I once had a St Bernard come out after me, scary *^%$#!!, I took my Silca pump with a Campagnolo head ( http://rayhosler.wordpress.com/2010/...lo-pump-heads/ ) and stuck the prongs in his nose and twisted, He yelped? (was years ago) ran behind his fence and barked like hell.
A medium sized dog chased me like he htought I wads gonna be lunch on one of my regular training loops, ( Spokane, Tum-Tum, Reardan, Medical Lake WA) in 1980 and I out sprinted hiim, while drawing it out a bit to make him work as I recall. He made me kinda mad so I went back and out sprinted hime a few more times till he decided, "aw the helll wiith it and walked home. I never saw him again