With bicycles, as with virtually all wheeled vehicles, the front brake is the more important and effective brake. The front brake by itself will stop a standard bicycle twice as fast as a rear brake by itself. The front brake by itself will stop a standard bicycle as fast as both brakes used together, except on very slippery surfaces.
Unfortunately, many casual cyclists and non-cyclists have the mistaken idea that using the front brake is dangerous, and that you are likely to lock up the front wheel, pitch over the handlebars and crack you skull. This type of accident is extremely rare, and unlikely on a bicycle that is in good repair, ridden by a cyclist who has learned to use the front brake sensitively.
The danger is more real for bicycles with damaged rims, or mis-adjusted brakes. The danger is even greater for the cyclist who habitually relies on the rear brake alone when suddenly faced with the need for a panic stop. A panicky rider who is unused to the front brake may indeed grab it full-force as a last resort, and may take a header.
If you will forgive an automotive analogy in these green pages, a driver who has never driven a car with power brakes is likely to skid a few times the first time he or she tries driving a car that has them. This does not mean that there is something wrong with power brakes, however, it means that the driver needs to learn how to use them.
— Sheldon Brown, in Braking and Turning Your Bicycle
Boom. I kind of want to print this out and hand it round whenever I get shit for having a front brake.
