Prepare the grill for indirect cooking over very low heat (225° to 275°F). If using wood chips, soak them in water for at least 30 minutes prior to putting on fire.
In a small bowl stir together all the rub ingredients.
Place the racks, bone side up, on a cutting board. If the racks still have the membrane attached, remove it: Slide the tip of a knife under the membrane covering the back of each rack. Lift and loosen the membrane until you can pry it up, then grab a corner of it with a paper towel and pull it off.
Season the racks on both sides with the rub putting the majority of it on the meaty side. Rub and press it into the meat. Let sit 20 minutes to sweat a bit.
Drain the wood chips and add four handfuls to the grill, following manufacturer's instructions, and close the lid. When the wood begins to smoke, add the rib racks and cook, meaty side up, over indirect very low heat (225° to 275°F), with the lid closed, for 1½ hours.
Add the remaining wood chips to the grill. Cook the racks for 1½ to 3 hours longer. After each hour, rotate the position of each rack so they cook evenly. Keep the temperature of the grill between 225° to 275°F.
The ribs are done when the meat has shrunk back from the bones by at least ½ inch and you can easily slide a skewer through the meat between the bones.
Transfer the racks to a cutting board. Let rest for 5 to 10 minutes. Cut between the bones into individual ribs. Serve warm, and if you prefer, with sauce on the side.
Notes
(Adapted from Weber) Another good test is to lift a rack by picking up one end with tongs. If the rack bends in the middle and the meat tears easily, the ribs are ready.