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Suspension is the term given to the system of springs

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

, shock absorber

Shock absorber

A shock absorber in common parlance is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy.- Explanation :...

s and linkages

Linkage (mechanical)

A mechanical linkage is a series of rigid links connected with joints to form a closed chain, or a series of closed chains. Each link has two or more joints, and the joints have various degrees of freedom to allow motion between the links. A linkage is called a mechanism if two or more links are...

that connects a vehicle

Vehicle

A vehicle is a mechanical means of conveyance, a carriage or transport. Most often they are manufactured , although some other means of transport which are not made by humans also may be called vehicles; examples include icebergs and floating tree trunks.Vehicles may be propelled or pulled by...

to its wheel

Wheel

A wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load , or performing labour in machines. Common examples are found in transport applications. A wheel, together with an axle overcomes friction by facilitating motion by...

s. Suspension systems serve a dual purpose – contributing to the car's roadholding/handling

Car handling

Car handling and vehicle handling is a description of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving in a straight line. Handling and braking are the major components of a vehicle's...

and braking

Brake

A brake is a device for applying a force against the friction of the road, slowing or stopping the motion of a machine or vehicle, or alternatively a device to restrain it from starting to move again. The kinetic energy lost by the moving part is usually translated to heat by friction...

for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations,etc. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the forces acting on the vehicle do so through the contact patches of the tyres. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension

Rear suspension

For front-wheel drive cars, rear suspension has few constraints and a variety of beam axles and independent suspensions are used.For rear-wheel drive cars, rear suspension has many constraints and the march to the superior but more expensive independent suspension layout has been a long slog...

of a car may be different.

This article is primarily about four-wheeled (or more) vehicle suspension. For information on two-wheeled vehicles' suspensions see the suspension (motorcycle)

Suspension (motorcycle)

A motorcycle's suspension is similar to the suspension in an automobile in its purpose: Suspension systems serve a dual purpose - contributing to the vehicle's handling and braking for good active safety and driving pleasure, and keeping vehicle passengers comfortable and reasonably well isolated...

, motorcycle fork

Motorcycle fork

A motorcycle fork connects a motorcycle's front wheel and axle to its frame, typically via a pair of triple trees. It typically incorporates the front suspension and front brake, and allows the bike to be steered via handlebars attached to the upper triple tree.The fork and its attachment points...

, bicycle suspension

Bicycle suspension

Bicycle suspension refers to the system or systems used to suspend the rider and all or part of the bicycle in order to protect them from the roughness of the terrain over which they travel...

, and bicycle fork

Bicycle fork

A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two dropouts which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...

articles.

History

Leaf springs have been around since the early Egyptians.

Ancient military engineers used leaf springs in the form of bows to power their siege engines, with little success at first. The use of leaf springs in catapults was later refined and made to work years later. Springs were not only made of metal, a sturdy tree branch could be used as a spring, such as with a bow.

Horse drawn vehicles

By the early 19th century most British horse carriages were equipped with springs; wooden springs in the case of light one-horse vehicles to avoid taxation, and steel springs in larger vehicles. These were made of low-carbon steel and usually took the form of multiple layer leaf springs.

The British steel springs were not well suited for use on America's rough roads of the time, and could even cause coaches to collapse if cornered too fast. In the 1820s, the Abbot Downing Company of Concord, New Hampshire

Concord, New Hampshire

The city of Concord is the capital of the state of New Hampshire in the United States. It is also the county seat of Merrimack County. As of the 2000 census, its population was 40,765. Its estimated population in 2007 was 42,392.Concord includes the villages of Penacook, East Concord and West...

developed a system whereby the bodies of stagecoach

Stagecoach

A stagecoach is a type of four-wheeled closed coach for passengers and goods, strongly sprung and drawn by four horses, usually four-in-hand. Widely used before the introduction of railway transport, it made regular trips between stages or stations, which were places of rest provided for stagecoach...

es were supported on leather straps called "thoroughbraces", which gave a swinging motion instead of the jolting up and down of a spring suspension (the stagecoach itself was sometimes called a "thoroughbrace")

Automobiles

Automobiles were initially developed as self-propelled versions of horse drawn vehicles. However, horse drawn vehicles had been designed for relatively slow speeds and their suspension was not well suited to the higher speeds permitted by the internal combustion engine.

In 1903 Mors

Mors (automobile)

The Mors automobile factory was an early French car manufacturer. It was one of the first to take part in automobile racing, beginning in 1897, due to the belief of the company founder, Émile Mors, in racing's technical and promotional benefits...

of Germany first fitted an automobile with shock absorber

Shock absorber

A shock absorber in common parlance is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy.- Explanation :...

s. In 1920 Leyland

Leyland Motors Ltd

Leyland Motors Limited was a British vehicle manufacturer of lorries and buses. It gave its name to the British Leyland Motor Corporation formed when it merged with British Motor Holdings, later to become British Leyland after effectively becoming nationalised...

used torsion bars in a suspension system. In 1922 independent front suspension was pioneered on the Lancia Lambda

Lancia Lambda

The Lancia Lambda is an innovative automobile produced from 1922 through 1931. It was the first car to feature a load-bearing monocoque-type body, and it also pioneered the use of an independent suspension . Lancia even invented a shock absorber for the car...

and became more common in mass market cars from 1932.

