My two cents; The friction force and the normal force are both somewhat proportional to the contact area however the cof of friction is not. That is what I beleive KENAT's high school experiments proved. Think in terms of the units
Another thing to consider is that with an 8 groove pully you should be useing a matched set of belts. Other wise the slight differance in belt lenght cause both vibration and lower belt life. I also agree fully with the other guys the pully needs a qd or eq bushing and a tight fitting key
