Re: overslung vs. underslung
"Underslung" = when supported from below. The support definition applies to the springs -- not the axle. When supported from below that's an under slung axle. Springs are under the axle. The reason you can't just turn the axle over to work both ways is that axles have camber (a slight bend) put in them so the wheels are vertical when loaded. If you flipped the axle the camber would be in the wrong direction. Be carerful when looking at axle pictures. Manufacturers tend to show one type. Even though they say its an underslung, they may show an axle with the spring perches on top of the axle.
