Types of Non-Independent Suspensions
In order to explain the basics of how automotive suspensions work, I’ve split up this topic into two parts. In this first article, I will only explain how non-independent suspensions work. This is meant to cover the basics of how these systems operate and why they exist. This is by no means a comprehensive guide.
What Is A Non-Independent Suspension?
A non-independent suspension is a system which links two wheels together. When there is a force on one wheel, the other wheel will be affected in some way. This often comes in the form of camber changes while the opposite wheel lifts. Many SUV’s, trucks, and utility vehicles use this type of suspension for its simplicity and durability. However, some modern sports cars, like the Ford Mustang, also use non-independent suspensions for increased traction in a straight line due to a lack of camber change under rebound. It's important to note that almost all non-independent suspensions will suffer from large jacking forces due to a high roll center.
The Purpose of Suspension
Before discussing any one type of suspension design, it’s important to understand the basic purpose of a car’s suspension. When a car travels over a rough or bumpy surface the wheels must travel up and down to maintain traction and to avoid transferring forces through the car. As you can imagine, there are many different methods of moving wheels up and down, but there are a few common components in each suspension design. Springs of some sort are used to control the vertical motion of the wheels. Without springs, the car would just squat and our suspension linkage wouldn’t do much. Springs constrain the wheel’s motion in both the up and down directions to maintain control whether the tires are loaded or unloaded. Shocks or dampers are used in conjunction with springs to prevent vibrations from disrupting vehicle dynamics. Imagine what would happen if you drop a spring on the floor. It would continue to bounce and vibrate long after its initial compression. Shocks keep the springs from vibrating out of control and prevent our suspension from bouncing around.
Solid Axle With Leaf Springs
In this type of design, the axle is attached to the center of leaf springs. As shown in the figure, the other two ends of the leaf springs are attached to the chassis. Shocks are also attached to both the axle and the chassis. This design is simple and cheap, making it a great choice for utility vehicles or low budget projects. However, the leaf springs are performing double duty in that they also serve as location points for the axle. This sort of suspension suffers from lateral movement of the axle and, as a result, compromised ride comfort and traction.
Solid Axle With Coil Springs
This type of suspension design is very similar to a solid axle with leaf springs. The primary difference here is that leaf springs are replaced with coil springs. This frees the springs from acting as locating links. Instead, control arms are used to locate the axle and limit its motion. There are many different variations of control arm design and spring/shock placement. A few of the most common designs are discussed below.
The term multi-link refers to the control arms and other linkages used to locate the axle. The most common multi-link setup with solid axles is a 4 link. This uses two lower control arms and two upper control arms to locate the axle. If the upper control arms run parallel to each other, than a separate linkage, called a Panhard bar, must be used to locate the axle in the lateral direction. Alternatively, the two upper control arms can mount towards the center of the axle and run at an angle from each other. This is called a triangulated four-link. Such a design effectively locates the axle without need for a separate Panhard bar.
Instead of using a Panhard bar, some designs use a Watt’s linkage. This is a separate four-bar linkage used in conjunction with the control arms, making the total design a six-bar linkage. The Watt’s linkage is generally considered superior to the Panhard bar because it approximates straight-line motion more accurately (four-bar linkages generally travel on a very wide arc, not an actual line). A Watt’s linkage also locates the axle from its center, rather than the ends like in a Panhard design.
Other designs also use only three links. In a three-link suspension, there is one upper control arm and two lower control arms. With this design a separate linkage must also be used to locate the axle in the lateral direction. However, the reduced number of control arms decreases weight and may make the system easier to package.
De Dion Suspension
A suspension utilizing a de Dion tube can be based on any variation of the multi-link solid designs described above. A de Dion suspension differs from using a de Dion tube or dead axle to keep the wheels parallel in place of a solid axle. This is generally used to mount a differential between the wheels to allow for independent wheel speeds. In this design, the differential would be mounted to the chassis and would attach to the wheels via half shafts and CV joints. The de Dion tube would wrap around the differential and mount to each upright. This design is superior to a solid axle with a differential because the differential’s weight is transferred to the chassis rather than the suspension. Without the de Dion tube the differential would move up and down with the axle, adding significantly more unsprung weight. However, since the de Dion suspension requires two half shafts, two cv joints, and a dead axle, overall weight, complexity, and cost are increased.
Suspension dynamics are complicated, but everything boils down to simple linkages. By thinking about how solid axles move in relation to our car, we can get a grasp on a how suspension affects vehicle dynamics. Now that we’ve covered how the most simple suspension designs operate, we can begin to think about how fully independent suspensions work. Look for an upcoming article on this topic.