I have used a bike to excersize dogs - not as regular excersize but as extra conditioning for the show ring. No more than 1/2 to 3/4 mile at a time at a brisk trot. I did buy a "Springer", but never used it! I couldn't get it mounted on my bike where it would not interfere with the pedals. I bought an old one-speed Columbia at an auction just for this activity. With the coaster brakes, you don't have to worry about juggling a leash, the handlebars and hand brakes all at once. Now, mind you, I live in a rural area, where a (temporarily) loose dog isn't usually a disaster. I hold the leash very lightly in one hand, and if the dog pulls, I simply let go. However, I found that they don't generally pull once they get used to the idea that they are working; they will trot along and mostly ignore the distractions. Most important - I would never do forced excersize such as this with a dog under a year old!!
Pauline Merrick Brookfield, MA USA