Rim brakes for bikes use the sidewall of the rim to reduce speed, this is done by a pad on either side applying friction against the rim, which reduces your momentum. Road brake callipers have been redesigned over the years and now use a system called 'dual pivot' which means the brake arms pivot off centre to apply a higher amount of pressure on the rim to increase your stopping power. Most manufacturers use dual pivot callipers on the front and rear, but some Campagnolo brakes use a traditional single pivot on the rear to 'balance' the braking power, the logic behind this is that as you brake your weight shifts on to the front of the bike and as much as 75% of the braking force is applied to the front.