After it reaches the Chantry Flats trailhead, the upper Santa Anita Canyon Road is gated but open to mountain bikes. In fact, the road seems to be a much more popular mountain bike ride than the nearby singletrack. After passing the helipad, the pavement turns to gravel and climbs at a steady grade that isn't too steep. It's a good climb to just gear down and spin and enjoy the views as the trees open up! Once you reach the radio towers at the top, turn around and come down. About 4 miles one way.