Peel the grapefruit and oranges, using a very sharp knife, making sure to slice off any white pith under the skin. Slice the fruit in ½ inch thick slices crosswise, then cut each slice into quarters.
In a small bowl combine the shallot, olive oil, vinegar, tarragon, and salt and pepper and stir to combine.
Place the endives and radicchio on a large shallow serving platter, drizzle over half of the dressing and toss lightly. Scatter the citrus over the top, drizzle the rest of the dressing over the salad, and sprinkle the bacon on top.