Learn all about Shocks & Struts!
- Shocks & Your Vehicle
- What Worn Shocks Do
- When To Replace Shocks
- All About Strut Mounts
- Ride Control Terms & Definitions
- Differences between Monotube & Twin Tube
- Frequently Asked Questions
Shocks & Your Vehicle
What do shocks and struts do? They keep you in control of your vehicle! As you drive, steer, brake, hit bumps, or accelerate, the body of your vehicle moves around, creating odd sensations and weight shifts that makes the car or truck respond differently in each situation. If you did not know how your vehicle would handle, any driver would be extremely nervous all the time! Shocks and struts eliminate this uncertainty, by keeping the vehicle body motions minimized and consistent, providing you with a smooth ride and predictable responses no matter what you encounter on the road.
How do they do that? Each tire on your vehicle has a shock or strut attached to it. Inside is a series of valves, oil, and nitrogen gas. As the suspension moves up and down, the valves control the rate of oil flow and this results in the suspension moving in a consistent manner. That consistency is what stability feels like, and it leaves you firmly in control since you know how the vehicle will respond.
What is vehicle stability? Vehicle stability is feeling a direct and predicable response to the inputs that you as the driver put in while driving. That means the vehicle turns, stops, swerves, accelerates, and responds to bumps in a way that you know and are comfortable with. Engineers spend a considerable amount of effort designing all of the steering, braking, and suspension systems in order to generate a level of performance and ride comfort that is appropriate for the intended uses of your vehicle. For example, an SUV is designed to carry heavier loads of passengers and cargo than compact sports cars, so the tuning and design of the systems are different to match how you actually would use it. If any of the components is not performing up to the engineering specifications, then the vehicle will not perform as designed.
What Worn Shocks Do
As shocks wear their ability to control wheel and body movement declines, which make it more difficult for the driver to control the vehicle. Braking distances tend to increase, body roll and nose-dive are more pronounced, and the driver has to make additional steering corrections while turning as weight shifts to different tires. In addition, since there is more suspension movement, every other component begins to wear out more quickly. That means the springs, control arms, tires, bushings and wheel alignment get sloppier and more out of the original engineer’s intended spec. Floppy body control may also mean underbody pieces may drag on the ground or impact curbing or other obstacles that they would not have touched otherwise. As the system becomes increasingly worn, the ride and control worsen.
Why Less Control? Worn shocks and struts allow more suspension movement than the engineers intended, which reduces tire grip on the ground. When there is less tire grip, then control and handling decline.
Why More Wear? The entire suspension of your vehicle is an interconnected system, controlled by the shock or strut. When the shock is worn, there is more movement than normal and each up/down cycle is additional wear on all of the components that was un-necessary, reducing the lifespan of each part.
Performance Categories impacted by worn shocks or struts
- Increased Braking Distance
- Faster Tire Wear
- Scraping / Bottoming Out
- Harsh or Floaty ride
- More steering adjustments needed mid-corner
- Acceleration squat / braking Nose Dive
- Poor traction
- Additional Body Roll
When To Replace Shocks
What does your Odometer read? Experts generally recommend that automotive shocks and struts be inspected at 50,000 miles. Suspensions can move up and down up to 2000 times per mile, and in 50,000 miles that would be over 100 million cycles. Through those cycles the bushings wear, the oil loses viscosity due to heat cycling, and the valves lose their strength, which results in their damping force gradually declining over time. That gradual decline makes it difficult to feel and notice that they have worn. We frequently hear from customers that have waited to 100-120,000 miles and they consistently say “wow, I wish I had changed them earlier, I didn’t realize how great my car can handle!”
You can also make the decision based on you wish your vehicle to handle. For example, if you consistently have a snowplow on your pickup or your minivan is always loaded with 7 passengers and a full load of soccer gear, then you may not be a normal user and would thus require earlier replacement, or replacement with a higher-performance upgrade shock or strut that can handle the way you use the vehicle.
Sloppy Handling or Broken components. If you feel sudden uncontrolled bounce, have inconsistent handling when turning one way versus the other, or hear clunking or grinding noises, then its definitely time to have your suspension inspected by a qualified technician. ASE and other certifications recommend having the suspension road tested and visually inspected every 12,000 miles. Failed components are a safety hazard. Leaking fluid, damaged bushings, tire wear, or rattles and noises definitely need to be inspected asap.
