My apologies, I was referring to the Poison Spider\Portal loop. Poison Spider is often ridden as an out and back from the Dinosaur Tracks trailhead. You can catch the Portal trail at the top of the rim near the turn around. The Portal is a 4 mile section of singletrack that decents from the Rim to the Colorado River and Potash Rd. From there, it is a short and easy road petal to the dinosaur tracks trailhead. I would not recommend ridinge the Portal as an out and back from Potash.
It should be noted that the Portal trail is on a narrow ledge and very exposed in places. Mountain bikers have lost their lives on this trail. At the most exposed portion of the trail there is an obvious sign warning you and reminding you of past tragedy. Even for the most skilled bikers, the penalty for error is too steep.
You can also combine Gold Bar Rim and Portal.