The Radial-ply tire looks more bulbous than the cross-ply and generally has a broader-looking tread. The difference between them in their behaviour on the road is governed by the difference in their carcass constructions.

The radial-ply has a stiffness and resistance in its tread area, so that in motion the tread in the contact patch (the section that touches the road surface) retains virtually all of its pattern and grip.

On the cross-ply, the contact patch becomes pinched and compressed.

The radial-ply tire corners more tightly than the cross-ply because it has a lower slip angle. This is the difference between the path in which the wheel is pointed on a corner and the path it actually follows.

The cross-ply steers better at low speeds and is easier for parking.

The deflection of a tire the extent to which the carcass is deformed radially under the vertical load which it carries— provides a cushioning effect. The higher the deflection, the greater the cushioning.

A radial-ply tire has a higher deflection than a cross-ply tire and it will usually give a more comfortable ride at 65 km/h and over. However, at low speeds, the comparative stiffness of the radial hoop tends to reflect the irregularities in the road surface.

Changing Tires

Tire technology is changing quickly with many new developments moving from the experimental stage to the production line in recent years. Some tires have been designed to run flat. These are usually made in conjunction with a wheel, or specially designed so that a car can continue to a garage fora puncture repair if the tire deflates. This type of tire is strictly limited to a greatly reduced speed when running flat, as driving on a deflated tire generates considerable heat which will eventually damage the tire’s carcass.

Two other developments have taken place to improve a tire’s ability to grip the road surface. One, using a dedicated wheel, permits the sidewall to flex through a greater angle permitting improved tolerance of road conditions. Another displaces the carcass radially for more precise tread contact patch control.

Space-saving (slimline) tires are made for cars with restricted space for storing spare tires. They are smaller than normal tires and designed strictly for emergencies, to be used at restricted speed, when one of the main tires has been punctured.

Low rolling resistance tires, using alternative carcass designs and specially formulated rubber compounds improve fuel economy.

Dual purpose tires for four-wheel drive vehicles that spend most of their operating time on sealed roads have a tread designed to be quieter, while still offering a degree of grip in mud, on grass or over sand. Most off-road tires have a speed limitation imposed on them.

Radial-ply tires are fitted to all new cars. A large proportion of the tires replaced on older cars are also radial-ply.

The construction of a radial-ply tire reduces cornering wear and considerably increases the overall life of the tire; but this may be at the expense of slight harshness at lower speeds.

Radial-ply tires are made in two parts; the layers, or plies, and a belt of cords.

The cords in the plies run from bead to bead across the crown at right angles, not diagonally as in the cross-ply tire. This gives great pliability and comfort, but little or no directional stability. Stability comes from a belt of cords running around the circumference of the tire beneath the tread. Belt cords are usually spun from rayon or fine steel wire and are flexible, but do not lose their tautness. So the belt firmly restricts any lateral stretching of the tire’s tread.

A recent development in the manufacture of radial-ply tires is the use of glass fibre for breakers. Glass fibre is very strong and surprisingly elastic; but special techniques have to be used to prevent the chafing of the filaments, and to achieve a satisfactory bond between them and the rubber casing. To help it keep its stiffness against sideways forces, a belt is made of a least two layers of cord which are slightly diagonal to the circumference. The angle between the cord varies from 18 to 22 degrees. The number of layers depends on the material used, the lateral stiffness needed and the load the tire is required to carry.

If steel cords are used in a belt, there are usually only two; but as many as four, five or six cords may be used if they are made of rayon or other fibre. Where four are used, they are folded over and sometimes interleaved, to increase their lateral stability. Steel makes a stronger cord and does not stretch under inflation pressure.

Six layers may be produced by interleaving two or three plies, suitably folded. Nylon, first used for breakers in aircraft tires, is stronger, more elastic and more flexible than rayon and keeps the tire cooler. It is used in car tires for high speeds or heavy loads, but only in combination with other materials.

When cold, nylon cords tend to lose their flexibility. If nylon cords alone were used, the section of the tire resting on the road would change its shape and give a bumpy ride until the casing had warmed up enough to recover its flexibility.

