Double wishbone suspension

In automobiles, a double wishbone (or “upper and lower A-arm”) suspension is an independent suspension design using two parallel wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting positions to the chassis and one joint at the knuckle. The shock absorber and coil spring mount to the wishbones to control vertical movement. Double wishbone designs allow the engineer to carefully control the motion of the wheel throughout suspension travel, controlling such parameters as camber angle, caster angle, toe pattern, roll center height, scrub radius, scuff and more.

The double wishbone suspension is also often referred to as double ‘A’ arm or short long arm (SLA) suspension. It is commonly used in sports cars, luxury cars and light trucks.

A single wishbones or A-arms are used in various other suspension types, such as MacPherson strut and Chapman strut.

The suspension consists of a pair of upper and lower lateral arms, roughly horizontal and of similar length. The upper arm is usually slightly shorter to induce more negative camber on the outside wheel as the vehicle body rolls in a turn. Between the arms there is a knuckle with a spindle or hub which carries the wheel bearing and wheel. Knuckles with an integral spindle usually do not allow the wheel to be driven. A bolt on hub design is commonly used if the wheel is to be driven.

In order to resist fore-aft loads such as acceleration and braking, the arms need two bushings or ball joints at the body.

At the knuckle end, single ball joints can be used, in which case the steering loads have to be taken via a steering arm, and the wishbones look A- or L-shaped. An L-shaped arm is generally preferred on passenger vehicles because it allows a better compromise of handling and comfort to be tuned in. The bushing in line with the wheel can be kept relatively stiff to effectively handle cornering loads while the offline joint can be softer to allow the wheel to recess under fore aft impact loads. For a rear suspension, a pair of joints can be used at both ends of the arm, making them more H-shaped in plain view.

In front view, the suspension is a 4-bar link, and it is easy to work out the camber gain (see camber angle) and other parameters for a given set of bush locations.

The various bushes do not have to be on horizontal axes, parallel to the vehicle centre line. If they are set at an angle, then antidive and antisquat can be dialled in.

The advantage of a double wishbone suspension is that it is fairly easy to work out the effect of moving each joint, so you can tune the kinematics of the suspension easily and optimize wheel motion. It is also easy to work out the loads that different parts will be subjected to which allows more optimized lightweight parts to be designed.

The disadvantage is that it is slightly more complex than other systems like a MacPherson strut.

SLAs are very common on front suspensions for larger cars, pickups, and SUVs; double wishbones are very common at both ends of racing cars.

Prior to the dominance of front wheel drive in the 1980s, many everyday cars used double wishbone front suspension systems, or a variation on it. Since that time, the MacPherson strut has become almost ubiquitous, as it is simpler and cheaper to manufacture. Double wishbones are usually considered to have superior dynamic characteristics, load handling capability and are still found on higher performance vehicles.

Watt’s linkage

Watt’s linkage was invented by James Watt (1736–1819) to constrain the movement of a piston in a steam engine to move in a straight line.

The idea of its genesis using links is contained in a letter he wrote to Matthew Boulton in June 1784.

I have got a glimpse of a method of causing a piston rod to move up and down perpendicularly by only fixing it to a piece of iron upon the beam, without chains or perpendicular guides and one of the most ingenious simple pieces of mechanics I have invented.

This linkage does not generate a true straight line motion, and indeed Watt did not claim it did so. In a letter to Boulton on 11th September 1784 he describes the linkage as follows.

The convexities of the arches, lying in contrary directions, there is a certain point in the connecting-lever, which has very little sensible variation from a straight line.

The Watt’s linkage is also an automotive rear suspension designed in the early twentieth century as an improvement over the Panhard rod as a means of locating a rear beam axle of an automobile relative to the body and preventing relative movement side to side. Whereas the Panhard rod is pivoted at both axle and body forcing the axle to move in an arc, thus introducing a sideways component into the vertical movement of the axle, the Watt’s linkage ensures pure vertical motion.

It consists of two almost symmetrically arranged long rods mounted one at each side of the chassis and running parallel to and behind the rear axle, where they attach to the ends of a short vertical bar whose center is mounted to the center of the axle and which is free to rotate in the plane at right angles to the automobile’s longitudinal dimension. As in the Panhard rod, the sideways arms themselves are free to pivot vertically at either end. Thus, each sideways member acts as a shorter Panhard rod mounted to the center vertical member providing lateral location. In contrast to the Panhard rod’s action, however, the sideways components of the motion of the two arms as they pivot around their outboard mountings cancel each other in their effect on the axle and are instead taken up by the center member’s rotation about its axis.