Spring rate

The spring rate (or suspension rate) is a component in setting the vehicle's ride height or its location in the suspension stroke. Vehicles which carry heavy loads will often have heavier springs to compensate for the additional weight that would otherwise collapse a vehicle to the bottom of its travel (stroke). Heavier springs are also used in performance applications where the loading conditions experienced are more extreme.

Springs that are too hard or too soft will both effectively cause the vehicle to have no suspension at all. Vehicles that commonly experience suspension loads heavier than normal have heavy or hard springs with a spring rate close to the upper limit for that vehicle's weight. This allows the vehicle to perform properly under a heavy load when control is limited by the inertia

Inertia

Inertia is the resistance of any physical object, to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by...

of the load. Riding in an empty truck used for carrying loads can be uncomfortable for passengers because of its high spring rate relative to the weight of the vehicle. A race car would also be described as having heavy springs and would also be uncomfortably bumpy. However, even though we say they both have heavy springs, the actual spring rates for a 2000 lb race car and a 10,000 lb truck are very different. A luxury car, taxi, or passenger bus would be described as having soft springs. Vehicles with worn out or damaged springs ride lower to the ground which reduces the overall amount of compression available to the suspension and increases the amount of body lean. Performance vehicles can sometimes have spring rate requirements other than vehicle weight and load.

Mathematics of the spring rate

Spring rate is a ratio

Ratio

A ratio is an expression that compares quantities relative to each other. The most common examples involve two quantities, but any number of quantities can be compared. Ratios are represented mathematically by separating each quantity with a colon – for example, the ratio 2:3, which is read as the...

used to measure how resistant a spring is to being compressed or expanded during the spring's deflection

Deflection

Deflection or deflexion may refer to:*Deflection *Deflection *Deflection *Electrostatic deflection*Deflection...

. The magnitude of the spring force increases as deflection increases according to Hooke's Law

Hooke's law

In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit...

. Briefly, this can be stated as
where

F is the force the spring exerts

k is the spring rate of the spring.

x is the displacement from equilibrium length i.e. the length at which the spring is neither compressed or stretched.

Spring rate is confined to a narrow interval by the weight of the vehicle,load the vehicle will carry, and to a lesser extent by suspension geometry and performance desires.

Spring rates typically have units of N/mm (or lbf

Pound-force

The pound-force or simply pound is a unit of force.- Definitions :The pound-force is approximately equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth...

/in

Inch

An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

). An example of a linear spring rate is 500 lbf/in. For every inch the spring is compressed, it exerts 500 lbf. A non-linear spring rate is one for which the relation between the spring's compression and the force exerted cannot be fitted adequately to a linear model. For example, the first inch exerts 500 lbf force, the second inch exerts an additional 550 lbf (for a total of 1050 lbf), the third inch exerts another 600 lbf (for a total of 1650 lbf). In contrast a 500 lbf/in linear spring compressed to 3 inches will only exert 1500 lbf.

The spring rate of a coil spring may be calculated by a simple algebraic equation or it may be measured in a spring testing machine. The spring constant k can be calculated as follows:
where d is the wire diameter, G is the spring's shear modulus

Shear modulus

In materials science, shear modulus or modulus of rigidity, denoted by G, or sometimes S or μ, is defined as the ratio of shear stress to the shear strain:...

(e.g., about 12,000,000 lbf/in² or 80 GPa for steel), and N is the number of wraps and D is the diameter of the coil.

Wheel rate

Wheel rate is the effective spring rate when measured at the wheel. This is as opposed to simply measuring the spring rate alone.

Wheel rate is usually equal to or considerably less than the spring rate. Commonly, springs are mounted on control arms, swing arms or some other pivoting suspension member. Consider the example above where the spring rate was calculated to be 500lbs/inch, if you were to move the wheel 1 inch (without moving the car), the spring more than likely compresses a smaller amount. Lets assume the spring moved 0.75 inches, the lever arm ratio would be 0.75 to 1. The wheel rate is calculated by taking the square of the ratio (0.5625) times the spring rate. Squaring the ratio is because the ratio has two effects on the wheel rate. The ratio applies to both the force and distance traveled.

Wheel rate on independent suspension is fairly straight-forward. However, special consideration must be taken with some non-independent suspension designs. Take the case of the straight axle. When viewed from the front or rear, the wheel rate can be measured by the means above. Yet because the wheels are not independent, when viewed from the side under acceleration or braking the pivot point is at infinity (because both wheels have moved) and the spring is directly inline with the wheel contact patch. The result is often that the effective wheel rate under cornering is different from what it is under acceleration and braking. This variation in wheel rate may be minimized by locating the spring as close to the wheel as possible.

Roll couple percentage

Roll couple percentage is the effective wheel rates, in roll, of each axle of the vehicle just as a ratio of the vehicle's total roll rate. Roll Couple Percentage is critical in accurately balancing the handling of a vehicle. It is commonly adjusted through the use of anti-roll bars, but can also be changed through the use of different springs.

A vehicle with a roll couple percentage of 70% will transfer 70% of its sprung weight transfer at the front of the vehicle during cornering. This is also commonly known as "Total Lateral Load Transfer Distribution" or "TLLTD".

Weight transfer

Weight transfer during cornering, acceleration or braking is usually calculated per individual wheel and compared with the static weights for the same wheels.

