I'll throw another idea in here - steam...
I know its nothing new, but after steaming my wheels & arches this weekend and achieving outstanding results with just a small amount of water, NO chemicals and nothing hitting the floor beyond a few drips of condensation, I think this is another way to compliment other wash methods. I know some (me too) sometimes struggle slightly with ONR and wheels/undercarriage, and I shall certainly be cracking out the steamer on the tougher jobs I face in this area