Make shallow cuts between the ribs on the bone side with a sharp knife, piercing just the membrane.
Mix 1 tbls salt, the brown sugar, garlic, thyme, chili powder, 1/4 tsp black pepper and the cayenne in a bowl. Rub the spice mixture evenly over the ribs. Place the ribs on a rack in a roasting pan, loosely cover and refrigerate at least 4 hours or overnight.
Meanwhile, combine the cider, ketchup, mustard and worcestershire sauce in a saucepan over medium heat and simmer until reduced to about 2 cups, abotu 30 minutes. Cover and set aside.
Preheat grill to low heat on one side. Cook the ribs bone-side down on the cooler side of the grill, covered, until tender, about 2 hours and 30 minutes. Continue to cook, uncovering every 10 minutes to baste with sauce, until the ribs are glazed, about 40 more minutes. Remove from frill and cut into pieces.