Originally Posted by bikesatwork

At a minimum, the distance from the front of the trailer to the axle should be at least half the length of the load. This will put the center-of-mass of your cargo ahead of the axle. So, for an 8' load, you want a front-to-axle distance at least 4' long. For a 12' load, at least 6' long.

Below is a picture carrying an extension ladder a little over 12' in length on a B@W #96A trailer. Note how the axle is behind the center of the ladder.



If the distance is too short, and if you load is heavy, the trailer will be back-heavy and lift your bike in the air when you get off. A back-heavy trailer also reduces your traction.