Originally Posted by cny-bikeman

Even on an upright/mountain bike it's possible to make a hard stop with only the front brake. It's very difficult to make a hard stop with just the rear - as soon as the tire skids you're toast. The trick is to use the amount of force needed, not to just clamp down as hard as you can, and to shift as far back in the saddle and as low down as you can. That way you significantly move the center of gravity down and back. Two brakes are still far better, because the rear brake allows you a bit less pressure on the front, and in extreme stops the rear wheel will tell you by starting to skid when to let off the front a bit. It helps to practice - braking well is a skill.