Preheat your grill to medium-high heat.
In a small bowl, combine the brown sugar, Worcestershire sauce, soy sauce, apple cider vinegar, minced garlic, Dijon mustard, paprika, salt, black pepper, and cayenne pepper. Stir until well combined.
Rub the spice mixture all over the racks of ribs, covering them completely.
Place the racks of ribs on the grill, bone side down. Cook for about 1 hour, or until the ribs are tender and cooked through, turning occasionally.
During the last 10 minutes of cooking, brush the ribs with the glaze mixture, turning and brushing every few minutes to create a sticky glaze.
Remove the ribs from the grill and let them rest for a few minutes before serving.
Serve the glazed ribs hot with any remaining glaze on the side.