In small bowl, combine mayonnaise, lime juice, chili powder, cayenne and salt. Set aside.
Spread 1/4 of the mayonnaise mixture over bottom half of each tortilla within 1 inch from edges. Divide roast beef, bacon, avocado, lettuce and onion evenly over bottom half of tortillas. Tuck in sides of tortillas; roll up tightly to enclose filling. Cut in half to serve.