In this configuration, the axle hub, spindle and trailing arm are combined into a single assembly with the trailing arm attached to the chassis at two mounting points, and these define both the front view instant centre (roll centre) and side view instant centre.

Generally, the semi-trailing arm has a pivot axis angle around 20 to 30 degrees.

If both pivots are on, or very close to, a line at right angles to the vehicle centreline, then this is referred to as a “Trailing arm”. The rear suspension on the original Austin/Morris Mini is a typical example.

If both pivots are on, or very close to, a line parallel to the vehicle centreline, then this is referred to as a “Swing axle”.

Note that the original Volkswagen rear suspension, which is usually called a “Swing axle” is actually more of a “Semi-swing axle” as the pivot axis angle is around 60 degrees. The trailing arm which connects to the torsion bar acts as the outboard pivot, with the driveshaft universal joint acting as the inner pivot.