All vehicles are equipped with some form of suspension damping control. They may use hydraulic fluid, air, gas or be electrically controlled. The two most widely used internal designs are twin-tube and monotube.
The twin tube design is the most common one in use on cars, light trucks, SUV’s and vans. It’s a cost effective unit that provides excellent handling & control characteristics for most driving conditions. The monotube design offers additional performance and can have a more aggressive ride.
A twin-tube shock or strut has two cylinders:
The inner, or working cylinder, is where the piston and shaft move up and down. The outer cylinder serves as a reservoir for the hydraulic fluid. There are fluid valves in the piston and in the stationary base valve. The base valve controls fluid flow between both cylinders and provides some of the damping force. The valves in the piston control most of the damping. The term Gas Shock usually refers to the same twin-tube design, but with one improvement. Low pressure Nitrogen gas is added to replace any oxygen air. The effect lessens aeration and performance fade.
The monotube design has a single cylinder:
The cylinder is divided into sections: A fluid area and a gas chamber. The piston and shaft move in the fluid portion. It uses a single fluid valve assembly in the piston. The diameter of the single working cylinder and piston valve is larger than in a twin tube even though the outside dimensions of each may the same. There is no need for an air or gas in the fluid area so the valve can operate more responsively and without any aeration or performance fade. The high pressure gas chamber is separated from the fluid area by a floating piston & seal. That provides an expansion area for the excess fluid movement during the compression stroke. On more aggressive movement the floating piston is pushed further into the gas chamber which increases gas pressure quickly and provides additional damping force. Because of its higher performance capabilities, the monotube design is used as original equipment on some vehicles and offered as an upgrade on vehicles that came equipped with the twin tube design.
As the name (strut mount) implies, it’s a component that attaches the suspension strut to the vehicle. In addition, it insulates the tire noise & vibrations from the vehicle. Many (front) strut mounts also contain a bearing or bearing plate that serves as the steering pivot.
A strut mount is like a sandwich. One side bolts to the vehicle, the other side to the strut. In the middle is a rubber-like insulating material. So as the vehicle moves and goes over bumps, the up & down impact pushes and pulls at the mount. The mount’s job is to cushion the impacts to reduce the jarring effect, noise and vibration that could transmit into the vehicle.
On many front struts, the mount also includes a bearing that the strut attaches to. With one on each side of the vehicle, these bearings act as the steering pivots. The bearing is a critical component that affects steering movement smoothness and response.
Why replace it?
There are two main reasons; age changes the density of the insulating material so more noise and vibration can be transmitted into the vehicle and the bearing portion of the mount wears.
When to replace them?
A simple rule of thumb is that if the struts or coil springs are being replaced because of age or wear, then also replace the mounts. A worn or damaged mount can cause a variety of symptoms so they should be inspected by a qualified technician. Disassembly of the entire strut & spring assembly is required to remove & replace the mount. Therefore replacing all worn components at the same time could save you from doing the work a second time.
How to tell if the mount is worn. Drivability symptoms may include:
Visual indication of a worn mount may be:
What’s important to you: Vehicle safety, comfort or performance? Most people would say that all three are important, but at different times. A comfortable ride on a straight, smooth road without much traffic on a clear, dry day is easy. But change the conditions slightly and suddenly your priorities change. You know what I’m talking about if you’ve ever had to quickly dodge something in the road, fight strong winds or experienced white-knuckle driving on irregular or slippery roads.
Your vehicle’s ability to offer the utmost driving safety, comfort & performance directly relates to its Ride Control design & the condition of its components. Ride control is comprised of four inter-related systems: The tires, brakes, suspension and steering. Each system affects the other, such as; a worn suspension component adversely affects braking, steering and tire performance. The same is true in every combination when any other ride control component is worn.
Shocks and struts are part of your vehicle’s suspension system. Technically speaking, they control coil spring oscillations. But for a better understanding of why that’s important to you, consider what it would be like if excessive vehicle body weight movement affected the steering? And, what if the tires bounced more and stayed on the pavement less? That’s basically what shocks and struts are designed to control. The result should be vehicle stability and maximum tire to road contact: Safer, smoother and designed to keep the driver in control.
What is it?
It's called Electronic Stability Control or ESC. In a nutshell, it's an amazing on-board electronic system that helps prevent spin-outs and roll-overs. It's a fact: Most accidents that involve losing control of the vehicle occur when the vehicle is driven beyond its traction limits like during over-steer or under-steer conditions or driving too fast for conditions. ESC senses when the vehicle is rolling or leaning too far or when the tires begin to lose traction. It instantly reduces engine speed and applies one of the individual wheel brakes in just the right amount to keep the vehicle in control.
How many vehicles have ESC?