Cross-ply tires

Cross-plytires have a casing of two or more layers of fabric. They are rarely fitted to modern cars, but are sometimes still found on commercial vehicles. Originally, the threads, or cords, were at right angles to the direction of rotation.

This design gave a comfortable ride but had side-effects on the steering. It was found that placing the cords parallel to the direction of rotation greatly improved the directional stability; but it also reduced the comfort.

As the tires have to provide a comfortable ride, help with the steering and cope with the different forces exerted in acceleration, braking and cornering, a compromise had to be found.

The plies were accordingly superimposed at an angle so that their cords ran across each other diagonally in a lattice pattern. This allows the layers to retain their strength while being stretched in different directions during rotation.

For many years, cords were placed at an angle of 45 degrees; but this was later reduced to 40 degrees, and even less in some high-speed tires where accurate and easy steering response is of more importance than comfort.

Some cross-ply tires are now being made with what is known as a depressed- crown contour moulding.

When new and uninflated, they have a concave tread. This becomes flat when inflated, to give a uniform pressure over the whole contact area.

In addition to a tire’s inner lining, various strips and fillers are incorporated to strengthen the casing.

A tire’s strength and load-carrying ability were at one time indicated by its number of plies: a four-ply tire indicated that the casing was built up from four layers of inner lining material and could carry a specific load with safety.

Modern materials and man-made fibres, such as polyester and glass fibre, embedded in the rubber, are much stronger than the cotton fibres originally used; and the ply rating can no longer be related to the number of plies.

Ply-ratings are still used, but only to indicate the strength and load-carrying capacity: a four-ply rating may well be given to a tire which has only two plies in the casing.

Two steel-wire hoops, or beads, which prevent the tire stretching and parting from the wheel, are common to all tires. But most tires vary considerably in casing and tread construction.

Treads are designed to help tyres grip the road and to enable the car to move with maximum comfort, speed, safety and economy in any kind of weather.

Tyre treads are made of natural or synthetic rubber bonded directly on to the casing, or on to the breaker strip (the belt of cords running around the circumference of radial tyres to stiffen them).

A wide variety of patterns are moulded into the tread to drain away water and cope with various other factors dictated by the road surface and the type of vehicle being used.

On a dry road the best grip is afforded by a fairly smooth tyre, as this provides the greatest possible contact patch, that part of the tyre in direct touch with the road at any moment. But on a wet road a smooth tyre has hardly any grip, and the least trace of water acts as a lubricant. If there is a lot of water on the road, it will form a wedge in front of and underneath a smooth tyre, causing a form of skidding known as aquaplaning. The tyre, lifted off the road and riding on the water, stops turning and steering control is lost.

If there is a thin film of water on the road, the tread pattern breaks through it and grips the hard road surface.

With larger amounts of water, the tread pattern does three things. It first pushes the water aside or pumps it through zigzag grooves and channels in the tread which run parallel to the wheel. In this way the water is driven to the back of the contact patch, where it spins off behind the tyre.

Secondly, the film of water left is mopped up by sipes—slits like tiny knife-cuts in the tread which act like a sponge.

Thirdly, the tread pattern grips the remainder of the now-dry contact area.

As speed increases, contact time decreases and the first two parts of the process take up more of the contact patch, leaving less for a dry grip.

At 100 km/h on a wet road, the tread pattern on an average-sized tyre needs to move more than four and a half litres of water out of the way every second—nine litres in heavy rain—to dry enough surface to provide a grip.

Even with a moderate amount of tread pattern, a tyre’s grip on a wet road decreases appreciably as the speed increases, and it will still aquaplane if there is plenty of water about.

A bold and rugged tread gives a reasonable grip in bad conditions, because the tread can quickly channel away snow, slush, mud or loose stones.

Variations in Tread

Tread patterns differ to suit different purposes. A tyre that is to be used for long periods in wintry conditions or on unmade roads needs a tread with a deep bite to get a grip, and with channels deep enough to get rid of snow, slush or mud.

