My biker Dad taught me how to yell at them. Timing is everything. When the dog gets about 10 feet away, yell out a very fierce, low, ferocious "YYYYaaahh!" Begin the yell very suddenly and percussively. Don't be one bit civilized nor lady-like. Translated, the yell should mean, "Get away from me, dog, before I rip your stinkin' gizzard out!'" The dog will invariably jump and reverse his direction! I haven't be bitten yet, nor have I really had to rip out any gizzards. It does make my throat sore for a mile or so!