All About Strut Mounts
Strut Mounts are additional parts that connect a strut to the vehicle body. They often have rubber pieces, bearings, seals, and other components combined into one unit. Their purpose is to provide additional cushioning to sharp road impacts, provide precise steering feel, and eliminate road noises from being transmitted into the vehicle cabin. As they wear, you may hear clunking, feel stiffness or soft spots in the ride, or have alignment issues that cannot be fixed.
When should I replace them?
Its always smart to change strut mounts when the struts are being changed. Even if they still appear to be usable, they do wear along with the rest of the suspension and frequently a new strut will cause an old mount to break even faster. Its also good insurance to do the job once, by changing the mounts with the new struts, so that you don’t have to do the job a second time if the old mounts fail in the near future.
Ride Control Terms & Definitions
- Active Suspension ... A suspension system that pro-actively responds to the road surface and / or handling settings to adjust how the suspension responds. Typically these are hydraulically operated through either a pressure pump and valving, or a series of lines that link all of the shocks together and then the fluid flow between them is controlled in order to have the vehicle perform as desired.
- Air Shock ... A shock with an overload air bladder can be inflated to increase the suspension's load carrying ability. This can also be automated from the factory (auto Load Leveling) with a mini air compressor.
- Air Suspension ... Vehicles are typically held up by steel coil or leaf springs. In an air-suspension system, air-filled pressurized rubber bags provide the necessary support. The main advantages are a softer ride and the ability to raise/lower ride height based on the driver demands and vehicle load.
- All Wheel Drive AWD ... All four wheels are driven at all times, whether needed or not.
- Body Roll ... The leaning or tipping of a vehicle's body to one side while turning sharply. Body roll reduces the vehicles stability and handling by allowing weight to shift to the outer tires increasing the amount of work they have to do and eventually overloading them, causing a skid or other loss of control. Roll is typically controlled by swaybars and spring stiffness.
- Bounce & Jounce Test ... A basic testing method where you push down on a vehicle and see how quickly it bounces up and is stopped by the shocks or struts. There is no OE specification, however it can provide some wear indications.
- Bushing ... Typically a rubber component or sleeve that isolates one component from another, alowing controlled movement and/or isolating noise vibration and harshness (NVH).
- Cartridge ... When a vehicle has serviceable struts, a replaceable shock absorber insert is used inside of a reusable housing.
- Coilover Shock ... A shock absorber that includes a coil spring and an adjustable coil spring seat. The adjustment increases or decreases firmness and ride height.
- Coil Spring ... Heavy-gauge steel wire wound into a circular coil shape and used to support the weight of the vehicle.
- Compressed Length ... The total length of a shock or strut when the shaft is fully depressed into the unit's body, as measured from the mounting points.
- Compression Stroke ... Also called Jounce: The process of the shock compressing, like when the tire rises from hitting a bump.
- Corner Unit (Module, Assembly) ... A pre-assembled assembly of suspension strut, coil spring, mount, spring insulators and any required hardware, typically ready to install on the vehicle out of the box.
- Damper ... A generic name for any device (shock, strut, cartridge, stabilizer, etc.) who's primary function is to resist movement and control movement ocillations.
- Damping Force ... The amount of resistence a damper provides when acted upon by an outside force.
- Electronic Stability Control ESC ... ESC technology improves the safety of a vehicle's stability by detecting and minimizing skids by applying the brakes to particular wheels, reducing power, or adjusting steering angle. It helps prevent common issues like spinning while braking and turning, or hitting small patches of ice.
- Extended Length ... The total length of a shock or strut when the shaft is fully extended from the units body, as measured from the mounting points.
- Eye Rings ... Round bands on the end of a shock or strut that typically house a mounting bushing.
- Four Wheel Drive 4WD ... Four Wheel Drive vehicles allow all four wheels to be driven when needed. Sometimes that is through a transfer case that is driver selectable with different gear ratios (4hi, 4lo, etc), or it can by automatically engaged by the vehicle whenever slippage is detected (audi quattro, etc). A traditional four wheel drive system drives all four wheels at the exact same speed, whether the vehicle is turning or not and whether the wheel is on the ground or not, so that off road and/or snow performance is maximized. Such systems can not be used on dry pavement without damage to the system.
- Front Wheel Drive FWD ... Engine power is routed through the two front wheels.
- Gas Shock ... Any shock or strut that uses pressurized Nitrogen gas instead of oxygen: In a twin tube design, a low pressure Nitrogen gas is used to reduce aeration. The monotube design has a seperate high pressure Nitrogen gas chamber that reacts to driving conditions.