Other tyres may be needed for use in generally wet weather, when the most important requirement is a tread that ensures rapid drainage. A tyre with a heavy rugged tread intended for rough roads would quickly wear out if used on dry roads at high speed.

Below are three tread patterns, each designed for a different job. On the left is a high-speed nylon cross-ply tyre with through drainage in the main channels and many sipes to give a good grip in wet weather.

The centre tyre is a radial-ply with the outer shoulder built up to withstand heavier wear at high speed and to give the tyre a longer life.

The cross-country or winter tyre (right) has a deeply-grooved tread to provide good grip on loose or slushy surfaces.

If road engineers could build perfect roads, there would be no need for cars to have a complex suspension system to cushion the occupants.

A good suspension system must have springiness and damping. Springiness is an elastic resistance to a load; damping is the ability to absorb some of the energy of a spring after it has been compressed. If this energy is not absorbed, a spring will seriously overshoot its original position and continue to bounce up and down until eventually its oscillations die out.

Damping converts work energy into heat energy. To reduce noise and add further softness, springs are mounted on rubber; and a car’s suspension system incorporates a final cushioning in seating the occupants’ bodies, as a last-stage protection against vibration.

The size of the wheels is an important factor. A large wheel will ride over most road irregularities. A wheel big enough to iron out all irregularities would be impracticable; but a wheel should not be so small that it will roll into every hollow.

Types of Auto Springs and their uses

In fulfilling their primary functions of cushioning the body and occupants of a car from road shocks, the springs act as reservoirs of energy.

Steel springs store this energy by being bent, as in the case of leaf springs, or by being twisted, as in the case of coil springs or the rod in a torsion-bar spring. The energy is released by the spring resuming its normal state.

Leaf springs are referred to as semi- elliptic, although they are almost flat.

Usually the two ends are attached to the car frame or structure by bolts supported in rubber bushes, and the middle of the spring is clamped to the axle. If the spring is fitted across the body, the middle is located on the frame and the ends on the wheel carriers.

The best energy-storing shape for a given weight of spring is circular; and a coil spring efficiently stores the energy produced by up-and-down movement.

Where coil springs are used, the end coils usually sit square, for stability, upon the surfaces through which the load is applied, and act as the lever for the twist to be applied to the rest of the spring.

A torsion bar, which has one end anchored to the car structure and the other to a component subjected to loading, stores energy when it is twisted.

The torsion bar is often used as an anti- roll device. A steel bar is mounted in rubber bushes across the vehicle, with its ends bent round to act as levers, which in turn are connected to the suspension.

When the wheels go up and down as a pair, the anti-roll bar merely rotates in its bearings without effect on the suspension. But when only one wheel rises or falls, or the body rolls on a bend, the torsion bar twists, reacting against the movement.

Rubber is used in various ways for springing. It is most effective in a combination of shear (side-to-side movement of successive layers) and compression.

An important example is the Moulton Hydrolastic system; in it, the main suspension medium is still rubber springiness, but fluid is used to transmit the movement of the wheels from front to rear, or vice versa.

One-piece front axles, supporting the body on leaf springs, were superseded years ago on cars by independent suspension for each front wheel. This provides better steering and is a great improvement on the old beam-axle arrangement in terms of a comfortable ride.

In an independent front-suspension system, each front wheel is connected to the car’s body by its own linkage and springing, so that its movement does not affect that of the other wheel. However, modern suspensions are joined by an anti- roll bar which is hinged across the monoframe, bent, and joined to the lower wishbone on each side to resist body roll when the car is cornering.

A vital feature of front suspension is that the wheels have to be steered as well as moving up and down. When a car corners or goes over bumps, it is subjected through the wheels to a variety of forces.

The suspension must not allow these forces to deflect the car from the course chosen by the driver. Nor must they cause the wheels to wobble, move any significant distance backwards, forwards or sideways, or alter their angle of tilt to any serious degree. Any such effects would interfere with the handling of the car.

Action of double Auto Wishbones

One common form of independent front suspension is the double-wishbone system. In this, two wishbone links, so called because of their shape, are hinged at their broad ends to the body or sub-frame of the car, and at their narrow ends to the swivel members which have stub axles to carry the wheels. Between each wishbone assembly and the car structure are a spring and a hydraulic damper to intercept road shocks.