The total amount of weight transfer is only affected by 4 factors: the distance between wheel centers (wheelbase in the case of braking, or track width in the case of cornering) the height of the center of gravity, the mass of the vehicle, and the amount of acceleration experienced.

The speed at which weight transfer occurs as well as through which components it transfers is complex and is determined by many factors including but not limited to roll center height, spring and damper rates, anti-roll bar stiffness and the kinematic design of the suspension links.

Unsprung weight transfer

Unsprung weight

In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...

transfer is calculated based on the weight of the vehicle's components that are not supported by the springs. This includes tires, wheels, brakes, spindles, half the control arm's weight and other components. These components are then (for calculation purposes) assumed to be connected to a vehicle with zero sprung weight. They are then put through the same dynamic loads. The weight transfer for cornering in the front would be equal to the total unsprung front weight times the G-Force times the front unsprung center of gravity height divided by the front track width. The same is true for the rear.

Sprung weight transfer

Sprung Weight Transfer is the weight transferred by only the weight of the vehicle resting on the springs, not the total vehicle weight. Calculating this requires knowing the vehicles sprung weight (total weight less the unsprung weight), the front and rear roll center heights and the sprung center of gravity height (used to calculate the roll moment arm length). Calculating the front and rear sprung weight transfer will also require knowing the roll couple percentage.

The roll axis is the line through the front and rear roll centers that the vehicle rolls around during cornering. The distance from this axis to the sprung center of gravity height is the roll moment arm length. The total sprung weight transfer is equal to the G-force

G-force

The g-force experienced by an object is its acceleration relative to free-fall. The term g-force is considered a misnomer, as g-force is not a force but an acceleration....

times the sprung weight times the roll moment arm length divided by the effective track width. The front sprung weight transfer is calculated by multiplying the roll couple percentage times the total sprung weight transfer. The rear is just the total minus the front transfer.

Jacking forces

Jacking forces are the sum of the vertical force components experienced by the suspension links. The resultant force acts to lift the sprung mass if the roll center is above ground, or compress it if underground. Generally, the higher the roll center

Roll center

The roll center of a vehicle is the notional point at which the cornering forces in the suspension are reacted to the vehicle body.-Theory:There are two definitions of roll center...

, the more jacking force is experienced.

Travel

Travel is the measure of distance from the bottom of the suspension stroke (such as when the vehicle is on a jack and the wheel hangs freely), to the top of the suspension stroke (such as when the vehicles wheel can no longer travel in an upward direction toward the vehicle). Bottoming or lifting a wheel can cause serious control problems or directly cause damage. "Bottoming" can be either the suspension, tires, fenders, etc. running out of space to move or the body or other components of the car hitting the road. The control problems caused by lifting a wheel are less severe if the wheel lifts when the spring reaches its unloaded shape than they are if travel is limited by contact of suspension members. (See Triumph TR3B

Triumph TR3B

The Triumph TR3"B" is a sports car and was produced by the Triumph Motor Company in 1962. It followed the TR3A and was offered concurrent with the TR4, which started production in 1961...

Damping

In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...

is the control of motion or oscillation, as seen with the use of hydraulic gates and valves in a vehicles shock absorber. This may also vary, intentionally or unintentionally. Like spring rate, the optimal damping for comfort may be less than for control.

Damping controls the travel speed and resistance of the vehicles suspension. An undamped car will oscillate up and down. With proper damping levels, the car will settle back to a normal state in a minimal amount of time. Most damping in modern vehicles can be controlled by increasing or decreasing the resistance to fluid flow in the shock absorber.

Camber control

See dependent and independent below.

Camber

Camber angle

frame|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheel of an automobile; specifically, it is the angle between the vertical axis of the wheel and the vertical axis of the vehicle when viewed from the front or rear. It is used in the...

changes due to wheel travel, body roll and suspension system deflection

Deflection

Deflection or deflexion may refer to:*Deflection *Deflection *Deflection *Electrostatic deflection*Deflection...

or compliance. In general, a tire wears and brake

Brake

s best at -1 to -2 degrees of camber from vertical. Depending on the tire and the road surface, it may hold the road best at a slightly different angle. Small changes in camber, front and rear, can be used to tune handling. Some race cars are tuned with -2~-7 degree camber depending on the type of handling desired and the tire construction. Often times, too much camber will result in the decrease of braking performance due to a reduced contact patch size through excessive camber variation in the suspension geometry. The amount of camber change in bump is determined by the instantaneous front view swing arm (FVSA) length of the suspension geometry, or in other words, the tendency of the tire to camber inward when compressed in bump.

Roll center height

This is important to body roll and to front to rear roll stiffness distribution. However, the roll stiffness distribution in most cars is set more by the antiroll bars than the RCH. The height of the roll center is related to the amount of jacking forces experienced.

Instant center

Due to the fact that the wheel and tire's motion is constrained by the suspension links on the vehicle, the motion of the wheel package in the front view will scribe an imaginary arc in space with an “instantaneous center" of rotation at any given point along its path. The instant center for any wheel package can be found by following imaginary lines drawn through the suspension links to their intersection point.

