Dissolve sugar and yeast in warm water in large bowl; let stand 5 minutes.
Weigh or lightly spoon flours into dry measuring cups; level with a knife.
Add bread flour, 3 ounces (about 3/4 cup) whole-wheat flour, yogurt, oil, and salt to the yeast mixture; beat with a mixer at medium speed until smooth.
Turn the dough onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining whole-wheat flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover an let rise for 45 minutes or until doubled in size.
Position the oven rack on lowest shelf. Preheat the oven to 500.
Divide dough into 8 portions. Working with one portion at a time, gently roll each into a 5-1/2 inch circle. Place 4 dough circles on each of 2 baking sheets heavily coated with cooking spray. Bake, 1 sheet at a time, at 500 for 8 minutes or until puffed and browned. Cool on a wire rack.