By leaf springs, I mean a straight beam axle with the hum on the end. Ifor Williams work this way, maximum strength for heavy work. Independants either have a beam axle bolted directly to the chassis for strength with small trailing arms off the end that the hubs are attached to, or separate trailing arms are bolted to the chassis. These units tend to have a + shaped pivot arm, surrounded by rubber inside them to provide the springing rate and damping function. The other reason for independent units is to lower the chassis height, especially for car trailers. They are great on road, but if you tow off road ( diggers, soil etc) when the going gets tough you tend to bend them a bit!