The chart below shows the rapid growth of ESC. (The U.S. government mandated that all 2012 vehicles must have ESC safety systems).
How does it work?
An array of sensors provide vehicle operation conditions to one of the vehicle's on-board computers. When the computer determines that the vehicle is losing control, it instantly begins reducing engine speed through the engine management system and then applies the appropriate individual brake in just the right proportions to keep the vehicle in control.
The seat belt of the 21st century
Numerous studies have shown ESC is highly effective at preventing loss-of-control and fatal crashes. The Insurance Institute for Highway Safety estimates 10,000 fatal crashes and up to 238,000 injuries could be avoided each year*. Vehicles with ESC reduce the risk a fatal loss-of-control by half, and reduces rollover risk by up to 80%. ESC has been described as the most important advancement in safety by automotive experts. (*in the United States).
Required by Law
Because of this significant reduction in roadway accidents and fatalities, the United States, Canada, and Australia are implementing new legislation requiring ESC in all new passenger vehicles.
Since 2005 the United States has had legislation requiring a percentage of new passenger vehicles to have ESC. This percentage requirement has increased with each model year. By the 2012 model year, all passenger vehicles sold in the U.S. will be required to have ESC.
Now entering service bays
Estimates show that 60% of all vehicles in service bays are 4 to 12 years old. There are scores of 10 year old ESC-equipped vehicles on the road today. Almost a third of 2005 model year vehicles have ESC and more than half of all 2007 light duty vehicles are equipped with ESC.
It’s important for both motorist and service provider to know whether the vehicle has ESC or not. Why? Two big reasons:
Maintenance suggestions for ESC-equipped vehicles
While the electronic components of your ESC system won't wear like tires, shocks, struts, or brakes; they should be inspected on a regular basis. Check your Owner's Manual for a schedule provided by the manufacturer.
Wearable parts like the brakes, shocks, struts, and tires all require regular maintenance and periodic replacement. The Motorist Assurance Program and the Automotive Maintenance and Repair Association (AMRA) suggest replacing OE shocks and struts after 50,000 miles (80,000 kilometers).
ESC-equipped vehicles, like all vehicles, require periodic replacement of brake pads, shocks and struts, tires, and other wearable parts. These replacement parts can be purchased and installed at a automotive service center near you. Be sure to choose replacement parts that are designed to restore the vehicle's originally designed handling and control capabilities. Restoring original handling and control is critical to ESC performance.
The KYB OEA advantage
Restoring designed performance is critical. Even with ESC, stopping and turning still depends on good tire traction. Good tire traction depends greatly on well-functioning shocks and struts. The ability of ESC to do its job can be compromised by worn, low-quality, or under-calibrated shocks or struts.
What does OEA parts mean?
OEA stands for Original Equipment for the Aftermarket. It is the baseline for all of KYB's products. KYB don't believe in manufacturing a product that delivers less than the original design that it is replacing.
How is ESC different from ABS or Traction Control?
In a nutshell: ABS works great at preventing skidding and sliding in a straight line, and Traction Control prevents wheel slippage, but only ESC prevents vehicles from skidding during curves.
ESC uses the same sensors as ABS and Traction Control, then adds a yaw rate sensor and in some cases a steering angle sensor. This basically results in a computer system that knows the speed of every tire, the intended engine speed, the lean & roll of the vehicle on its axis, the intended steering direction, and when any tire is losing traction. The ESC Computer System is capable of more operational input and can make more driving corrections than any human will ever be capable of.
ABS, Traction Control, and ESC all control the vehicle by modulating brakes on individual wheels in precise amounts. Unlike the others, ESC also has the ability to modulate the throttle (engine speed) and transmission.
Gravity and weight keep your tires on the ground… until you hit a bump. And, your vehicle may seem stable and ride evenly… until you swerve, brake or turn.
What shocks and struts do? They keep the tires from bouncing which causes loss of road contact and they also keep the vehicle body weight from shifting and rolling which causes a loss of vehicle control and handling. You could say then, that their main job is to resist both tire and vehicle body motion. When shocks and struts are working well, the vehicle maintains traction on the road and has the optimum stability in every driving condition: stopping, turning, swerving, over bumps and potholes.
How do they do that? A shock or a strut operate the same but mount differently. Each of the four tires has either a shock or strut connected to that corner’s suspension and vehicle body or frame. They are hydraulic cylinders that provide motion damping by restricting fluid flow through a series of internal valves. As the wheel and body moves up and down, the shock valving slows and stabilizes the movements.
But what is vehicle stability?
Answer: Vehicle stability means being able to stop, turn, and swerve at exactly the moment and in the amount you need. Your shocks react to every wheel and body movement; every bump and every corner.
