Do like Ceasar Millan the Dog Whisperer... Turn your shoulders toward the dog and shout.. SSSHHHHHHHHT!!!! NO!!.... Really really loud in a deep commanding voice. Then Growl back at them "Aaaaaaarrrrrggh!!" when they startle.

That worked for me this weekend when we were chased by a large spaniel that came bounding across a lawn/ditch/and was about to crash into us. The loud SSSHHHHHHHHT!!!! stopped him in for a second and then the Aaaaaaarrrrrggh!!! kept him there (looking rather confused I might add) while we pedaled safely away

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