Combine brown sugar, garlic, salt, pepper and if desired, liquid smoke, in a small bowl. Rub over the ribs coating completely. Layer the ribs and onion in a greased 5-quart slow cooker. Pour Coca-cola over the ribs, cover, and cook on low for 8-10 hours or until ribs are fork tender.
Drain liquid from ribs and pour barbecue sauce over ribs and continue cooking for one hour longer. Serve while hot.