- Jounce ... When you drive over a bump and the suspension is momentarily compressed, that's called jounce.
- Independent Suspension ... A suspension system that allows each wheel on a vehicle to move up and down independently of the other wheels.
- Leaf Spring ... A type of spring made out of a flat strip or individual leaves to support the weight of a vehicle. Most are steel, but some are made of lightweight composite materials.
- MacPherson Strut ... A suspension system that consists of a combination coil spring and shock absorber (strut) in one compact unit at each wheel. With this "independent" suspension design, road shocks at one wheel are not transferred to the opposite wheel. MacPherson struts use fewer parts, meaning a reduction on weight and fewer elements that could wear out.
- Monotube ... A damping unit design that uses a single cylinder and incorporates a seperated high pressure gas chamber. In this design the gas does not mix with the hydraulic fluid. The gas area acts as the fluid expansion area and provides additional damping on demand. The fluid-only valve area provides more responsiveness compared to the twin tube design.
- Mounting Hardware ... Nuts, bolts or other fasteners.
- Mounting Stud ... A shaft with a threaded end. A mounting component.
- Nivomat ... A trademarked type of shock absorber that controls ride height. (self-leveling) It contains a mechanical hydraulic pump activated by vehicle movement.
- Non-Serviceable Struts ... A sealed strut that does not have a replaceable cartridge. This unit must be replaced as an assembly.
- Nose Dive ... Occurs during vehicle braking: The front of the vehicle surges downward and the rear of the vehicle surges upward.
- Pinch Bolt ... A fastener (bolt) that secures,squeezes or prevents a component from turning.
- Rear Wheel Drive RWD ... A drive system where the engine applies the driving force to the rear wheels only. This pushes the vehicle from the rear wheels
- Rebound Stroke ... The motion of a wheel that extends the suspension. The opposite of jounce or compression stroke.
- Ride Control ... Four separate vehicle systems (Tires, suspension, steering & brakes) that work together to control a vehicle's stopping, turning, handling, stability control and ride comfort.
- Ride Height ... The amount of space between the base of an automobile tire and the underside of the chassis.
- Self Leveling Shock ... A damping unit that automatically adjusts suspension balance and height to keep the vehicle level in all driving conditions.
- Serviceable Strut ... A strut assembly that is designed with a replaceable cartridge. Some designs have an upper hex nut while others require a special cutting tool. See the vehicle manucaturers manual before attempting to disassemble.
- Shock Absorber ... A device that converts motion into heat, usually by forcing oil through small internal passages in a tubular housing to dampen suspension oscillations.
- Shock Fade ... A condition where loss of dampening action occurs because of fluid foaming inside a shock absorber. The rapid oscillations of the piston moving through the fluid churns it into foam, which reduces the amount of resistance encountered by the piston. This causes the dampening action to fade, resulting in loss of control, excessive suspension travel and reduced handling.
- Shock Travel ... The measurable difference between the extended and compressed lengths of a shock or strut.
- Spring Compressor ... A tool for compressing and holding a coil spring so it can be removed or replaced, or to allow the disassembly of a MacPherson strut.
- Spring Seat ... The mounting area for a suspension coil spring. This may be located on the vehicle or on the damping unit.
- Spring Insulator ... A cushion between a coil spring and spring seat that helps reduce noise and vibration.
- Stability Control ... A type of advanced antilock brake/traction control system that uses the brakes to assist steering maneuvers and to help improve vehicle handling and stability as driving conditions change. The system includes various sensors that monitor the driver's steering inputs and the position of the body with respect to the road. A "yaw sensor" can tell if the vehicle is starting to understeer or oversteer in a turn. The stability control system is active fulltime and will apply individual brakes to create a counter-steer effect that brings the vehicle back under control.
- Stabilizer Link ... The bolt, stud, bushings and washers used to attach a stabilizer (sway) bar to a suspension control arm.
- Static Height Measurement ... A measurement taken when the shock or strut is installed on a vehicle (on level ground): From the lower measuring point to the upper measuring point.
- Steering Damper Stabilizer ... A hydraulic device similar to a shock absorber attached to the steering linkage to absorb road shock and steering kickback.
- Steering Knuckle ... A forging that usually includes the spindle and steering arm, and allows the front wheel to pivot. The knuckle is mounted between the upper and lower ball joints on a double wishbone (upper & lower control arms) suspension, and between the strut and lower ball joint on a MacPherson strut suspension.
- Steering Stabilizer Damper ... A hydraulic device similar to a shock absorber attached to the steering linkage to absorb road shock and steering kickback.
