Remove the membrane from underside of ribs: Slide a screwdriver tip along each bone and under one end of membrane to loosen, then grab membrane with a paper towel and pull off (it's okay if membrane breaks and bits remain).
In a small bowl, mix salt, pepper, and cayenne. Put ribs on a baking sheet and sprinkle on both sides with salt mixture. Snugly wrap each rack in heavy-duty foil. Let sit 30 minutes at room temperature.
Meanwhile, prepare a charcoal or gas grill for indirect medium-low heat (300° to 350°; you should be able to hold your hand 5 in. above cooking grate only 6 to 7 seconds). If using charcoal, light 60 briquets in a chimney on firegrate. When coals are covered with ash, about 20 minutes, bank evenly on opposite sides of firegrate and let burn to medium-low. Set a sturdy drip pan on grate between mounds. The area over the drip pan is the indirect heat area. Add 3 or 4 more unlit briquets to each mound when ribs go on and every 30 minutes while cooking. If using gas, turn all burners to high, close lid, and heat 10 minutes. Then turn center burner(s) off and reduce heat of other burner(s) to medium-low. Place a sturdy drip pan under the turned-off burner(s). The area above the drip pan is the indirect heat area.
Place rib packets, bone side down, on cooking grate over indirect heat, overlapping slightly if necessary. Cover grill and cook ribs until fairly tender when pierced through foil, 50 to 70 minutes.
Transfer rib packets to a rimmed pan. Carefully remove ribs from foil. Set ribs, bone side up, on grill over indirect heat.
Spoon about 1/4 cup glaze into a small bowl and set aside. Using a silicone brush, baste ribs with remaining glaze. Cover grill and cook ribs 10 minutes. Brush melted glaze from center of each rack up along sides of meat, turn ribs over, and baste with more glaze. Repeat brushing and turning every 10 minutes until ribs are browned and tender and meat has shrunk back from ends of the bones, 30 to 40 minutes total.
Remove ribs from grill. Cover loosely with foil and let sit about 10 minutes. Stir reserved 1/4 cup glaze and brush over ribs. Cut between bones to serve.