there are several brands of bear repellent on the market now. REI carries one at about $40.00 a can.
Range according to the label 30 ft. Garunteed (did I spell that rgiht?) to stop a grizzly bear. Label also warns that it is not for use on humans. Supposedly it can cause death due to inflamation of mucus membranes. Here is the link http://www.counterassault.com/Bear_D...deterrent.html
IF IT WILL STOP A GRIZZLY most likely it will stop a dog four legged or other.
Mine is velcroed to my top tube, just in case a grizz wanders into town on my daily commute. Grizzly bears can sprint at 30 mph+ and I doubt I could out ride one.
But seriously as has been suggested before, at what point in time do you take defensive action ? That is a tough one. My daily commute takes me through a bad hood into a really bad hood. I do not stop at lights, standing still you are a target.