http://www.sheldonbrown.com/cantilever-geometry.html
V-brakes require more cable pull than standard cantilever brakes, so if you are using road brake levers, you should use cantilever brakes or V-brakes with a travel agent (an inline adaptor which doubles the amount of cable pull by use of concentric pulleys). V-brakes without the travel agent or long-pull levers requires setting the brake pads too close to the rim and also changes the braking characteristics.