Trailing arm and Semi-trailing arm suspension

Trailing arms were a big step in the evolution of front suspension. These were first used with a transverse torsion bar or leaf spring. The 1936 Auto Union car used them and was the first car with an independently sprung front. when coil springs began to be used on cars the design of trailing arms improved and carried Aston Martin to the first and only World Sports Car Championship for Britain until 1987.

Some manufacturers of the time did the opposite to trailing arms and used leading arms instead. The problem with leading arms is that they are not suited to angle compression loads and steering movements. The Citroen 2CV utilizes this design of front suspension and anyone who has driven one hard knows about the short comings of this design.

Compared with the following rear suspensions, Trailing arm / Semi-trailing arm suspensions are rather old. It was commonly used in nearly all mid-price to high-price Saloons before multi-link rear suspension became popular in the 1990s. From '82 BMW 3-series to Mercedes 560SEC, and even the Porsche 911, trailing arm / semi-trailing arm suspensions dominated half the world.

Trailing arm suspension (the upper picture) employs two trailing arms which are pivoted to the car body at the arm's front edge. The arm is relatively large compare with other suspensions' control arms because it is in single piece and the upper surface supports the coil spring. It is rigidly fixed to the wheel at the other end.

Note that it only allows the wheel to move up and down to deal with bump. Any lateral movement and camber change (with respect to the car body) is not allowed. Nevertheless, when the car rolls into a corner, the trailing arm rolls to the same degree as the car body, thus changes the camber angle with respect to the road surface. So both wheels lean towards the outside of the corner, thus leading to understeer. Because of this reason, pure trailing arm was forgotten by car makers long long ago. Instead of it, they adopted semi-trailing arm.

Semi-trailing arm suspension has the trailing arm pivoted at inclined angles - about 50 to 70 degrees. Otherwise they are the same as trailing arm suspension. Apparently, the semi-trailing arms are half trailing and half transverse. 

The trailing component leads to understeer, as mentioned. On the other hand, the transverse component  equals  a swing axle suspension. which always introduce oversteer due to body roll. As a result, the two components can cancel each other and result in near neutral steering response.

Semi-trailings disadvantage - when the wheel moves up and down, camber angle changes, unlike double wishbones suspension.

No matter whether semi-trailing arm or pure trailing arm, since they are rigidly attached to the wheels, inevitably more shock and noise could be transferred to the car body, especially under hard cornering or running on bumpy roads. Moreover, a lot of unsprung weight in the trailing arm leads to a poorer ride quality. Therefore most modern Saloons replace it with multi-link or double wishbones suspension. 

Trailing / Semi-trailing arms are disappearing in the Auto industry.

Check out yer old Mini for Trailing arm rear suspension.