Actually, I studied trucks and other cargo vehicles of various types for this commission. Take a look at the next truck that goes by.... flatbed, cube, semi-trailer, etc. The rear axle(s) will be behind the middle of the loaded area.

There are several design reasons I can think of why Xtracycle puts their rack over the rear axle. One primary consideration is shipping weight and size. Moving the load forward entails making the extension kit even longer, and thus harder to ship. Moreover, because it is an addition to the existing bike, the designer cannot be certain exactly if the bike frame being modified is even strong enough to carry extra weight. By putting all the cargo weight on the rear axle, Xtracycle guarantees that any bike frame can be used, regardless if it is a Bianchi or a Flying Pigeon. Finally, the load itself is almost never large enough to affect the handling of the overall vehicle, especially if using the Wideloader wings to carry the weight down low. IME, only loads in excess of 100 lbs negatively affect the handling of the typical longtail with wings, and only when moving slower than 2 mph. If I had to guess, I'd say that the load has to reach 200 lbs before the bike requires constant attention while in motion. For what it is supposed to do, it is a good design.

:)ensen.