A component of the tire's force vector points from the contact patch of the tire through instant center. The larger this component is, the less suspension motion will occur. Theoretically if the resultant of the vertical load on the tire and the lateral force generated by it points directly into the instant center, the suspension links will not move. In this case all weight transfer at that end of the vehicle will be geometric in nature. This is key information used in finding the force-based roll center as well.

In this respect the instant centers are more important to the handling of the vehicle than the kinematic roll center alone, in that the ratio of geometric to elastic weight transfer is determined by the forces at the tires and their directions in relation to the position of their respective instant centers.

Anti-dive and anti-squat

Anti-dive and anti-squat are expressed in terms of percentage and refer to the front diving under braking and the rear squatting under acceleration. They can be thought of as the counterparts for braking and acceleration as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and right of the vehicle.

Anti-dive and anti-squat percentage are always calculated with respect to a vertical plane that intersects the vehicle's center of gravity. Consider anti-dive first. Locate the front instant centers of the suspension from the vehicle's side view. Draw a line from the tire contact patch through the instant center, this is the tire force vector. Now draw a line straight down from the vehicle's center of gravity. The anti-dive is the ratio between the height of where the tire force vector crosses the center of gravity plane expressed as a percentage. An anti-dive ratio of 50% would mean the force vector under braking crosses half way between the ground and the center of gravity.

Anti-squat is the counterpart to anti-dive and is for the rear suspension under acceleration.

Anti-dive and anti-squat may or may not be desirable depending on the suspension design. Independent suspension using multiple control arms can be an issue if the percentage is too high (say over 30%). A percentage of 100% in this case would indicate the suspension is taking 100% of the weight transfer under braking instead of the springs. This effectively binds the suspension and turns the independent suspension into no suspension like a go-cart. However, in the case of leaf spring rear suspension the anti-squat can often exceed 100% (meaning the rear may actually raise under acceleration) yet because there isn't a second arm to bind against and the suspension can freely move. Traction bars are often added to drag racing cars with rear leaf springs to increase the anti-squat to its maximum. This has the effect of forcing the rear of the car in the air and the tires onto the ground for better traction.

Flexibility and vibration modes of the suspension elements

In modern cars, the flexibility is mainly in the rubber bushings.

Isolation from high frequency shock

For most purposes, the weight of the suspension components is unimportant, but at high frequencies, caused by road surface roughness, the parts isolated by rubber bushings act as a multistage filter to suppress noise and vibration better than can be done with only the tires and springs. (The springs work mainly in the vertical direction.)

Contribution to unsprung weight and total weight

These are usually small, except that the suspension is related to whether the brakes and differential(s) are sprung.

Space occupied

Designs differ as to how much space they take up and where it is located. It is generally accepted that MacPherson strut

MacPherson strut

The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot, widely used in modern vehicles and named after Earle S. MacPherson who developed the design, based in part on designs created by Guido Fornaca of FIAT in the mid-1920s...

s are the most compact arrangement for front-engined vehicles, where space between the wheels is required to place the engine.

Force distribution

The suspension attachment must match the frame design in geometry, strength and rigidity.

Air resistance (drag)

Certain modern vehicles have height adjustable suspension

Height adjustable suspension

Height adjustable suspension is a feature of certain automobile suspension systems that allow the motorist to vary the ride height or ground clearance...

in order to improve aerodynamics and fuel efficiency. And modern formula cars, that have exposed wheels and suspension, typically use streamlined tubing rather than simple round tubing for their suspension arms to reduce drag. Also typical is the use of rocker arm, push rod, or pull rod type suspensions, that among other things, places the spring/damper unit inboard and out of the air stream to further reduce air resistance.

Cost

Production methods improve, but cost is always a factor. The continued use of the solid rear axle, with unsprung differential, especially on heavy vehicles, seems to be the most obvious example.

Springs and dampers

Most conventional suspensions use passive springs

Spring (device)

to absorb impacts and dampers (or shock absorber

Shock absorber

A shock absorber in common parlance is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy.- Explanation :...

s) to control spring motions.

Some notable exceptions are the hydropneumatic systems, which can be treated as an integrated unit of gas spring and damping components, used by the French manufacturer Citroën

Citroën

Citroën is a French automobile manufacturer. Founded in 1919 by André Citroën, it was the world's first mass-production car company outside of the USA. Since 1976 it has been part of PSA Peugeot Citroën, and its headquarters is on rue Fructidor, Paris...

and the hydrolastic

Hydrolastic

Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation and its successor companies....

, hydragas and rubber cone systems used by the British Motor Corporation

British Motor Corporation

The British Motor Corporation was a UK vehicle company, formed by the merger of the Austin Motor Company and the Nuffield Organisation in 1952...

, most notably on the Mini

Mini

The Mini is a small car that was produced by the British Motor Corporation and its successors from 1959 until 2000. The original is considered an icon of the 1960s, and its space-saving front-wheel-drive layout influenced a generation of car-makers...

. A number of different types of each have been used:

Passive suspensions

Traditional springs and dampers are referred to as passive suspensions - most vehicles are suspended in this manner.

Springs

Leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times....

