In this section, we will take a look at the last type of automobile suspension, which is usually used for rear wheels.
In the H-arm system, one end of the arm is connecting to two locations of the car body; another end connects to two locations of the wheel hub. The H-arm alone can limit 4 degrees of freedom, so it need another horizontal control arm to limit one more degree to make the whole system with 1 degree of freedom.
However the above design has non-optimal wheel movement trajectory, most automaker will not use such design without modifications. On the other hand, with proper design enhancements which creates variations of the H-arm system, superior handling performance can be achieved. In fact some of the luxury cars and high-end exotic sports cars use such design.
For example Audi changes one of the wheel hub – H-arm connections with flexible joint, which releases one degree of freedom (makes the H-arm limits 3 degrees of freedom), and compensate it by one extra control arm. See the below image for reference.
BMW use another approach, by using an intermediate control arm to connect one location of the wheel hub to the H-arm, which releases one degree of freedom; and at the same time place another control arm to gain control to the released degree of freedom. Below is the rear suspension of the 2012 BMW M5.
Even Ferrari is using the similar design to BMW, see how the Ferrari FF’s rear suspension is constructed.
Not only luxury cars are using the H-arm suspension. Mainstream products such as Ford’s Fusion and the latest generation of Mustang, are all using the H-arm suspension for the rear wheels.
In this series of articles, we reviewed 4 major types of independent suspension. Of course, there are many types of suspension which are not covered in these articles (for example semi-independent type, and live axle suspensions) ; but anyway, no matter what type of a suspension belongs to, they all share one common property and requirement: limit 5 of the 6 degrees of freedom. I hope my article can help you understand more facts of your car.