Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose - contributing to the car's handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.
This may vary with deflection. For active suspensions, it may depend on other things. The softer the springs, the more important the other requirements are. Spring rate is often a compromise between comfort and handling, but when other things are compromised instead, as in the 1960s Lotus Elan, both may be achieved.
Spring rates typically have units of lbf/in. or N/mm. 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 (typically increasing) is one that the force exerted increasess exponentially. For example, the first inch exerts 500 lbf, the second inch exerts an additional 550 lbf, the third inch exerts another 600 lbf.
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.
This may also vary, intentionally or unintentionally. Like spring rate, the optimal damping for comfort may be less than for control.
Damping controls the body movement of the car. 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.
See dependent and independent below.
Camber changes with wheel travel and with body roll.
A tyre wears and brakes best perpendicular to the road. Depending on the tyre, it may hold the road best at a slightly different angle. Small changes in camber, front and rear, are used to tune handling.
Roll Center HeightEdit
This is important to body roll and to relative weight transfer, front and rear. It may affect tendency to rollover. All other things being equal the end of the car with the higher roll center will have more weight transfer and therefore more slip in a turn. However, the roll stiffness in most cars is set more by the antiroll bars than the RCH.
Flexibility and vibration modes of the suspension elementsEdit
In modern cars, the flexibility is mainly in the rubber bushings.
Isolation from high frequency shockEdit
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 weightEdit
Designs differ as to how much space they take up and where it is located.
The suspension attachment must match the frame design in geometry, strength and rigidity.
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 dampersEdit
All suspensions use springs to absorb impacts and dampers (or shock absorbers) to control spring motions. A number of different types of each have been used:
Passive, Semi Active, and Active SuspensionsEdit
Traditional springs and dampers are referred to as passive suspensions. If the suspension is externally controlled then it is a semi-active or active suspension.
Semi-active suspensions include devices such as air springs and switchable shock absorbers, various self-levelling solutions, as well as systems like Hydropneumatic, Hydrolastic, and Hydragas suspensions. Delphi currently sells shock absorbers filled with a magneto-rheological fluid, whose viscousity can be changed electromagnetically, thereby giving variable control without switching valves, which is faster, and probably cheaper and better. An Australian company, Kinetic, is having (as of 2005) some success with various semi-active systems, which provide adjustable roll control and damping, by using cross linked shock absorbers, and other methods. They have now been bought out by Tenneco and Alcorn.
For example, a hydropneumatic Citroen 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. Citroen's system adds about 1% to the cost of the car versus passive steel springs.
Fully active suspensions 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 developed several prototypes, 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. Citroen has also developed several active suspension models.
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.
- Leaf spring - AKA Hotchkiss, Cart, or semi-elliptical spring
- Torsion beam suspension
- Coil spring
- Rubber bushing
- Air spring
Dampers or shock absorbersEdit
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 of a wheel, hub, axle and sometimes brakes and differential bounces up and down on the springiness of a tire. The "corduroy" bumps found on dirt roads are caused by this wheel bounce. These bumps are more common on US dirt roads, where solid rear axles are common, than they are in e.g. French dirt roads, where unsprung weight tends to be low and suspensions well damped.
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 live axle (a simple beam or 'cart' axle) that holds wheels parallel to each other and perpendicular to the axle. When the camber of one wheel changes, the camber of the opposite wheel changes in the same way.
An independent suspension allows wheels to rise and fall on their own without affecting the opposite wheel. Suspensions with other devices, such as anti-roll bars that link the wheels in some way are still classed as independent.
A third type is a semi-dependent suspension. In this case, jointed axles are used, on drive wheels, but the wheels are connected with a solid member, most often a deDion axle. This differs from "dependent" mainly in unsprung weight.
Interconnected suspensions (mechanically interconnected, such as anti-roll bars; and hydraulically or pneumatically interconnected, e.g., SAE 2005-01-3593, SAE 2003-01-0312) have also been used to achieve a better compromise among vertical, roll and pitch properties.
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:
- Trailing arms
- Satchell link
- Panhard rod
- Watts linkage
- Mumford linkage
- leaf springs 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 on some US cars and on 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, before and soon after World War II, even on expensive models. It had the advantages of simplicity and low unsprung weight (compared to other solid axle designs), as well as the other advantages of solid axles.
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 cary 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. However the use of dependent front suspension has become limited to a few trucks.
Independent suspensions Edit
The variety of independent systems is greater and includes:
- Swing axle
- MacPherson strut/Chapman strut
- A arm or wishbone suspension
- multi-link suspension
- semi-trailing arm suspension
- swinging arm
- leaf springs
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 hight, and therefore the camber, independent of load. In sports cars, 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 compromize between the geometries of swinging arm and swing axle.
Armoured fighting vehicle suspensionEdit
Military AFVs, including tanks, 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 mines and antitank weapons. Tracked AFVs can have as many as nine road wheels on each side. Many wheeled 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 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 was developed, which allowed the use of coil springs inside a vehicle's armoured hull, by redirecting the direction of travel using a bell crank. Horstmann suspension 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.
Torsion bar suspensions have been the dominant heavy armored vehicle suspension since the Second World War, sometimes but not always including shock absorbers.