Peel cucumbers. Using a mandolin, a food processor with the thin slicer attachment, or the slicer opening on a cheese grater, slice the cucumbers very thinly.
Add vinegar, sugar, salt and pepper in a serving bowl and whisk until the sugar dissolves completely.
Add the sliced cucumber, red onion rings and chopped dill, and toss well. Let sit and marinade for at least 20 minutes.