When most people think of automotive performance, they think of power, the need for speed, the roaring sound of the engine, and how fast the vehicle will go from zero to 60 miles per hour. However, all this power and speed is useless if the driver is unable to control the vehicle and is not comfortable while driving. Therefore, the automotive suspension is a crucial vehicle system.
The primary functions of the suspension system include maximizing the contact between the tires and the road surface, providing steering stability and good handling, evenly supporting the weight of the vehicle (including the frame, engine, and body), and ensuring the comfort of passengers by absorbing and dampening shock. Your vehicle’s suspension system works hard to withstand a considerable amount of stress compared to other major vehicle systems.
The suspension system consists of tires, the air in the tires, springs, shock absorbers, struts, arms, bars, linkages, bushings, and joints. The suspension system components are located between the frame of the vehicle and the road. Well-tuned suspensions will absorb bumps and other imperfections in the road allowing the people inside the vehicle to travel safely and comfortably.
The tires and the amount of air in the tires are a fundamental part of the suspension system. The tires are the only part of the vehicle that come in contact with the road. This means they have to steer and put power to the ground simultaneously while also being responsible for stopping the vehicle. The suspension system requires the wheels and tires to travel up and down to absorb the shock from bumps. The rubber tires and air in the tires also soften the ride over hard surfaces and conform to slightly uneven and rough surfaces.
Shock absorbers, also known as shocks, are hydraulic oil-filled cylinders that force the suspension to compress and decompress at a consistent rate to prevent the springs and vehicle from bouncing up and down. Shocks are velocity-sensitive, meaning they are smoother when dealing with light bumps and put up more resistance to big bumps. The primary purpose of the shock absorbers is to control spring and suspension movement and make sure tires maintain contact with the road.
Many vehicles use struts, which are similar to shocks, that sit in the center of a coil spring. Strut assemblies consist of a coil spring to support the vehicle’s weight, a strut housing to provide rigid structural support for the assembly, and a strut cartridge within the strut housing and spring to control spring and suspension movement and make sure tires maintain contact with the road. A strut assembly is a major structural part of a suspension. It takes the place of the upper control arm, upper ball joint, and shock used in conventional suspension systems.
Struts perform a damping function like shocks. Internally, a strut cartridge is similar to a shock. A strut assembly provides structural support for the vehicle suspension, supports the spring, holds the wheel and tire in the aligned position, and ensures the tire maintains contact with the road. Struts also bear much of the side load placed on a vehicle’s suspension. As a result, strut assemblies affect riding comfort and handling as well as vehicle control, braking, steering, wheel alignment, and wear on other suspension components and the tires.
Most modern vehicles have an independent suspension on the front and back allowing each wheel to travel independently of the others. Some vehicles use a more basic beam axle. The only beam axles still being used in new vehicles are live axles. A live axle is one that supports part of the weight of a vehicle and drives the wheels connected to it. The problem with rear tires that don’t move independently is they always keep the same angle relative to one-another rather than relative to the road surface. This means less traction and less predictability in handling. This is one reason why an independent suspension is almost universally adopted by automobile manufacturers for the front and rear wheels on new vehicles.
An independent front suspension allows each front wheel to travel up and down with the spring and strut assembly bolted to the frame on one end and a control arm or wishbone on the other end. A control arm is attached to the front of the vehicle near the center at one end of the arm and the steering knuckle at the other. A wishbone does the same thing except it attaches to the frame at two points, causing the piece to resemble a wishbone. The movement at connection points is softened and absorbed by bushings. The positioning of every component in independent front suspension systems is very important as the front wheels have to steer and maintain consistent alignment to provide safe vehicle operation.
An independent rear suspension uses the same technology as the front without consideration taken for steering dynamics, as the rear wheels usually do not steer. Rear-wheel and all-wheel drive vehicles have a differential mounted to the frame in the middle of the control arms or wishbones, while front-wheel drive vehicles have a very simple rear suspension, needing only springs and shock absorbers. Shock absorbers and springs provide all the cushioning and compressing when the suspension moves. Springs provide force to hold the sprung weight up off the wheels and to resist compressing.
