Originally Posted by mihlbach

Watch another rider in the rain and note where the water flys off the wheels and at what angle. You will quickly realize that partial fenders are basically worthless. Only a full fender, that comes down at least 90 degrees down from the top of the rear wheel, will effectively prevent water from spraying onto your back. The fender has to come down ever further than 90 degrees to prevent spraying a following cyclist. Likewise, the front fender has to come down much more than 90 degrees to prevent spray from hitting your shoes and drivetrain. In these cases, long mud flaps do the trick.