It should be noted that the Watt’s linkage is not limited in use as an improved version of a Panhard rod. Many rearend racing suspension systems also use the Watt’s linkage to locate the rear axle from front to rear. This application usually requires 2 Watt’s linkages, one on the driver’s side and one on the passenger’s side. Suspension binding during deflection is prevented by floating the center linkage around the axle instead of welding it directly to the axle. Some means of preventing the rear axle from rotating is then needed which is a form of “caster” control. Often times only one side is “floated” which eliminates the axle rotation issue.

Unsprung weight

In a ground vehicle with a suspension, the unsprung weight (or, more properly, the unsprung mass) is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them, rather than supported by the suspension. (The mass of the body and other components supported by the suspension is the sprung mass.) Unsprung weight includes the mass of components such as the wheel spindles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle’s brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.

The unsprung weight of a wheel controls a trade-off between a wheel’s bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression–which induces a force on the unsprung weight. In time, the unsprung weight then responds to this force with movement of its own. The amount of movement is inversely proportional to the weight – a lighter wheel which readily moves in response to road bumps will have more grip when tracking over an imperfect road. For this reason, lighter wheels are often sought for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality is deteriorated.

Pneumatic or elastic tires help by providing some springing for most of the (otherwise) unsprung mass, but the damping that can be included in the tires is limited by considerations of fuel economy and overheating. The shock absorbers, if any, damp the spring motion also and must be less stiff than would optimally damp the wheel bounce. So the wheels execute some vibrations after each bump before coming to rest. On dirt roads and perhaps on some softly paved roads, these motions form small bumps, known as washboarding or “corduroy” because they resemble smaller versions of the bumps in roads made of logs. These cause sustained wheel bounce in subsequent vehicles, enlarging the bumps.

High unsprung weight also exacerbates wheel control under hard acceleration or braking. If the vehicle does not have adequate wheel location in the vertical plane (such as a rear-wheel drive car with Hotchkiss drive, a live axle supported by simple leaf springs), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control.

Though this is usually not considered important, at least in the popular literature, there is a positive effect. High frequency road irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the body more completely because the tires and springs act as separate filter stages, with the unsprung weight tending to uncouple them. This can improve overall safety.

Unsprung weight is largely a function of the design of a vehicle’s suspension and the materials used in the construction of suspension components. Beam axle suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung weight than independent suspension systems, in which the wheels are suspended and allowed to move separately. Heavy components such as the differential can be made part of the sprung weight by connecting them directly to the body (as in a de Dion tube rear suspension). Lightweight materials, such as aluminum, plastic, carbon fiber, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility.

Inboard brakes make a big difference, but put more load on half axles and (constant velocity) universal joints and require space that may not be easily accommodated.

Trailing arm

A trailing-arm suspension is an automobile suspension design in which one or more arms (or “links”) are connected between (and perpendicular to and forward of) the axle and the chassis. It is usually used on rear axles. A ‘leading arm’ as used on a Citroen 2CV, has an arm connected between (and perpendicular to, and to the rear of) the axle and the chassis. It is used on the front axle.

Trailing-arm designs in live axle setups often use just two or three links and a Panhard rod to locate the wheel laterally. A trailing arm design can also be used in an independent suspension arrangement. Each wheel hub is located only by a large, roughly triangular arm that pivots at one point, ahead of the wheel. Seen from the side, this arm is roughly parallel to the ground, with the angle changing based on road irregularities.

A semi-trailing arm suspension is an independent rear suspension system for automobiles in which each wheel hub is located only by a large, roughly triangular arm that pivots at two points. Viewed from the top, the line formed by the two pivots is somewhere between parallel and perpendicular to the car’s longitudinal axis; it is generally parallel to the ground. Trailing-arm and multilink suspension designs are much more commonly used for the rear wheels of a vehicle where they can allow for a flatter floor and more cargo room. Many small vehicles feature a MacPherson strut front suspension and trailing-arm rear axle.

Transaxle

A transaxle, in the automotive field, is a component that combines the functionality of the transmission, the differential and the drive axle into one integrated assembly. Transaxles are near universal in all automobile configurations that have the engine placed at the same end of the car as the driven wheels: the front wheel drive, rear-engined and mid-engined arrangements.

