21 Oct Education Station: Suspension
Welcome to the third installment of Education Station — the suspension system!
The suspension system’s main purpose is threefold – it’s meant to maximize the friction between the tires and road, to provide stability in steering and handling, and to minimize the discomfort of the passengers. Since the inception of the road-going vehicle, there have been many different types of suspension systems, but for simplicity’s sake, we’ll be discussing the most popular type of suspension.
The two most common types of suspensions on cars today are based on coil springs and air. The two less common types, torsion bars and leaf springs, have been phased out over the years; while torsion bars have gone the way of the dodo bird, leaf springs still exist on some trucks and heavy-duty vehicles. Air-based suspension systems use the compressive qualities of air to provide stability, and can be seen most readily on higher-end cars with on-the-fly adjustable ride heights.
The coil spring suspension is based on the physics behind – you guessed it – springs and their ability to compress and expand. The suspension system relies on several major components to keep everybody stable and comfortable – the frames, the springs, the shock absorbers, and the various other parts that connect in between the first three. Essentially, the vertical movement of the wheel has its motion transferred into energy, which moves through the spring and is dampened by the shock absorber and its various connecting parts before transmitting the remaining energy to the frame (and the vehicle as a whole).
The frame is the principal load-carrying structure of the vehicle. The engine, passenger compartment, and the body as a whole, rest on the frame. It’s definitely the strongest point on the car, and for good reason; were the frame unable to support the thousands of pounds of car on top of it, nobody would be going anywhere.
If the suspension was based on a solid, rigid connection to the wheels and tires, every bump and small inconsistency in the road would transmit its energy through the wheels and directly to the frame. The result would be a terribly bumpy ride, causing large amounts of distress to the frame, contributing to an awfully sore back and a quickly deteriorating vehicle. Were the frame not rigid, movement would be transferred in all sorts of directions and it would essentially equate to driving on a waterbed, the physics of which wouldn’t generate any stability whatsoever. Therefore, to make a happy medium exist, the springs, shocks and other suspension components take some of the load off, pun intended.
Broken down to its most simple terms, the spring is essentially a torsion bar (a bar built for the purpose of flexing and transferring energy) coiled around an axis, which in motor vehicles is a vertical axis. The spring’s main job is to absorb the vertical motion of the wheel going over bumps.
Springs come in a variety of stiffness. Each side of the stiffness spectrum (loosely sprung or tightly sprung) has both benefits and detriments. Loosely sprung springs are typically found on larger, more comfort-based vehicles, like a Lincoln Town Car. Its looseness allows it to absorb bumps and road inconsistencies, providing for a very smooth ride for the driver and passengers. However, this smoothness comes at the expense of handling; with springs that loose, there is limited rigidity in the system, allowing for a greater amount of body roll (the sideways dipping felt when turning) and a reduced ability for higher-speed handling. Tightly sprung springs are more oriented for sports cars and other vehicles where handling at speed is a serious factor. While the spring’s taut nature allows for a greater transfer of bumps to the occupants, it greatly reduces sway and body roll, allowing for a tighter, more controlled feeling in overall handling.
Trying to find a perfect balance of comfort and stability can seem like a Herculean task for those that design these systems, and for all intents and purposes, it’s quite an undertaking. But thankfully, not all the stress lies on the shoulders of the springs.
The Shock Absorbers
Shock absorbers, also known as dampers (due to their ability to dampen the vertical forces from the wheels and springs), exist to complement the spring’s job in reducing the overall forces acting on the vehicle’s frame. It accomplishes this task by using pistons and hydraulic action to transfer the kinetic energy in the system to heat energy, which is dispersed through the oil in the damper.
The most common type of shock is the hydraulic shock, which relies solely on the piston pushing against the oil to create resistance and heat. Gas shocks also exist, which incorporate nitrogen gas to further aid compression. In terms of differences between the two, a hydraulic shock has equal dampening force in both directions, whereas a gas shock will extend on its own, due to the charged nitrogen. Hydraulic shocks have a tendency to fade as the oil heats up excessively and gets agitated, creating air bubbles. The nitrogen in gas shocks keeps this under control. As a rule of thumb, hydraulic shocks tend to ride better, but gas shocks are stiffer with a greater longevity and tend not to fade. For now, we’ll just discuss the hydraulic style of shock absorber.
