Dissolve yeast and sugar in warm water in a large bowl. Let sit until creamy, about 10 minutes.

Stir whole wheat flour, oil, and salt into yeast mixture, then stir in 4 cups bread flour. Add remaining 1 1/2 cups bread flour, 1/2 cup at a time, stirring well after each addition. When dough pulls together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes.

Lightly oil a large mixing bowl. Place dough into the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in size, about 1 hour.