This trail is located east of Moab. And in this area I can only recommend to get the Charles A. Wells Guide.
Porcupine Rim Trail is in the Sand Flats area which is a fee area. The fee right now is $6- for day use and $8- including camping. The Sand Flats Area features a few 4x4 trails as well as mountain bike trails. Most of the time the 4-wheel and mountain bike trails are separate but sometimes they cross each other.

We drive east on Sand Flats Road to get to the trail head of Porcupine Rim Trail. This trail is one of the few that combines 4-wheelers and mountain bikers. It starts out rocky and the road follows a fairly narrow ridge. Along the way are plenty of rock shelves to conquer and some boulders to overcome. It is a moderate trail and like all trails in the Moab area it is very scenic. After about three miles there is an overlook on the right with incredible views of Castle Valley and the La Sal Mountains. Further on the trail we meet the rim several times before the trail gets so narrow that it only allows bicyclists to continue. This is where we turn around.

Although this trail is not very challenging the views are definitely worth the trip.
Be considerate to bicyclists (they have the right-of-way) and PLEASE, stay on the trails. This is true for all trails. Even if it might not seem this way, there is life (micro organisms) in the desert soil which is crucial to plant and animal development. If you drive on it you kill it.

Check out the 4x4 Gallery

Directions: From Main Street in Moab go east on Center Street to 400 East Street. Turn right and go south. Turn left on Millcreek Drive and go straight through the Stop sign. Follow Sand Flats Road to the Sand Flats Recreation Area. Take this road through the park to the sign for the Porcupine Rim Trail on the left.