– AKA Hotchkiss, Cart, or semi-elliptical spring
Torsion beam suspension
Torsion beam suspension
thumb|250px|A front [[VW beetle]] suspension crossectionA torsion bar suspension, also known as a torsion spring suspension or incorrectly torsion beam, is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring...
Coil spring
Coil spring
A Coil spring, also known as a helical spring, is a mechanical device, which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces...
Rubber
Rubber
Natural rubber is an elastomer that was originally derived from a milky colloidal suspension, or latex, found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically...

bushing
Bushing
A bushing is a type of bearing. It is a cylindrical lining designed to reduce friction and wear inside a hole, or constrict and restrain motion of mechanical parts.- Threaded Insert :...
Air spring
Air suspension
Air suspension is a type of vehicle suspension powered by an engine driven or electric air pump or compressor. This pump pressurizes the air, using compressed air as a spring. Air suspension replaces conventional steel springs. If the engine is left off for an extended period, the car will settle...

Dampers or shock absorbers

The shock absorbers damp out the (otherwise resonant) motions of a vehicle up and down on its springs. They also must damp out much of the wheel bounce when the unsprung weight

Unsprung weight

of a wheel, hub, axle and sometimes brakes and differential

Differential (mechanics)

A differential is a device, usually but not necessarily employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways. In one way, it receives one input and provides two outputs; this is found in most automobiles...

bounces up and down on the springiness of a tire. The regular bumps found on dirt roads (nicknamed "corduroy

Corduroy road

A corduroy road or log road is a type of road made by placing sand-covered logs perpendicular to the direction of the road over a low or swampy area. The result is an improvement over impassable mud or dirt roads, yet is a bumpy ride in the best of conditions and a hazard to horses due to loose...

", but properly washboarding

Washboarding

Washboarding is the name of the process which results in unmetalled roads, particularly gravel roads or dirt roads, developing a series of regular bumps with short spacing in the road surface. The result is called a washboard road...

) are caused by this wheel bounce.

Semi-active and active suspensions

If the suspension is externally controlled then it is a semi-active or active suspension - the suspension is reacting to what are in effect "brain" signals. As electronics have become more sophisticated, the opportunities in this area have expanded.

For example, a hydropneumatic Citroën will "know" how far off the ground the car is supposed to be and constantly reset to achieve that level, regardless of load. It will not instantly compensate for body roll due to cornering however. Citroën's system adds about 1% to the cost of the car versus passive steel springs.

Semi-active suspensions include devices such as air springs

Air suspension

Air suspension is a type of vehicle suspension powered by an engine driven or electric air pump or compressor. This pump pressurizes the air, using compressed air as a spring. Air suspension replaces conventional steel springs. If the engine is left off for an extended period, the car will settle...

and switchable shock absorbers, various self-levelling

Self-levelling suspension

Self-levelling refers to an automobile suspension system that maintains a constant ride height of the vehicle above the road, regardless of load....

solutions, as well as systems like Hydropneumatic, Hydrolastic

Hydrolastic

Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation and its successor companies....

, and Hydragas suspensions. Mitsubishi

Mitsubishi Motors

is the fifth largest automaker in Japan and the fifteenth largest in the world by global unit sales. It is part of the Mitsubishi keiretsu, formerly the biggest industrial group in Japan, and was formed in 1970 from the automotive division of Mitsubishi Heavy Industries.Throughout its history it...

developed the world’s first production semi-active electronically controlled suspension system in passenger cars; the system was first incorporated in the 1987 Galant

Mitsubishi Galant

The Mitsubishi Galant is an automobile manufactured by Mitsubishi Motors since 1969. The name was derived from the French word galant, meaning "chivalrous". There have been nine distinct generations, and cumulative sales now exceed five million. It began as a compact sedan, but over the course of...

model. Delphi currently sells shock absorbers filled with a magneto-rheological fluid, whose viscosity can be changed electromagnetically, thereby giving variable control without switching valves, which is faster and thus more effective.

Fully active suspension

Active suspension

Active or adaptive suspension is an automotive technology that controls the vertical movement of the wheels via an onboard system rather than the movement being determined entirely by the surface on which the car is driving...

systems use electronic monitoring of vehicle conditions, coupled with the means to impact vehicle suspension and behavior in real time to directly control the motion of the car. Lotus Cars

Lotus Cars

Lotus Cars is a British manufacturer of sports and racing cars based at Hethel, Norfolk, England. The company designs and builds race and production automobiles of light weight and high handling characteristics....

developed several prototypes, from 1982 onwards, and introduced them to F1, where they have been fairly effective, but have now been banned. Nissan introduced a low bandwidth active suspension in circa 1990 as an option that added an extra 20% to the price of luxury models. Citroën has also developed several active suspension models (see hydractive). A recently publicised fully active system from Bose Corporation uses linear electric motors, ie solenoids, in place of hydraulic or pneumatic actuators that have generally been used up until recently. The most advanced suspension system is Active Body Control

Active Body Control

Active Body Control, or ABC, is the Mercedes-Benz brand name used to describe fully-active suspension, that allows control of the vehicle body motions and therefore virtually eliminates body roll in many driving situations including cornering, accelerating, and braking.In the ABC system, a computer...

, introduced in 1999 on the top-of-the-line Mercedes-Benz CL-Class

Mercedes-Benz CL-Class

The Mercedes-Benz CL-Class is a full-sized grand tourer produced by the German automaker Mercedes-Benz, which stands for Comfort Leicht in German, or Comfort Light in English....

.