Many mid and rear-engined vehicles use a transverse engine and transaxle, similar to a front wheel drive unit. Others use both a longitudinal engine and transaxle. Ferrari’s 1989 Mondial t introduced a “t” arrangement with a longitudinal engine connected to a transverse transaxle, a design the company continues to this day.

Torsion beam suspension

Torsion beam suspension, also known as a torsion bar or torsion spring suspension, is a vehicle suspension system. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, mounted perpendicular to the bar, that is attached to the axle of the suspension arm or wishbone. Vertical motion of the wheel causes the bar to rotate along its axis and is resisted by the bar’s torsion resistance. The effective spring rate of the bar is determined by its length, diameter, and material.

Torsion Bar Suspensions are currently used on trucks and SUV’s from Ford, GM and Dodge. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for heavier or lighter engine packages. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock keys too far can bend the adjusting bolt and (more importantly) place the shock piston outside the standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump stop prematurely, causing a harsh ride. Aftermarket forged torsion key kits use re-clocked adjuster keys to prevent over-rotation, as well as shock brackets that keep the piston travel in the stock position.

The main advantages of torsion beam suspension are durability, easy adjustability of ride height, and small profile along the width of the vehicle. It provides a longer travel than leaf spring systems, and takes up less of the vehicle’s interior volume compared to coil springs. A major disadvantage is that torsion bars, unlike coil springs, usually cannot provide a progressive spring rate, forcing designers to compromise between ride quality and handling ability – progressive torsion bars are available, but at the expense of durability since they have a tendency to crack where the diameter of the bar changes. In most torsion bar systems, especially Chrysler’s, ride height (and therefore many handling features) may be adjusted by bolts which connect the torsion bars to the steering knuckles and require nothing more than crawling under the car with a wrench in hand. In most cars which use this type of suspension, swapping torsion bars for those with a different spring rate is usually an extremely easy task.

Some vehicles use torsion bars to provide automatic leveling, using a motor to tighten the bars to provide greater resistance to load and, in some cases (depending on the speed with which the motors can act), to respond to changes in road conditions. Height adjustable suspension has been used to implement a wheel-change mode where the vehicle is raised on three axles and the remaining wheel is lifted off the ground without the aid of a jack.

Before World War II, the front wheel drive Citroen Traction Avant (1934) had independent front torsion bar suspension and a trailing dead axle, also sprung by torsion bars. The Czechoslovakian Tatra cars designed by Professor Hans Ledwinka in the mid 1930s used all round independent torsion bar suspension, along with air cooled rear engines. Also in the 1930s, prototypes of the first Volkswagen Beetle incorporated torsion bars – especially its transverse mounting style. Ledwinka’s concept had been copied by Ferdinand Porsche, whose successors later had to acknowledge the influence of Ledwinka’s Tatra models on the Porsche-designed Kdf-Wagen of 1938 (later known as the VW Beetle), a post-war lawsuit resulting in a DM3,000,000 settlement paid by Volkswagen to Ringhoffer-Tatra in 1961.

The system was applied to many new armoured fighting vehicle designs during the Second World War. It was used extensively in European cars Renault, Citroen and Volkswagen, as well as by Packard in the 1950s. The Packard used torsion bars at both front and rear, and interconnected the front and rear systems to improve ride quality. The most famous American passenger-car application was the Chrysler system used beginning with the 1957 model year, although Chrysler’s “Torsion-Aire” suspension was only for the front; the same basic system (longitudinal mounting) was maintained until the 1981 introduction of the K-car. A reengineered torsion beam suspension, introduced with the 1976 Dodge Aspen, introduced transverse-mounted torsion beams (possibly based on the Volkswagen Type 3 passenger car) until production ended in 1989 (with Chrysler’s M platform). Light-duty Dodge trucks however continue to use torsion bars on their front suspension.

General Motors has used torsion bars since 1966, starting with the E-platform vehicles (Oldsmobile Toronado, Cadillac Eldorado), 4 wheel drive S-10 pickups, and since 1988, full size trucks (GMT400, GMT800, and GMT900 series).

Some front-wheel drive automobiles use a type of torsion bar suspension, usually called a Twist-beam rear suspension, in which the rear wheels are carried on trailing arms connected by a laterally mounted torsion beam. The torsion beam functions both as wheel-locating arm and as an anti-roll bar to resist lateral motion of the wheels as the body leans in turns. Its advantages are that it is inexpensive to manufacture and install, and engages a minimum amount of interior volume, leaving more space for the carriage of passengers, cargo, and other components. Because the torsion bar acts in the lateral plane, not vertically, the twist-beam axle cannot provide ride-height adjustment, and it suffers, to some extent, similar car handling limitations as other beam axle suspensions. However these limitations may not be apparent on the road, because of the trend towards firmer, more sporty suspension setups with more limited wheel travel. Twist-beam rear suspensions were pioneered on the Volkswagen Golf in the early 1970s, and remain common on compact cars and minivans.