- Striker Plate ... An area at the top of a strut housing where the travel limiting bumper would contact.
- Suspension Strut ... A suspension strut combines the primary function of a shock absorber (as a damper), with the ability to support sideways loads not along its axis of compression, thus eliminating the need for an upper suspension arm.
- Strut Boot ... A flexible protective boot that keeps dirt and debris away from the polished upper shaft and upper shaft seal.
- Strut Mount ... A mounting insulator between the vehicle and the strut. Front strut mounts often include a steering pivot bearing or bearing plate.
- Strut Tower ... The panels or structural members in a unibody to which the upper strut mounts are bolted.
- Sway Bar ... A component that's often used in a suspension system to control body roll. A sway bar may be used on the front and/or rear suspension to help keep the body flat during cornering/maneuvering.
- Tie Rod End ... A part of the steering linkage that connects the steering arms on the knuckles to the steering rack or center link.
- Trailing Arm ... A suspension element consisting of a longitudinal member that pivots from the body at its forward end and has a wheel hub rigidly attached to its trailing end.
Differences between Monotube & Twin Tube
Most vehicular shock absorbers are either twin-tube or mono-tube types with some variations on these themes.
Also known as a "two-tube" shock absorber, this device consists of two nested cylindrical tubes, an inner tube that is called the "working tube" or the "pressure tube", and an outer tube called the "reserve tube". At the bottom of the device on the inside is a compression valve or base valve. When the piston is forced up or down by bumps in the road, hydraulic fluid moves between different chambers via small holes or "orifices" in the piston and via the valve, converting the vertical movement energy into heat which must then be dissipated.
Twin-tube gas charged
Variously known as a "gas cell two-tube" or similarly-named design, this variation represented a significant advancement over the basic twin-tube form. Its overall structure is very similar to the twin-tube, but a low-pressure charge of nitrogen gas is added to the reserve tube. The goal is to make shocks perform consistently no matter how rapid or extreme the road surface is, by reducing the "foaming" or "aeration" of the oil. When the oil degrades or foams inside the shock, its ability to dampen the ride forces is reduced. Twin-tube gas charged shock absorbers represent the vast majority of original modern vehicle suspensions installations.
Position sensitive damping
Often abbreviated simply as "PSD", this design is another evolution of the twin-tube shock. In a PSD shock absorber, which still consists of two nested tubes and still contains nitrogen gas, a set of grooves has been added to the pressure tube. These grooves allow the piston to move relatively freely in the middle range of travel (i.e., the most common street or highway use, called by engineers the "comfort zone") and to move with significantly less freedom in response to shifts to more irregular surfaces when upward and downward movement of the piston starts to occur with greater intensity (i.e., on bumpy sections of roads— the stiffening gives the driver greater control of movement over the vehicle so its range on either side of the comfort zone is called the "control zone"). This advance allowed car designers to make a shock absorber tailored to specific makes and models of vehicles and to take into account a given vehicle's size and weight, its maneuverability, its horsepower, etc. in creating a correspondingly effective shock.
Acceleration sensitive damping
The next phase in shock absorber evolution was the development of a shock absorber that could sense and respond to not just situational changes from "bumpy" to "smooth" but to individual bumps in the road in a near instantaneous reaction. This was achieved through a change in the design of the compression valve, and has been termed "acceleration sensitive damping" or "ASD". Not only does this result in a complete disappearance of the "comfort vs. control" tradeoff, it also reduced pitch during vehicle braking and roll during turns.
Coilover shock absorbers are usually a kind of twin-tube gas charged shock absorber around which has been mounted a large metal coil. Though common on motorcycle and aftermarket performance suspensions, coilover shocks are uncommon in original equipment designs for vehicles. Coilover shocks for cars have been considered specialty items for high performance and racing applications where they allow for significant tailoring of the damping and spring forces to the exact requirements needed for that application.
The principal design alternative to the twin-tube form is the mono-tube shock absorber. As its name implies, the mono-tube shock consists of only one tube, which is divided into a gas chamber and an oil chamber. Since the chambers are separate, the each one can be specifically tailored to meet higher performance specifications. For example, the piston and valves can be made larger, resulting in more responsive handling. Mercedes became the first auto manufacturer to install mono-tube shocks as standard equipment on some of their cars starting in 1958, manufactured by Bilstein. Because the design was patented, no other manufacturer could use it until after the the patent expired in 1971.
Frequently Asked Questions
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