Several electromagnetic suspension

Electromagnetic suspension

Electromagnetic suspension is magnetic levitation achieved using servo-controlled electromagnets.-Suspension:The attraction from a fixed strength magnet decreases with increased distance, and increases at closer distances. This is termed 'unstable'...

s have also been developed for vehicles. Examples include the electromagnetic suspension of Bose, and the electromagnetic suspension developed by prof. Laurentiu Encica. In addition, the new Michelin wheel with embedded suspension working on a electromotor is also similar.

With the help of control system, various semi-active/active suspensions realize an improved design compromise among different vibrations modes of the vehicle, namely bounce, roll, pitch and warp modes. However, the applications of these advanced suspensions are constrained by the cost, packaging, weight, reliability, and/or the other challenges.

Interconnected suspensions

Interconnected suspension, unlike semi-active/active suspensions, could easily decouple different vehicle vibration modes in a passive manner. The interconnections can be realized by various means, such as mechanical, hydraulic and pneumatic. Anti-roll bars are one of the typical examples of mechanical interconnections, while it has been stated that fluidic interconnections offer greater potential and flexibility in improving both the stiffness and damping properties.

Considering the considerable commercial potentials of hydro-pneumatic technology (Corolla, 1996), interconnected hydropneumatic suspension

Hydropneumatic suspension

Hydropneumatic is a type of automotive suspension system, invented by Citroën, and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Rolls-Royce, Mercedes-Benz and Peugeot. It was also used on Berliet trucks...

s have also been explored in some recent studies, and their potential benefits in enhancing vehicle ride and handling have been demonstrated. The control system can also be used for further improving performance of interconnected suspensions. Apart from academic research, an Australian company, Kinetic, is having some success (WRC

World Rally Championship

The World Rally Championship is a rallying series organised by the FIA, culminating with a champion driver, co-driver and manufacturer. The driver's world championship and manufacturer's world championship are separate championships, but based on the same point system...

: 3 Championships, Dakar Rally

Dakar Rally

The Dakar Rally is an annual rally raid type of off-road automobile race, organised by the Amaury Sport Organisation...

: 2 Championships, Lexus GX470 2004 4x4 of the year with KDSS, 2005 PACE award) with various passive or semi-active systems, which generally decouple at least two vehicle modes (roll, warp (articulation), pitch and/or heave (bounce)) to simultaneous control each mode’s stiffness and damping, by using interconnected shock absorbers, and other methods. In 1999 Kinetic was bought out by Tenneco.

Historically, the first mass production car with front to rear mechanical interconnected suspension was the 1948 Citroën 2CV

Citroën 2CV

The Citroën 2CV was an economy car produced by the French automaker Citroën from 1949 to 1990. It was technologically advanced and innovative, but with extremely utilitarian and deceptively simple Bauhaus inspired bodywork, that belied the sheer quality of its underlying engineering...

. The suspension of the 2CV was extremely soft — it had low roll stiffness, but its pitch stiffness was increased by using an interconnected suspension. The leading arm / trailing arm swinging arm

Trailing arm

thumb|220px|Trailing arm rear suspension of [[Front-engine, front-wheel drive layout|FF]] carsA trailing-arm suspension is an automobile suspension design in which one or more arms are connected between the axle and the chassis. It is usually used on rear axles...

, fore-aft linked suspension system together with inboard front brakes had a much smaller unsprung weight

Unsprung weight

than existing coil spring or leaf designs. The interconnection transmitted some of the force deflecting a front wheel up over a bump, to push the rear wheel down on the same side. When the rear wheel met that bump a moment later, it did the same in reverse, keeping the car level front to rear. The 2CV had a design brief to be able to be driven at speed over a ploughed field. It originally featured friction dampers and tuned mass damper

Tuned mass damper

A tuned mass damper, also known as an active mass damper or harmonic absorber, is a device mounted in structures to prevent discomfort, damage, or outright structural failure caused by vibration...

s. Later models had tuned mass damper

Tuned mass damper

A tuned mass damper, also known as an active mass damper or harmonic absorber, is a device mounted in structures to prevent discomfort, damage, or outright structural failure caused by vibration...

s at the front with telescopic dampers / shock absorber

Shock absorber

A shock absorber in common parlance is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy.- Explanation :...

s front and rear.

Some of the last post war Packard

Packard

Packard was an American luxury automobile marque built by the Packard Motor Car Company of Detroit, Michigan, and later by the Studebaker-Packard Corporation of South Bend, Indiana...

models also featured interconnected suspension. The original Mini and some more recent British Leyland models also featured interlinking, when fitted with Moulton's Hydrolastic or Hydragas suspensions.

Suspension Geometry

Suspension systems can be broadly classified into two subgroups – dependent and independent. These terms refer to the ability of opposite wheels to move independently of each other.

A dependent suspension normally has a beam

Beam axle

A beam axle is a suspension system, also called a solid axle, in which one set of wheels is connected laterally by a single beam or shaft...

(a simple 'cart' axle) or (driven) live axle

Live axle

A live axle, sometimes called a solid axle, is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit....

that holds wheels parallel to each other and perpendicular to the axle. When the camber

Camber angle

of one wheel changes, the camber of the opposite wheel changes in the same way (by convention on one side this is a positive change in camber and on the other side this a negative change). De Dion suspensions are also in this category as they rigidly connect the wheels together.