Swing axle

A swing axle suspension is a simple type of independent suspension used in automobiles. Swing axles have universal joints connecting the driveshafts to the differential, which is attached to the chassis. They do not have universal joints at the wheels – the wheels are always perpendicular to the driveshafts. Swing axle suspensions traditionally used leaf springs and shock absorbers. Pre 1967 Volkswagens used torsion bars as their spring.

This type of suspension was considered better than the more typical solid axle for two reasons:

1. It reduced unsprung weight since the differential is mounted to the chassis
2. It eliminates sympathetic camber changes on opposite wheels

However, there are a number of shortcomings to this arrangement:

1. A great amount of single-wheel camber change is experienced since the wheel is always perpendicular to the driveshaft
2. “Jacking” on suspension unloading (or rebound) causes positive camber changes on both sides
3. Reduction in cornering forces due to change in camber can lead to oversteer instability and in extreme cases lift-off oversteer

These problems were evident on Volkswagen up until 1967 and others.

Swing axles were supplanted by deDion axles in the late 1960s, though live axles remained the most common. Most rear suspensions have been replaced by more modern independent suspensions in recent years, and both swing and deDion types are virtually unused today.

The First Production (1960-1964) Chevrolet Corvair used this design. The alleged unsafe behaviour of the Corvair was described in detail by Ralph Nader in his book Unsafe at Any Speed. Second Production Corvairs (1965-1969) used a true independent rear suspension system.

Sway bar

A sway bar (also stabilizer bar, anti-sway bar, roll bar, or anti-roll bar, ARB) is an automobile suspension device. It connects opposite (left/right) 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.

Sway bar: Principles

In a turn, the sprung mass of the vehicle’s body rotates around its roll axis. The roll axis is a line that joins the front and rear roll centers (SAEJ670e). If the vertical distance between the roll axis and the center of gravity is not zero, a torque (roll moment) equal to the centrifugal force times the distance between the center of gravity and the roll axis will be exerted on the sprung mass, causing the body to lean towards the outside of the turn. This force is called the roll couple. One effect of body (frame) lean, for typical suspension geometry, is positive camber of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip (especially with cross ply tires).

Roll couple is resisted by the suspension’s roll stiffness, which is a function of the spring rate of the vehicle’s springs and of the anti-roll bars, if any. The use of anti-roll bars allows designers to reduce body lean without making the suspension’s springs stiffer in the vertical plane, which allows improved body control with less compromise of ride quality.

The spring rate of an anti-roll bar is based on the fourth power of the torsion bar’s diameter, the stiffness of the material, the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar), the geometry of the mounting points, and the rigidity of the bar’s mounting points. Some anti-roll bars, particularly those intended for use in auto racing, are adjustable, allowing their stiffness to be altered by increasing or reducing the length of the lever arms. This permits the roll stiffness to be tuned for different situations without replacing the entire bar.

Anti roll bars provide 2 main functions:

The first is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.

The other function of anti roll bars is to tune the high g / limit understeer behavior of the vehicle. The limit understeer behavior is tuned by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total weight transfer that the front axle reacts and decrease the proportion that the rear axle reacts. This will cause the outer front wheel to run at a higher slip angle, and the outer rear wheel to run at a lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.

Sway bar: Drawbacks

Because an anti-roll bar connects wheels on the opposite sides of the vehicle together, the bar will transmit the force of one-wheel bumps to the opposite wheel. On rough or broken pavement, anti-roll bars can produce jarring, side-to-side body motions (a “waddling” sensation), which increase in severity with the diameter and stiffness of the sway bars. Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, will cause the inside wheels to lift off the ground during very hard cornering. This, of course, is only possible if the regular spring rate actually allows the outside wheels to handle the much increased load. This can be used to advantage, in fact many front wheel drive production cars will lift a wheel when cornering hard, in order to overload the other wheel on the axle, so providing limit understeer.

Some high-priced cars, such as the Mercedes S-class and BMW 7-series, have begun to use “active” anti-roll bars that can be connected or disconnected automatically by a suspension-control computer, reducing body lean in turns while improving rough-road ride quality.