As the car hits a bump, the motion is transferred through the spring to the shock absorber, and the piston begins to push through the oil. Small perforations allow the oil to flow through the piston at a controlled rate, creating a specific dampening force. Shocks are also velocity-sensitive, meaning that the faster the shock moves, the more resistance it supplies. This is important in adjusting to road conditions and keeping vehicle control optimal.
The following is a picture of a setup featuring a spring and shock seated independent from one another, taken from the rear of a Mercedes C-Class:
A strut is essentially a shock absorber and coil spring built into a single unit. More often than not, you’ll hear that a car comes with MacPherson struts, which is the de facto strut that you encounter on most cars. While struts use the hydraulic dampening force similar to shocks to keep the car stable, struts also provide structural support for the suspension system. Unlike standalone shock absorbers, struts also support the weight of the vehicle.
The following is a picture of a coilover setup on the same C-Class. A coilover is very similar to a MacPherson strut, except coilovers are adjustable in a variety of ways. More explanation about coilovers can be found later on in the article, but since they roughly look the same, here’s a shot of one:
-Control Arms, Steering Knuckle, Bushings and Ball Joints
A control arm is a piece of the suspension that looks similar to a wishbone and helps to transfer motion and energy between other components in the suspension. It is a flat and roughly triangular piece that pivots in two places, although the size and shape of a control arm can vary from car to car. The broad end of the control arm attaches at the frame and pivots on a bushing. The narrow end attaches to the steering knuckle and pivots on a ball joint. In a MacPherson strut design, there is only one control arm, the lower. In a double-wishbone suspension setup, there are two control arms, an upper and a lower, which is where the “double-wishbone” name comes from. The last type of suspension, multi-link, uses multiple lateral arms to control the aspects of the car’s handling. The control arms for a multi-link car will look more like a typical arm, rather than a wishbone.
The steering knuckle could technically be a part of both the steering and suspension systems. Its purpose is to allow the wheel to pivot, letting the car turn. On a double-wishbone setup, the knuckle is located in-between the upper and lower ball joints; on a MacPherson setup, the knuckle is located between the lower ball joint and the strut assembly. In a multi-link suspension setup, the steering knuckle is located somewhere between the lateral arms.
Bushings, typically made from rubber or polyurethane, are placed in between two metal parts, which dampen the energy transmitted between the two parts, while allowing for a certain amount of movement. This movement allows the suspension parts to move freely, such as when travelling over a large bump.
Ball joints are spherical bearings that connect the steering knuckle to the control arms, and are the main pivot between the wheel and the suspension. They allow motion in all three directions.
The following is a shot from the C-Class containing both upper and lower control arms, the steering knuckle and a ball joint. The shot after that is of an end link bushing (which is the black-ish piece, contained in the metal jacket, surrounding the bar):
-Sway Bar and End Links
The sway bar, also known as the anti-sway bar or the anti-roll bar, provides additional stability for the vehicle. The sway bar is a long, metal rod that essentially connects both sides of the suspension. As one wheel moves, the motion is transferred to the other end via the sway bar. This creates a more level ride and reduces sway in the suspension. It helps prevent body roll when cornering, as well.
The end links connect the sway bar to the rest of the suspension components. Since they attach at different points on different vehicles, they can vary in length and size. They are always rigid in design, helping to transfer the motion from one side of the vehicle to the other.
The following two shots of the C-Class include the frame and sway bar (in the first photo), and the sway bar end link (in the second):
Issues That Can Arise with the Suspension System
The most common symptom to arise when a suspension component begins to lose its effectiveness is a noise, originating from inside the wheel well. Noises may all sound similar, but the noise emitted during a specific part of your everyday driving can tell you the difference between a sway bar end link and a busted ball joint.
The typical noises related to suspension components are rattling and knocking noises. Everything in the wheel well exists in somewhat cramped conditions, especially as systems get more complicated. As something breaks or comes loose, any motion in the suspension system can cause the affected piece to bounce off other pieces or rattle around when there should be a solid connection. More often than not, any noise coming from that area (when there typically isn’t a noise) is a suspension issue, and should be looked at promptly. Loss of control is a serious issue when it comes to piloting thousands of pounds of metal and glass, so it should be obvious that maintaining a proper suspension setup is paramount to both your safety and the safety of everyone else on the road.