As well as positioning the wheels and transmitting loads to the spring, the wishbones must also resist acceleration, braking and cornering forces.

Since the first two of these forces act along the car, a simple sideways link would yield to them. It follows that a form of triangle is necessary to give a broad base, which is why links of triangulated or wishbone shape are used.

The geometry of the wishbone layout— the lengths, positions and angles of the links—governs the path of the wheels when the car rides over bumps ; this path in turn affects steering, road-holding and tyre wear.

If the upper and lower wishbones are parallel and of equal length, the wheels will not tilt as they move up and down over bumps. However, the track (distance between the wheels, across the car) does change somewhat, with bad effect on tyre life. On corners, the wheels lean outwards with the body, which reduces their cornering ability.

In most modern designs, the wishbones are neither of equal length nor parallel, the upper one being the shorter. Hence the wheel no longer remains upright as it moves up or down over bumps, but leans slightly inwards.

This gives better cornering characteristics than the other layout: when the body rolls outwards, the outside wheel—which carries the greater load and therefore exerts the higher cornering power of the two—remains more or less at right angles to the road surface.

MacPherson Strut Auto Assembly

Instead of using double wishbones, some suspension systems have a single arm, a diagonal stay and a telescopic strut for each front wheel; this is known as a MacPherson layout.

The stub axle carrying the wheel is part of the lower half of the strut.

The whole strut is turned by the steering linkage and is connected to the car’s body structure at the top by a flexible mounting and connected at the bottom by a joint to the lower arm.

To absorb road shocks, a coil spring surrounds the upper part of the strut, which contains a hydraulic damper.

This system has gained wide popularity for several reasons; it is mechanically simple; its light moving parts help the wheels to follow road irregularities; and it does not cause the camber (angle of wheel tilt) to vary much as the wheel moves up and down.

On the other hand, the body structure has to be really strong above the wheel arches, where the struts are attached, to absorb the full suspension loads.

In most cars, the rear suspension has to carry the greater part of the extra weight of passengers and luggage. If the suspension springs are stiff enough only for the car with driver, they will be too soft for the car when it is fully laden. Conversely, springs that are right for the laden car will be too stiff with only the driver to carry.

Designers solve this problem in a variety of ways. In fact, the variety of rear suspension systems is greater than that of front suspension. Most designs use coil springs, but others use leaf springs, torsion bars, rubber, air or compressed gas.

Auto Rear suspension, front-Wheel Drive

Front-wheel drive cars carry only 35-40 percent of the car’s mass on the rear wheels when unladen. Most such cars have independent rear suspension of either the trailing arm or three-link type. The alternative is a beam axle (sometimes called `dead’ axle).

Some trailing arm designs incorporate a torsion beam or anti-roll bar in the mounting, others have a separate anti-roll bar.

Auto Live axles

When the right-angle drive, differential, axle shafts and wheel-hub mountings are combined in one rigid unit, this unit is known as a live axle.

The live axle is connected to the propeller shaft and is attached to the vehicle structure in such a way that it can move up and down on its springs and can cope with the loads and the torque, or turning forces, imposed on it.

The rear suspension should be designed also to position the axle in a way that minimises the bouncing and associated vibrations to which it is liable, particularly when moving off, braking and cornering.

Dampers, which suppress bouncing of the springs, are mostly hydraulic, and the telescopic type is favoured in preference to the piston type widely used earlier.

Auto Hotchkiss Drive

The simplest design that combines springing and positioning, or locating, of the rear axle is known as the Hotchkiss drive. In this, a pair of leaf springs are set as far apart on the axle as possible.

The axle may be carried exactly in the middle of the springs, but many axles are set forward from the middle to give a downward tilt as the axle rises when riding over bumps. This reduces the amount by which the rear end of the propeller shaft lifts on a bump and, in turn, minimises the height of the propeller-shaft tunnel and the amount it intrudes into the car.