An independent suspension
Independent suspension
Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically independently of each other. This is contrasted with a beam axle, live axle or deDion axle system in which the wheels are linked - movement on one side affects...

allows wheels to rise and fall on their own without affecting the opposite wheel. Suspensions with other devices, such as sway bar

Sway bar

A sway bar is an automobile suspension device. It connects opposite wheels together through short lever arms linked by a torsion spring. A sway bar increases the suspension's roll stiffness—its resistance to roll in turns, independent of its spring rate in the vertical direction...

s that link the wheels in some way are still classed as independent.

A third type is a semi-dependent suspension. In this case, the motion of one wheel does affect the position of the other but they are not rigidly attached to each other. A twist-beam rear suspension

Twist-beam rear suspension

The twist-beam rear suspension, also known as torsion-beam axle is a type of automobile suspension based on a large H or C shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each stub-axle assembly, on each side of the car...

is such a system.

Dependent suspensions

Dependent systems may be differentiated by the system of linkages used to locate them, both longitudinally and transversely. Often both functions are combined in a set of linkages.

Examples of location linkages include:

Satchell link
Panhard rod
Panhard rod
A Panhard rod is a component of a car suspension system that provides lateral location of the axle...
Watt's linkage
Watt's linkage
Watt's linkage is a type of mechanical linkage invented by James Watt to constrain the movement of a steam engine piston in a straight line....
WOBLink
Mumford linkage
leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times....

s used for location (transverse or longitudinal)
- Fully elliptical springs usually need supplementary location links and are no longer in common use
- Longitudinal semi-elliptical springs used to be common and still are used in heavy-duty trucks. They have the advantage that the spring rate can easily be made progressive (non-linear)
- A single transverse leaf spring for both front wheels and/or both back wheels, supporting solid axles was used by Ford Motor Company
  Ford Motor Company
  The Ford Motor Company is an American multinational corporation based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford, Lincoln, and Mercury brands, Ford also owns Volvo Cars of Sweden, and a small stake...
  
  , before and soon after World War II
  World War II
  World War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
  
  , even on expensive models. It had the advantages of simplicity and low unsprung weight (compared to other solid axle designs).

In a front engine, rear-drive vehicle, dependent rear suspension is either "live axle" or deDion axle, depending on whether or not the differential is carried on the axle. Live axle is simpler but the unsprung weight contributes to wheel bounce.

Because it assures constant camber, dependent (and semi-independent) suspension is most common on vehicles that need to carry large loads as a proportion of the vehicle weight, that have relatively soft springs and that do not (for cost and simplicity reasons) use active suspensions. The use of dependent front suspension has become limited to heavier commercial vehicles.

Semi-Independent Suspension

In a semi-independent suspensions the wheels of an axle are able to move relative to one another as in an independent suspension but the position of one wheel has an affect on the position and attitude of the other wheel. This effect is achieved via the twisting or deflecting of suspension parts under load. The most common type of semi-independent suspension is the twist beam.

Twist beam
Twist-beam rear suspension
The twist-beam rear suspension, also known as torsion-beam axle is a type of automobile suspension based on a large H or C shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each stub-axle assembly, on each side of the car...

Independent suspensions

The variety of independent systems is greater and includes:

Swing axle
Swing axle
A swing axle is a simple type of independent suspension first used in early aircraft , such as the Sopwith and Fokker, usually with rubber bungee and no damping....
Sliding pillar
Sliding pillar
A sliding pillar suspension is one in which the stub axle and wheel assembly are attached to a fixed vertical "pillar" or kingpin which slides up and down through a bush or bushes which are attached to the vehicle chassis, usually as part of transverse outrigger assemblies.Sliding pillar...
MacPherson strut
MacPherson strut
The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot, widely used in modern vehicles and named after Earle S. MacPherson who developed the design, based in part on designs created by Guido Fornaca of FIAT in the mid-1920s...

/Chapman strut
Chapman strut
A Chapman strut is an automobile suspension device. Being a strut, it is designed to act as both a shock absorber and a wheel location device. That is, it is designed to resist lateral forces. Chapman struts are the term for such a device when used on rear wheels, while the very similar...
Upper and lower A-arm (double wishbone)
multi-link suspension
Multi-link suspension
A multi-link suspension is a type of vehicle suspension design typically used in independent suspensions, using three or more lateral arms, and one or more longitudinal arms. These arms do not have to be of equal length, and may be angled away from their 'obvious' direction.Typically each arm has a...
semi-trailing arm suspension
swinging arm
leaf springs
- Transverse leaf springs when used as a suspension link, or four quarter elliptics on one end of a car are similar to wishbones in geometry, but are more compliant. Examples are the front of the original Fiat 500
  Fiat 500
  The Fiat 500 is a car produced by the Fiat company of Italy between 1957 and 1975, with limited production of the Fiat 500 K estate continuing until 1977. The car was designed by Dante Giacosa....
  
