Desired Features of Suspension Systems

A suspension system should satisfy certain requirements for use in vehicles. The main desired features are as follows.

• Independency: It is desirable to have the movement of a wheel on one side of the axle to be independent from the movement of the wheel on the other side of the axle. Figure 1.2 (top) shows a vehicle with its left wheel going over a bump. At higher speeds, the wheel can negotiate the bump without disturbing the other wheel. This is only possible when each wheel has an independent suspension. Independency of wheel movement improves a vehicle’s ride comfort, road holding, and handling.

• Good camber control: The camber angle is the wheel angle about its longitudinal axis (this will be comprehensively discussed in Section 4.1.2). A negative camber is desired since it results in improved handling, however, the convex shape of roads tends toward a positive camber to reduce tire wear. Due to road bump and body roll, the camber will ultimately change. Using a well-designed suspension geometry, we can control the camber angle (Figures 1.3 and 1.4)

• Good body roll control: Each suspension system has a roll center. The hypostatical line that connects the front and rear suspensions’ roll centers is called the roll axis. The vehicle’s body rolls about this line during cornering maneuvers. It is necessary to analyze the roll axis because of its effect on the body roll and lateral vehicle behavior. The design of the suspension geometry should account for the best location of the roll axis in order to optimize vehicle body roll motion.

• Good space efficiency: It comes as no surprise that the space utilized by a suspension system may create difficulties for the installation of other components of the vehicle. Under the hood, the suspension system should leave enough room for an engine and other components. Also, the suspension of the rear axle should not interfere with the vehicle trunk and, instead, occupy only its internal space (Figure 1.5).

• Good structural efficiency: The suspension system should be able to handle the vehicle’s weight and all applied forces and moments in the contact area between the wheel and the road. The suspension mechanism must feed loads into the body in a well distributed manner and prevent the transfer of concentrated forces onto the vehicle’s body (Figure 1.6).

• Good isolation: Improving ride quality and isolating road roughness is one of the most important tasks of a suspension system.

• Low weight: Due to the road irregularities, the kinematic energy of a suspension system is proportional to its mass. Higher kinematic energy results in stronger transmitted shocks to the vehicle body. This effect clearly decreases the ride quality. To minimize this negative effect, we should minimize the suspension mass by using optimized designs and/or lightweight material. Lightweight materials may increase the cost and, therefore, a balanced design is needed for any suspension system.

• Long life: No one enjoys having to repair their car frequently, so the suspension must be as durable as any other part of a car. A durable system is able to resist wear, pressure, or damage, all of which play important roles in the success of a product.

• Low cost: While defining a low enough cost is a subjective matter, the suspension as a vehicle sub-system should be affordable. High performance suspension systems are more expensive, and mainly used in premium vehicles. Using a high number of bushings and lightweight materials certainly improves the ride quality, noise isolation, and performance of the system, but they also increase the cost of the product.

• Others: Other suspension features may include anti-dives and anti-squats. When a vehicle is breaking, a dive occurs where the front of the vehicle dips and the tail rises. A similar but opposite action, a squat, happens during acceleration. This rotational movement is slight but since the human body is very sensitive to pitch motion, mitigating for this movement in the passenger cabin allows for greater ride quality.