DefyInertia
Former '04 FZ6 Rider
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I personally believe a tire set should wear very nearly evenly, between the front and rear. As others have said, changing speed or direction results in friction, which wears away the tires.
My opinion is that the forces acting on the front and rear are accounted for, in the size of the tires fitted. The rear being twice as wide, it deals with roughly twice as much force. Most of the acceleration should wear the rear. Most of the braking forces should wear the front. Cornering loads should be nearly 50-50.
If you are wearing out rear tires 2:1 compared to your front(s), I suspect you are doing most of your braking with the back tire, rather than the front.