-Reduced Handling Capabilities
If your ride quality decreases a noticeable amount, it would be in one’s best interest to check the strut/spring/shock trifecta first. Shakiness or choppiness while cruising on the highway, or even a loss of complete control over the vehicle may be signs that those parts are beginning to wear out. Another common symptom is an unnatural tilt to the side during cornering. As time goes on, the shocks will lose both their ability to dampen and their ability to provide resistance. A deep dip when cornering is definitely cause for concern. Many times on the highway, you’ll notice cars that take a bump and slowly rise and fall, as if their car were a boat on an ocean. At that point, the shocks have completely lost their ability to provide resistance, and the piston is basically floating inside the oil, providing a ride that might feel smooth, but completely removes any notion of safety in handling. You can test this yourself by pushing down on a corner of your vehicle while it’s parked. If the rebound results in bouncing, then it’s time to look at replacing some components.
In theory, every part of your suspension helps out the handling. If something seems wrong or feels wrong, it’s probably best to get it checked out. Even something as simple as taking the tire off and looking around can potentially result in a full diagnosis of the problem. If you know what everything looks and feels like when it’s working, it could be easy to tell if a bolt has rattled loose or if your bushings are starting to lose their ability to perform.
How to Address Some of These Issues
When it comes to an issue involving your shocks, springs or struts, the best and most effective answer is replacement. More often than not, a blown shock cannot be fixed, so it will need to be replaced. While replacing one strut is a way to fix a single broken strut, odds are that the others will not be very far behind in failing. As a typical rule of thumb, it’s always best to replace these items in sets of four.
There are many aftermarket options available, should you desire a greater handling capability than stock. Springs and shocks are available as separate entities, and for many cars this is a good way to go. By choosing an aftermarket spring setup, you can lower your car (by virtue of the spring being shorter than the OEM spring) and provide a stiffer ride (as the aftermarket springs may compress less easily than the OEM ones).* Shocks also provide various ranges of dampening power, depending on whether or not you choose to tip the scale in favor of comfort or handling prowess. The last option is a coilover set. Coilovers are similar to struts in that they contain both a spring and a shock absorber and just like struts, if the shock absorber begins to leak, the whole coilover must be replaced. Using an adjustable nut at the top of the coilover and adjustable threads at the bottom, many sets allow for on-the-fly adjustment of dampening force and ride height, respectively. This can be especially useful if you prefer a different suspension setup between the track and the street, or if you want to lower your car during the warmer months and raise it up for winter. If you’re willing to spend the money (as coilover kits aren’t usually cheap, especially brands with racing pedigree and high quality build materials), then coilovers are a great modification for your car, or an excellent replacement should your struts fail.
But again, it’s important to think about your vehicle’s main use. If you don’t mind a sacrificed ride quality and more bumpiness on roadways, a stiffer suspension setup will make your car feel as if it’s on rails going through turns and will help the overall feeling of the car under most driving conditions.
*It’s incredibly important to note that, should you choose the do-it-yourself route on replacing springs, if you are removing the spring from a strut assembly, you must use a spring compressor. If not, removing the top perch will decompress the spring, and the force it contains is enough to shoot the spring off the damper with considerable (and often very, very harmful) kinetic energy. If you don’t have a compressor or access to one, it’s in your best interest to have a professional put the springs on for you. Safety first, folks.
Just like with the aforementioned suspension components, if something begins to break or become worn, the sole option is replacement. While OEM replacement parts are typically suggested for failures related to suspension components, aftermarket parts are always a suitable option if you would prefer to enhance the ride quality of your car.
Bushings are a very popular aftermarket replacement. More often than not, inclement weather and age will begin to cause your bushings to lose their lubrication and holding capabilities. Replacing stock rubber bushings with polyurethane bushings will reduce squeakiness (as rubber loves to squeak) and help to maintain proper alignment while driving, leading to a more controlled feeling behind the wheel.
If spirited driving is done frequently, the OEM parts may not be up to the task of consistently handling the abuse. A very common part to fail (and a part with a very widespread aftermarket presence) is the sway bar end link. Upgrading to beefier end links will help transmit the energy through the sway bar, evening out the ride quality. To compound this feeling, a thicker sway bar will help transmit a greater amount of force. The same can be said for control arms, as well; aftermarket control arms are useful in providing a stiffer, more controlled handling feeling. Again, it’s important to note that many aftermarket suspension components tend to deliver excellent handling results at a sacrificed ride quality, but for many people it’s definitely worth the tradeoff.
As always, when replacing a part, remember the theory of the weakest link. In the suspension system, there is always force and stress being transferred between different parts. If one of those parts fails and is replaced, the excess stress will transfer to the next weakest part, causing it to wear faster than normal. Preventative maintenance and a working knowledge of your car’s components are both excellent tools in helping keep your suspension system and handling in top-notch form.