There are two major disadvantages that begin to occur as your shocks (or struts) wear. First, as the shock’s ability to resist wheel & body movement diminishes, it becomes more difficult to control your vehicle. That becomes evident as quick-stopping distances increase and the driver has to compensate for conditions like body roll and nose-diving. The second is increased wear of every other related vehicle component. Since more suspension movement is allowed as the shocks wear, every attached part is subject to additional impacts and wear to those components occurs more quickly. The suspension springs, support arms, bushings, joints and wheel alignment all suffer. That translates to a shorter vehicle life, less vehicle satisfaction and additional repair expenses.
Worn shocks allow more dramatic up & down tire movement. As the tire bounces up, the available tread contact on the pavement decreases. If the tire isn’t gripping the road well, you have traction loss. Cornering, accelerating and stopping can become a problem. Additionally, when you add wet or slippery conditions… well you can imagine the trouble the driver can have.
It’s a little bit like the old song: “The knee bone is connected to the thigh bone; the thigh bone is connected to…” The extra impact and movement at each attached part causes accelerated deterioration or wear. One worn part causes another… and another… and so on.
The best way to talk about ride control conditions is to show you some examples. Actually all vehicles experience these movements or conditions to some degree. However as more wear occurs, each one finally becomes a control or safety concern. KYB developed a rating system that can be used to discuss various stages of wear.
Panic Stopping: What if the tires can’t grip well? As shocks/struts wear and the tire to road contact becomes less effective, the vehicle can’t stop as well.
When you apply the brakes, the tires begin stopping but the vehicle body inertia tries to keep going forward. The result thrusts the front end downward and the back end upward. This imbalance puts stress on the front brakes and reduces the rear brake’s efficiency. The effect then makes controlling and steering the vehicle more difficult.
During cornering the vehicle body weight tries to lift & roll toward the outside of the turn. Controlling the weight shift or slowing the vehicle speed are the only ways to reduce the effect.
Small bumps and tire movements are transmitted throughout the vehicle. The ride can feel rough, harsh and noisy. This abrupt action affects both driver comfort and tire performance. The result is traction loss and usually along with that, less vehicle satisfaction.
During initial acceleration, like when leaving a stop, the front end rises and the back end lowers. Front end rise is controlled by the rebound damping in the shock. On a front wheel drive vehicle this results in traction loss and reduces steering control.
This can occur during braking, accelerating or cornering. When the tires can’t grip the road, for any reason at all, the result is loss of control: When the tires are bouncing or the vehicle body inertia is lifting or pushing the tires; when the tire tread is worn; when the tires are improperly inflated; or when the shocks and struts aren’t controlling tire and body movements.
Reacting to a bump is normal. But when the tire is allowed to over-react and move upward too aggressively, the travel must be stopped by a cushion or bumper stop. This is an indication that either the vehicle is being operated beyond its normal ability, such as being over-loaded or being driven too harshly. Or it could mean that the shocks are worn and can no longer resist wheel movement adequately.
Excessive left to right or sideways movements from; wind gusts, road conditions and steering movements. The shock or strut on one side is controlling a compression (or downward) movement while the opposite side is controlling a rebound (upward) movement. Steering corrections are required and can become difficult as the condition worsens.
When the tire is allowed to move up & down excessively, a rhythmic movement develops and an evenly-spaced wear pattern begins. Unlike tire balance conditions, the wear pattern frequency is greater from shorter tire movements that are not being dampened by worn shocks or struts. As the condition worsens, tire noise occurs along with traction loss and of course… tire damage.
Perhaps the better question is “How well do you want your vehicle to perform?” If you can answer that question, the decision about “when to replace” becomes much easier. The simple answer then becomes a matter of choice:
Vehicles are generally designed to handle a broad range of purposes: Passenger ride comfort on one side, load and terrain capabilities on the other. When your typical use of a vehicle sometimes or always stresses heavier loads, top heavy, towing or off-road conditions, you may want to consider replacing the shocks with units that exceed original equipment capabilities. KYB offers many higher-performing “Monotube” shocks for this purpose.
Even on well paved roads the shocks can move up or down 1,500 to 1,900 times every mile: In 50,000 miles that can exceed 75 million cycles. Over time, performance tends to decrease gradually and the motorist usually doesn’t realize how much handling and control is lost. Replacing shocks and struts after 50,000 miles can keep the vehicle performing as well as it was designed and extend its life expectancy.
Worn shocks and struts eventually causes ride control conditions to worsen to a degree that impairs the drivers ability to control the vehicle. Road testing and vehicle inspections by a qualified technician every 12,000 miles is recommended to identify and keep the motorist informed about these conditions.
Shock or strut failures are a driving safety hazard. If you can’t control your vehicle, then it controls you. Evidence of failures includes things like; excessive bounce, units leaking to the point of fluid dripping, tires that are cupping and visual component damage.