the trail.
The trail conditions vary depending on the amount of maintenance done. A 5" travel bike is sufficient....I have friends that do it on hard-tails. there are some areas of extreme exposure (similar to the middle Sam Merrill trail which you rode in your shuttle article).
The trail is no slouch....and the 800 foot climb back out of the canyon is a grind...but offers some incredible vistas.
On warm days, plenty of water is definitely recommended. because of it being more remote/less used from other trails in the Mt. Lowe/Mt. Wilson trail network, I suggest riding the trail with a wing-man, in case anything goes wrong.