When the ride of a vehicle is comfortable, it means the suspension has good road isolation. The suspension is able to move up and down when needed without excessively jarring the vehicle. Just enough feeling from the road reaches the driver, so they will know of any alarming road conditions and feel a rumble strip if they enter the shoulder of a high-speed road. The feeling of the road is essential to keeping situational awareness while driving.
Body roll occurs when the body of the vehicle leans to the outside too much when cornering. All vehicles have some body roll when going around a corner, but if the body rolls too much, the shift in weight can cause the vehicle to lose traction on one or more wheels, steer out of the turn prematurely, or the vehicle to spin out of control. If the body begins to roll too much when cornering, the handling will be negatively affected resulting in a shift of the traction to one side of the vehicle more than the other. This causes the inside tires to lose traction and possibly leave the road surface. Suspensions that provide good road holding will, for the most part, help prevent this.
Bottoming out happens when the tires hit the body of the vehicle when the suspension is compressed. This happens when the vehicle doesn’t have enough suspension to absorb the force of the bump it is traveling over. Rubber bump-stops can prevent this by providing a cushion between the suspension and the frame that prevents the tire from moving up high enough to strike the body of the vehicle, but if the bump-stops are inadequate or missing, then this problem can occur. Bottoming out can easily damage the body or suspension system.
A vehicle’s road holding ability is measured by how well the vehicle can maintain good traction and even weight distribution when different forces are involved. To feel stable when stopping, a vehicle needs a suspension that will not let the front dive down when the brake pedal is depressed. For smooth acceleration, a suspension that prevents the vehicle from squatting down in back when accelerating is required. Shifting weight gives half of the wheels most of the traction, wastes power, and results in poor and inconsistent handling characteristics.
A traction problem called bump steer occurs when hitting a bump causes the vehicle to turn left or right without the driver turning the wheel. Poor alignment of the suspension can cause the wheels to be angled in a way that causes this issue.
A traction problem called oversteer occurs when the rear of the vehicle loses traction when rounding a corner. If the body rolls too much when cornering, the shift in weight may cause the rear wheels to lose traction. Having the rear wheels at an angle that doesn’t sufficiently allow the tire tread to meet the road when cornering can also cause this problem.
Understeer is another traction problem. This occurs when the front wheels lose traction when cornering and results in the vehicle drifting towards the outside of the turn. Similar to oversteer, excessive body roll or improperly angled wheels can cause the front wheels to have poor traction when cornering. Understeer is especially dangerous because front-wheel drive vehicles steer and provide power with the front wheels. The less traction the front wheels have, the less the vehicle can steer effectively. Both oversteer and understeer are amplified by slippery road conditions.
Technology is constantly improving suspension systems and addressing issues mentioned in this article. There are passive and active suspension systems. Passive systems are what most of us are accustom to and suspension movement is determined entirely by the road surface. Active systems proactively control vertical movement of the wheels relative to the vehicle frame and body with an onboard computer system. Active systems can be divided into two classes: pure active suspensions and adaptive/semi-active suspensions. While adaptive/semi-active suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions also use some type of actuator to raise and lower the chassis independently at each wheel.
Active suspension system technologies allow vehicle manufacturers to achieve a greater degree of ride quality and handling by keeping tires perpendicular to the road in corners, allowing better traction and control. An onboard computer detects body movement from sensors throughout the vehicle and controls the action of the suspension. This high-tech system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating, and braking.
There are few other components on modern vehicles as complex as the suspension system. A great deal of work goes into creating the suspension system which must be durable enough to ensure ride quality and maintain the handling of the vehicle, two elements that work at odds against one another, while simultaneously withstanding an enormous amount of stress. Still, with so much movement and force happening within the suspension system, parts inevitably wear out or become damaged. Noises are one of the first signs of suspension issues and commonly accompany the failing of bushings and other connections. Serious potholes can even cause the vehicle to bottom out so badly that suspension components can bend or break.
If you are experiencing squeaks when going over bumps or dips, unusual clunking or rattling noises while driving on uneven roads or over potholes, oversteer traction problems, understeer traction problems, bump steer traction problems, excessive body roll, bottoming out, excessive bouncing when going over bumps, leaking shocks or struts, or the handling of your vehicle just does not feel right, have your suspension system inspected right away by an ASE certified technician.