This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Arnie Berger <email@example.com>
There are varying degrees of defense against dogs.
1- Shout "NO!" as loud and authoritatively as you can. That works more
than half the time against most dogs that consider chasing you just
2- Get away from their territory as fast as you can.
3- A water bottle squirt sometimes startles them.
4- If you're willing to sacifice your pump, whump'em on the head when they
come in range.
If they're waiting for you in the road and all you can see are teeth
then you in a heap o' trouble. In those situations, I've turned around,
slowly, not staring at the dog, and rode away. When I have been in a stand
off situation, I keep the bike between me and the dog.
"Halt" works pretty well, and I've used it at times. It's range is about 8
I bought a "DAZER", from Heathkit. Its a small ultrasonic sound generator
that you point at the dog. My wife and I were tandeming on a back road and
used it on a mildly aggressive German Shephard. It seemed to cause the
dog to back off.
By far, without a doubt, hands down winner, is a squirt bottle full of
reagent grade ammonia, fresh out of the jug. The kind that fumes when
you remove the cap. When I lived in Illinois I had a big, mean dog that
put its cross-hairs on my leg whenever I went by. After talking to the
owner (redneck), I bought a handebar mount for a water bottle and loaded
it with a lab squirt bottle of the above mentioned fluid. Just as the
dog came alongside, I squirted him on his nose, eyes and mouth. The dog
stopped dead in his tracks and started to roll around in the street.
Although I continued to see that dog on my way to and from work, he
never bothered me again.
Finally, you can usually intimidate the most aggressive dog if there are
more than one of you. Stopping, getting off your bikes and moving towards
it will often cause it to back off. ( But not always ). My bottom line
is to alway ride routes that I'm not familiar with, with someone else.
As last resort, a nice compact, snubbed nose .25 caliber pistol will fit
comfortably in your jersey pocket. :-)