The stress on a rear motorcycle tire comes from acceleration. A rear tire is manufactured so the tire overlapping accommodates that stress. Conversely, the stress on a front motorcycle tire comes from braking and it is manufactured with those stresses in mind. So, if you mount a rear tire on the front of a motorcycle without reversing it, that tire will experience stresses that are opposite of what it was manufactured for. Probably not something that will be noticed by most of us recreational riders..... Jim