To reduce variations in axle tilt under different driving conditions, some rear suspensions have fore-and-aft torque- control links (that is, links controlling forward-and-backward twisting movement), to supplement the leaf springs.

Independent Designs

The advantage of independent rear suspension is the saving in unsprung mass, which should give better ride and handling.

The primary advantage of connecting front and rear springing in this way is that it greatly reduces any tendency of the car to pitching (rocking in a fore-and-aft direction); this gives the occupants a more nearly level ride. Outstanding examples of linked suspension on cars of recent times are the Moulton Hydrolastic and Hydragas suspensions used by Austin- Morris and the spring-linkage system used by Citroën on their smaller models.

The main difference between the two is that the Hydrolastic and Hydragas systems are hydraulically operated, while the Citroën system works mechanically.

Auto Hydrolastic system

In the hydrolastic system, each wheel has a `displacer‘ which fulfils the functions of spring and damper. The displacer is mounted on the car body and inside it, at one end, is a bonded-on conical rubber spring.

Closing the other end of the displacer is a flexible diaphragm, in the middle of which is a piston connected to the wheel- suspension linkage. The chamber between spring and diaphragm is divided by a metal separator plate containing a rubber two- way valve. A pipe connects the front end rear chambers on each side of the car; chambers and pipes are filled with fluid.

When the front wheel rises over a bump, the diaphragm moves inwards, forcing fluid through holes in the separator plate past the two-way valve; the resistance of which provides the damping effect.

The diaphragm movement reduces the chamber volume and increases pressure, displacing some fluid along the connecting pipe. This causes the diaphragm of the other displacer to be pushed outwards, extending the rear suspension. Any lifting of the front end of the car over the bump is thus matched by a rise of the rear end, and vice versa. The Hydragas system is a further development, using pressurized nitrogen gas as a damping and springing medium instead of rubber.

Auto Gas Compression provides Springing Effect

ALTHOUGH not available on currently produced South African cars, hydro- pneumatic suspensions warrant some mention. On their larger cars, Citroen, among others, use this design.

Each wheel has its own independent suspension unit. In each unit, nitrogen is contained under pressure in the upper half of a metal sphere above a fluid-resistant diaphragm clamped in position.

The lower half of the sphere is connected to a hydraulic cylinder. Sliding inside this cylinder is a piston with a push-rod joined to a suspension arm by a pivot pin.

When a wheel moves up to ride over a bump in the road, this movement is passed on by the suspension arm to the piston. Movement of the piston exerts pressure on the hydraulic fluid, and this in turn compresses the gas, which acts as a spring. Downward movement of the wheel causes the piston to travel downwards also, reducing pressure on the gas.

Height adjustment is achieved by the flow of fluid into or out of the cylinder. The fluid is kept under pressure in a reservoir supplied by a pump, and its flow is controlled by a slide valve.

If the load on the car is increased, the body at first sinks in relation to the wheels and suspension arms. Each arm then operates a linkage that opens the slide-valve connection to the reservoir, and fluid enters the cylinder to push the body up. The linkage returns to neutral when the body regains its original height.

The reverse happens if the load on the car is decreased. The body rises, the valve linkage opens the return pipe to the reservoir, and excess fluid flows back until the original body height is reached.

A damper is fitted at the top of the cylinder. It consists of a valve restricting the flow of fluid into and out of the sphere.

Auto Self-levelling suspension

Self- levelling suspension systems prevent a car from ’squatting down’ on its suspension when it has a full load of passengers and luggage. They are used on some cars with limited suspension travel and on luxury models like the BMW 7 series. These systems automatically compensate for changes in load by modifying the springing characteristics of the car keeping it the same height off the road.

This ensures that, even when the car is fully laden, the full upward travel of the suspension system is available for absorbing bumps. It also means that the headlights are kept in proper focusing alignment, whatever the distribution of load. A weakness of conventional suspension systems is the way in which the addition of rear-seat passengers, or luggage in a rear boot, makes the car ’sit down’ at the back and so tilts the headlight beams upwards.

