On the surface it appears the suspension of a car has a pretty straightforward job. If the bumps end up being less bumpy then it’s all good, right?
In reality, the suspension system has a massive amount to do and the components have to withstand an enormous amount of stress compared to other major systems in a car. The suspension system is located between the frame and the wheels and serves multiple important purposes. Ideally, well-tuned suspension will absorb bumps and other imperfections in the road so the people inside the car can travel comfortably. While this is very important from a passenger’s perspective, the driver will notice certain other attributes of the suspension system. This system also is responsible for keeping the wheels on the ground as much as possible.
The wheels are extremely important for performance and vehicle safety. The wheels are the only part of a car that touches the road. This means they have to put power to the ground and steer simultaneously while also being responsible for stopping the vehicle. Without a system to absorb the bumps and potholes present in roads, the vehicle would shake and wobble over rough surface and make the vehicle essentially unusable from lack of traction. While the suspension system is a great solution for bumpy roads, it does add in a lot of complexity when you consider that the wheels now are responsible for all of their standard duties and now have to travel up and down to absorb shock from bumps while making the vehicle handle like it isn’t on springs and being tossed about with every turn.
This is why the suspension system is very complex. There are many parts involved and a single broken or bent piece can sabotage the whole setup.
How does the suspension system work?
For the most part, modern cars have independent suspension front and back, allowing each wheel to travel independently of the others. Some cars, though, use a more basic beam axle because of lower cost and simpler design. The only beam axles still seen being used in new cars are live axles. Live axles have powered wheel at each end and dead axles have free-spinning tires at each end. The problem with rear tires that don’t move independently is that 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 the handling. Up until the newest iteration, the Ford Mustang utilized a live axle and received heavy criticism for sacrificing performance for nostalgic handling.
Beam axles also contribute unnecessary unsprung weight. Unsprung weight is weight that is not resting on the suspension. Weight resting on the suspension is called sprung weight. Having low unsprung weight compared to the sprung weight makes a car feel lighter and more lively. The opposite provides a harsh ride and a feeling of having less control of the vehicle. If the differential sending power to the wheels via the axles is attached to the frame or body of the car rather than the axle itself, then there is a significant amount less unsprung weight. This is one big reason, aside from the many other advantages to having one wheel able to move without greatly affecting the other wheels, why independent suspension is almost universally adopted by car makers for the front and rear wheels of their vehicles.
Independent front suspension allows each front wheel to travel up and down with the spring and shock absorber bolted to the frame on one end and a control arm or wishbone on the other end. A control arm is attached 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 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.
Independent rear suspension uses the same technology as the front without the consideration taken for the steering dynamics, as the rear wheels usually don’t steer. Rear-wheel and all-wheel drive cars have a differential mounted to the frame in the middle of the control arms or wishbones, while front-wheel drive cars have very simple rear suspension, needing only springs and shock absorbers.
Shock absorbers and springs provide all of the cushioning and compressing when the suspension moves. Springs provide force to hold the sprung weight up off of the wheels and to resist compressing. Shock absorbers are oil-filled cylinders that force the suspension to compress and decompress at a consistent rate to prevent the springs from bouncing up and down. Modern shock absorbers (or shocks) are velocity-sensitive, meaning they are smoother when dealing with light bumps and put up more resistance to big bumps. Think of the springs as guard dogs, ready to viciously defend the car from bumps. The shock absorbers would be the ones holding onto the leashes of the guard dogs, making sure that they don’t take the job too far and do more harm than good.
Many cars, small ones in particular, use MacPherson Struts that sit in the center of a coil spring and act like shock absorbers. This saves space and is lighter as well.
How does the suspension system increase passenger comfort?
When the ride or ride comfort of a car is good, it means that the suspension has good road isolation. The suspension is able to move up and down when needed without 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.
Older luxury cars, American luxury cars to be specific, have a reputation of having such cushy suspension that the driver would feel as if they were driving a boat. This is not optimal, as feeling the road (at least a little) is essential to keeping situational awareness while driving. Sports cars and factory-tuned compacts often are criticized for their poor road isolation. The manufacturers of these cars assume their demographic is looking for fast lap times at a track over road comfort. Also, vehicles traveling at racetrack-speed are getting a lot more downforce from the air that could make comfortable highway-oriented suspension act unpredictably, especially when cornering.
Some possible problems with the body or ride to look out for include:
Body Roll: When the body of the car leans to the outside when cornering. All cars do this to some extent when going around a corner, but if the body of the car rolls too much then the shift in weight can cause the vehicle to spin, steer out of the turn prematurely, or lose traction on one or more wheels.
Bottom Out: When the tires hit the body of the car when the suspension is compressed. This happens when the car doesn’t have enough suspension to absorb the force of the bump it is traveling over. 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 car, but if they are inadequate or missing then this problem can occur. Bottoming out can easily damage the body, wheels, or suspension system.
How does the suspension system help the car stay connected to the road?
A car’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 car needs suspension that doesn’t let the front dive down whenever the brakes are pushed. For smooth acceleration, suspension that prevents the car from squatting down in back when the throttle is opened is required. Shifting weight gives half of the wheels most of the traction, wasting power and causing inconsistent handling characteristics.
As mentioned above, the body rolling too much in the corners is bad for handling. Body roll is also bad because it shifts traction to one side of the car more than the other when rounding a corner. This causes the inside tires to lose traction and possibly leave the road surface. Suspension that provides good road holding will prevent this for the most part.
Some traction problems that may be attributed to a less-than-ideal suspension system arrangement include:
Bump Steer: When hitting a bump causes the car 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.
Oversteer: When the rear of the car loses traction and comes loose rounding a corner. If the body rolls too much cornering then 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 to stick to the road when cornering can also cause this problem.
Understeer: When the front wheels lose traction rounding a corner and cause the vehicle to drift 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 going around corners. Understeer is especially dangerous because front-wheel drive cars steer and provide power with the front wheels. the less traction the front wheels have, the less the car can steer effectively.
Both Oversteer and Understeer are exacerbated by slippery road conditions.
As the the suspension system’s main job is to absorb shock in order to protect the vehicle and its occupants, parts are built to be fairly durable. There are a few other components on modern vehicles that are as over-engineered as those found in the suspension system.
Still, with so much movement and force happening within the suspension, parts inevitably wear out or become damaged. Serious potholes can even bottom the car out so badly that the towers holding the springs in place bend or break.
Squeaking noises usually accompany bushings and other connections failing. If one corner of the vehicle becomes too bouncy when going over bumps, then have the shock absorbers or struts inspected right away. Problems with the suspension should be addressed immediately, so if the handling or shock absorption in a vehicle changes it should be inspected as soon as possible.