In a medium bowl, stir together all of the spices and set aside. Using a sharp knife, cut and pull off the membrane from the back of each rack of ribs. Once the membrane is removed, rub the spice mixture all over the ribs. Place ribs on a baking sheet and wrap tightly with plastic wrap. Refrigerate overnight.
To grill: heat coals or light grill to medium heat. Remove racks from baking sheet and wrap individually in two layers of heavy duty aluminum foil. Place the two foil packets on the grill and cover. Cook 25 minutes, then turn over the foil packets and cook another 25 minutes. At this point, check the ribs for doneness. The meat should be very tender if you poke a knife between the bones. If not quite done, return to the grill for an additional 10-15 minutes. Carefully remove the foil packets from the grill (using oven mitts) and unwrap the ribs. Transfer the ribs to a cutting board and allow the racks to rest for 10 minutes before cutting.