Any form of self-levelling is operated by one or more load-sensing devices which measure the vertical distance between, say, the floor and a point on a suspension arm. The greater the load, the smaller this distance tends to become. An initial variation in the distance operates a valve which controls the height-adjusting system. Usually, height adjustment takes place at both ends of the car, but it can be at the rear end only.

Auto Air Suspension

Control of the riding height of a car is simple with air suspension, a system that has been used with considerable success on the Mercedes 600.

It could become more widely adopted in the future, because of the ease with which the spring characteristics can be suited to individual vehicles. Air suspension is simple in principle. Collapsible, pressurized air containers take the place of conventional springs; the upward movement of the wheel reduces the volume of the air spring, raising its pressure, so that it tries to extend itself again.

If the air spring is inflated more, it can carry a heavier load before contracting to a given height.

A height-control valve connects the air spring to a high-pressure air reservoir when the load is increased; and pressure is released through the valve to the atmosphere when the load is reduced.

All cars are steered by turning the front wheels in the required direction and allowing the rear wheels to follow.

There would be several disadvantages in trying to steer a car by its rear wheels, the main one being that the car would be directionally unstable.

On a bicycle, the steering is controlled directly by the handle-bars. But in a car, the driver would not be strong enough to control the front wheels if they were connected directly to the steering wheel. So the steering must include a gearbox, and sometimes power assistance, to multiply the driver’s effort.

Major requirements in any steering mechanism are that it should be precise and easy to handle, and that the front wheels should have a tendency to return to the straight-ahead position after a turn.

The steering must not ‘kick back’ from road shocks, although there must be some degree of reaction, or `feed-back’, from the road to the driver.

The steering column, which encloses and supports the steering shaft, often carries controls, such as the horn, light and wiper switches.

A combined headlamp-flasher and dip- switch unit is frequently fitted just below the steering wheel, with a direction indicator switch opposite it; or the two functions may be controlled by a single lever.

Some cars have an adjustable steering column. The top part—with the steering wheel—can be moved up and down in telescopic fashion and, in some cases, can be tilted at an angle to suit the driver’s height and driving position.

Considerable research has been carried out in recent years to safeguard the driver against injuries inflicted by the steering wheel (or column) when the car is in a front-end collision.

The steering column can be designed to collapse on impact. For example, in the AC Delco design, the tubular column is made of an ‘expanded-metal’ grid so that, although strong in twist, it collapses and absorbs energy if compressed lengthwise. A telescopic joint is provided in the steering shaft.

Another design divides the steering shaft into sections which are connected by joints, but not placed in line.

The steering wheel is ‘dished’ and provided with a large boss and wide spokes in order to spread the load of impact over the driver’s chest. The wheel or boss can also be designed to collapse when under a pre-determined load.

Different Steering Systems

A CAR is steered through a steering gearbox and a linkage—a system of rods and levers —designed to give the driver directional control with minimal effort.

The steering wheel itself is attached to a shaft which is enclosed in a supporting tube known as the steering column.

The shaft is connected to a steering gearbox, which converts the turning motion of the steering wheel into a to-and-fro movement of the steering linkage and provides the driver with the extra leverage he needs to steer the road wheels without excessive effort.

Various types of steering boxes used over the years include those known as the cam and peg, worm and nut, and recirculating ball; but most cars today use a rack-and-pinion system.

With this design, a toothed rack is moved by a small pinion at the lower end of the steering shaft. When the steering wheel is turned, the rack moves from side to side and causes the stub axles—the two short shafts on which the front wheels are mounted—to swivel.

On earlier designs, the steering box carried a short lever known as a drop-arm, which operated the rest of the linkage.

On old cars fitted with a beam front axle, and on some types of independent front suspension, the stub axles swivel on a spindle called a king-pin. On cars with modern types of independent front suspension, the stub axles swivel on two widely separated hemispherical bearings.

Steering Gearbox reduces Effort needed by Driver

THE AMOUNT of reduction of effort—or leverage—to be provided by the steering box depends on the weight, type and use of the car.

A light sports car requires little reduction, as the driver needs quick control to correct skids or `drifts’; but a heavy car, with ‘fat’ tyres, requires a big reduction and/or some form of power assistance to make low-speed turns.

