In a small jar, combine cumin, chilli powder, paprika, salt and pepper. Close the lid and shake to mix.
Trim the membrane sheath from the back of each rack. Run a small, sharp knife between the membrane and each rib then snip off the membrane as much as possible.
Sprinkle as much of the spice mixture onto both sides of the ribs as desired. To prevent the ribs from becoming too dark and spicy, do not rub the spices into the ribs. Store any unused spice mix for future use.
Place aluminium foil on lower part to capture the dripping and prevent flare-ups, and lay ribs on the upper shelf of barbecue if possible. Reduce heat to low, close lid, and leave undisturbed for 1 hour. Do not lift lid at all.
Brush ribs with desired amount of barbecue sauce, and cook an additional 5 minutes. Serve ribs as whole rack or cut between each rib bone and pile on a serving platter.