About one hour before smoking, make the dry rub. Combine chili powder, salt, garlic powder and pepper in small bowl. Set aside 2 tablespoons of dry rub. Pat ribs dry with paper towels; sprinkle both sides of each rack with the remaining rub. Set aside at room temperature for 1 hour.
For the mop, whisk together vinegar, water and reserved 2 tablespoons chili rub in a bowl. Set aside.
Heat charcoal or gas grill for indirect cooking at 250 to 300 degrees F. Add drained wood chips, if using.
Place the ribs, meaty side down, over a drip pan and cook for 1 hour. Add about 12 coals to a charcoal grill every 45 minutes or so to maintain the heat.
Continue cooking 4 1/2 to 5 1/2 hours more over indirect heat, until very tender. Brush both sides of ribs with mop mixture every 45 minutes. Add more coals as needed to maintain temperature for a charcoal grill.
Brush ribs with barbecue sauce. Continue cooking for about 5 minutes, until the sauce is set. Turn ribs and repeat with more sauce.
Cut the racks into 3- or 4-rib sections and serve.