The steering ‘box and the linkage also pass back to the steering wheel the reaction of the wheels to the road surface.

This reaction gives immediate warning to the driver of changing conditions, but designers have different ideas as to how much reaction there should be.

Some mechanisms are efficient in transmitting the driver’s effort to the road wheels but are less effective in feeding back information about irregularities in the road surface to the driver.

Long before the advent of the car, the German inventor Rudolf Ackerman patented a device based on the principle of geometrically correct steering. He stated the principle that when a vehicle travels in a curved path, its wheels should describe circles round the same centre. A wheel which follows a markedly different path will slide to some extent; and this will cause tyre wear.

The application of Ackerman’s principle makes it possible to arrange for imaginary lines through the axes of all the wheels, front and back, to pass through, or very near, the same point—the centre of the curve on which the car is travelling. This is achieved by turning the inner front wheel through a greater angle than the outer front wheel.

However, modern car designers no longer need to follow the Ackerman principle strictly, because of improvements in suspension and tyres. When a car rounds a bend at speed the deflection of the tyres on the road surface creates a sideways force which assists steering. Designers consider it more important to obtain the smallest turning circle.

Toe-in and toe-out

ALTHOUGH in theory the front wheels should be parallel when pointing straight ahead, the best practical results are usually obtained by setting them slightly out of parallel. This gives the steadiest steering and least tyre wear. On most cars, when the steering is centralized, the front wheels point inwards by a fraction at the front. This is known as toe-in. It can be regarded as a compensation for the fact that no steering and suspension can be perfect and no steering linkage is free from a certain amount of ‘give’. Cars front-wheel drive usually have the wheels pointing slightly outwards. This is known as toe-out. The amount of toe-in or toe- out can be adjusted.

The term ‘wheel alignment’ describes the overall steering geometry, including toe-in / out as well as castor and camber angles. The amount of toe-in or toe-out rarely exceeds 5 mm. Incorrect rear-wheel alignment can also affect steering in cars with independent suspension.

Vehicle Steering Ratio

The STEERING RATIO is the ratio between the rotation of the steering wheel and the angle of movement imparted to the steering arms—those parts of the steering linkage connected directly to the stub-axles and which move the wheels.

If, for instance, a full turn (360 degrees) of the steering wheel moves the steering arms through 30 degrees, the steering ratio is 12:1 (360:30).

Most popular lightweight saloon cars have a steering ratio of about 15:1. To move the front wheels from lock to lock (about 60 degrees) might take two-and-a‑half turns of the steering wheel. But a heavy car may need four or five turns—a steering ratio of at least 24:1.

All cars are fitted with positive lock stops, to limit the steering movement of the wheels and so ensure that the tyres do not rub against any part of the car. The stops may be at the wheel pivots or on the steering box.

The minimum turning circle of a car is either the diameter of the circle traced by the extreme outer corner of the car or, more usually, the diameter of the circle traced by the outer front wheel.

Camber Angle

The front wheels of most cars, when seen from the front, lean slightly—either inwards or outwards. The tilt of the wheel is called the camber, and the amount that it tilts is called the camber angle.

The camber is usually ‘positive’, with the wheels leaning slightly outwards, so that they are further apart at the top than at the bottom. Wheels which are closer together at the top than at the bottom have a ‘negative’ camber.

The object is that, when viewed from the front, the centre of the tyre’s area of contact with the road is brought close to the point where an imaginary extension of the steering pivot’s axis cuts the ground. This is called centre-point steering.

Camber is a condition forced on the car designer because he cannot relieve stress on the steering linkage by placing the pivot directly over the wheel, as on a bicycle. The best the designer can do is to used a ‘dished’ (saucer-shaped) wheel and slope either the wheel or the steering pivot. or both.

It has been found that a small amount of offset, which reduces steering effort when parking and eliminates feedback judder from the road wheels to the steering wheel at high speeds is a desirable feature and is incorporated on practically all modern cars.

If the camber angle is too great the tyre’s outer edge will wear excessively. Cars with front-wheel drive often have less offset.