  , the Panhard Dyna Z
  Panhard Dyna Z
  The Panhard Dyna Z was a light weight motor car made by Panhard of France. It was first presented to the press at a Paris restaurant named "Les Ambassadeurs" on 17 June 1953 and went into production the following year...
  
  and the early examples of Peugeot 403
  Peugeot 403
  The Peugeot 403 is a car produced by French automobile manufacturer Peugeot from 1955 to 1966.Styled by Pininfarina, the 403 featured an enlarged version of the Peugeot 203's 1290 cc petrol engine...
  
  and the back of the AC Ace
  AC Ace
  AC Ace is a car made by AC Cars of Thames Ditton, England.AC came back to the market after the Second World War with the staid Two-litre range of cars in 1947, but it was with the Ace sports car of 1953 that the company really made its reputation in the post war years...
  
  and AC Aceca
  AC Aceca
  The Aceca is a closed coupé from the British AC Cars company, produced from 1954 until 1963. The car originally had an AC engine but the similar Bristol-engined Aceca-Bristol was also available alogside the original from 1956 to 1963 when production of the engine ceased...
  
  .

Because the wheels are not constrained to remain perpendicular to a flat road surface in turning, braking and varying load conditions, control of the wheel camber is an important issue. Swinging arm was common in small cars that were sprung softly and could carry large loads, because the camber is independent of load. Some active and semi-active suspensions maintain the ride height, and therefore the camber, independent of load. In sports car

Sports car

The term sports car has been defined as "an open, low-built, fast motor car." The term describes a class of automobile with two seats, two doors, precise handling, brisk acceleration, and sharp braking — trading practical considerations such as passenger space, comfort, and cargo capacity...

s, optimal camber change when turning is more important.

Wishbone and multi-link allow the engineer more control over the geometry, to arrive at the best compromise, than swing axle, MacPherson strut or swinging arm do; however the cost and space requirements may be greater. Semi-trailing arm is in between, being a variable compromise between the geometries of swinging arm and swing axle.

Armoured fighting vehicle suspension

Military AFV

Armoured fighting vehicle

An armoured fighting vehicle is a military vehicle, protected by armour and armed with weapons. AFV's can be wheeled or tracked.Armoured fighting vehicles are classified according to their intended role on the battlefield and characteristics. This classification is not absolute; at different times...

s, including tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility and tactical offensive and defensive capabilities...

s, have specialized suspension requirements. They can weigh more than seventy tons and are required to move at high speed over very rough ground. Their suspension components must be protected from land mine

Land mine

A land mine is a target triggered explosive weapon. Their non-explosive predecessors have been used on the battlefield since ancient times. Landmines were designed to be used to deter, channel, delay and kill an enemy. They have been used in various formats, for centuries and have featured in all...

s and antitank weapons. Tracked

Caterpillar track

Continuous tracks are large tracks used on the so-called caterpillar tanks, construction equipment and certain other off-road vehicles. Unlike the Kégresse tracks which use a flexible belt, most continuous tracks are made of a number of rigid units that are joined to each other...

AFVs can have as many as nine road wheels on each side. Many wheel

Wheel

ed AFVs have six or eight wheels, to help them ride over rough and soft ground.

The earliest tanks of the Great War had fixed suspensions—with no movement whatsoever. This unsatisfactory situation was improved with leaf spring

Leaf spring

Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times....

suspensions adopted from agricultural machinery, but even these had very limited travel.

Speeds increased due to more powerful engines, and the quality of ride had to be improved. In the 1930s, the Christie suspension

Christie suspension

The Christie suspension is a suspension system developed by American engineer Walter Christie for his tank designs. It allowed considerably longer movement than conventional leaf spring systems then in common use, which allowed his tanks to have considerably greater cross-country speed and a lower...

was developed, which allowed the use of coil spring

Coil spring

A Coil spring, also known as a helical spring, is a mechanical device, which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces...

s inside a vehicle's armoured hull, by redirecting the direction of travel using a bell crank

Bell crank

A bell crank is a type of crank that changes motion around a 90 degree angle. The name comes from its first use, changing the vertical pull on a rope to a horizontal pull on the striker of a bell, used for calling staff in large houses or commercial establishments.The bell crank consists of an "L"...

. Horstmann suspension

Horstmann suspension

Horstmann suspension is a type of tank and other tracked armoured fighting vehicle suspension devised by the British engineer Sidney Horstmann in 1922....

was a variation which used a combination of bell crank and exterior coil springs, in use from the 1930s to the 1990s.

By the Second World War the other common type was torsion-bar suspension, getting spring force from twisting bars inside the hull—this had less travel than the Christie type, but was significantly more compact, allowing the installation of larger turret rings and heavier main armament. The torsion-bar suspension, sometimes including shock absorbers, has been the dominant heavy armored vehicle suspension since the Second World War

World War II

World War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...

External links

Tire Rack article: Understanding Basic Suspension Terminology
How Car Suspensions Work

Suspension (vehicle)

History

Horse drawn vehicles

Automobiles

Spring rate

Mathematics of the spring rate

Wheel rate

Roll couple percentage

Weight transfer

Unsprung weight transfer

Sprung weight transfer

Jacking forces

Travel

Damping

Camber control

Roll center height

Instant center

Anti-dive and anti-squat

Flexibility and vibration modes of the suspension elements

Isolation from high frequency shock

Contribution to unsprung weight and total weight

Space occupied

Force distribution

Air resistance (drag)

Cost

Springs and dampers

Passive suspensions

Springs

Dampers or shock absorbers

Semi-active and active suspensions

Interconnected suspensions

Suspension Geometry

Dependent suspensions

Semi-Independent Suspension

Independent suspensions

Armoured fighting vehicle suspension

See also

External links