With many independent suspension systems the camber changes from positive to negative as a wheel rises or falls on hitting a bump or pothole.

Wheels with a permanent negative camber are used on some continental cars and on some sports-car conversions.

Castor Angle

A car should have an inbuilt tendency to travel straight and to return to the straight-ahead position after a turn.

This tendency, which makes a car stable in motion and makes the steering wheel spin back after a corner, is controlled by many factors, including the suspension and resilience of tyres. One of the most important direction-controlling factors is the castor angle.

The effect of this is most simply seen on the castor used on furniture. On a tea trolley, for example, the castor wheels trailing behind their pivots, swing round to follow the direction in which the trolley is pushed, so that it travels in a straight line unless it is deliberately steered. In a car, the castor angle has exactly the same effect of making a wheel trail behind its steering pivot. The central point of contact of the wheel on the mad is behind that of an imaginary line extending the steering-pivot axis to the road.

The castor angle is the angle between the line extending the steering-pivot axis and a vertical line through the centre of the wheel. As with camber angle, it needs checking after accident damage.

An excessive amount of castor angle or trail, coupled with very freely moving joints in the steering linkage, could lead to violent front-wheel wobble.

Many large cars are fitted with power- assisted steering as standard equipment. It reduces the effort needed to turn the steering wheel and it makes manoeuvring at low speed—when parking in a confined space, for instance—much easier than with conventional steering.

Power assistance also contributes to safety. With conventional steering, the steering wheel may be wrenched out of a driver’s grip if a tyre bursts or if the car hits a large bump. Power-assisted steering stops the steering wheel ‘kicking back’ and helps the driver to correct a sudden swerve.

It is essentially a power-assisted, not a power-operated, system. The driver is always left with some work to do, to retain the ‘feel’ of the steering.

Most systems use hydraulic fluid or a light oil, supplied under pressure by an engine-driven pump from a separate hydraulic tank. If the system fails, the car can still be steered manually but with greater effort. When the steering is not being moved, the fluid passes through two equal-sized ports, giving equal pressure to both sides of a piston in a ram connected to the linkage.

Movement of the steering wheel first takes up a small amount of slack, which is used to open one port and close the other.

The fluid then applies pressure to one side of the piston, which moves the steering linkage in the required direction. The amount of fluid pressure applied to the piston depends on the force applied to the steering wheel by the driver.

Hydraulic pressure to operate the power steering is provided by a pump. On a typical system the pump is powered by a V- belt turned by the crankshaft pulley. One type of pump uses a series of vanes which sit snugly between the rotor and the pump housing.

The Adwest Steering power-assisted

In the Adwest power-assisted rack-and- pinion steering system, the hydraulic assisting pressure is controlled and directed by a rotary valve.

Rotation of the steering column turns this control valve, directing the hydraulic pressure to one side or the other of a piston mounted on the rack member itself.

This hydraulic pressure then moves the rack member to the left or right, augmenting the effort applied by the driver to the steering wheel.

A torsion bar connecting the steering column to the valve ensures that the assistance provided varies with the resistance of the tyres to steering.

Cornering Steering

Effects of oversteering and understeering

Adhesion between the tyres and the road makes it possible to steer a car around curves; it is persuaded to leave a straight path by the lateral (sideways) forces applied to it by the rolling tyres.

Because a tyre is flexible and springy, the direction in which it rolls on a curve diverges to some extent from the direction in which the wheel is pointed. This divergence is called the slip angle, although it would be more accurately described as creep, or drift.

At low speeds and on gentle curves, the slip angles are small; sharp, fast cornering results in much larger slip angles up to a breakaway point when the tyre slides sideways in a skid. Slip angles also depend upon tyre design, inflation pressure, the load carried by the tyre, wheel camber and other factors, such as road surface conditions.

A car is said to understeer when the slip angles of the front tyres are greater than those of the rear tyres ; the car tries to run wide on a bend in the road and the driver has to correct this by extra movement of the steering wheel.

A car which oversteers is one in which the slip angles at the rear are greater than the slip angles of the front tyres. Consequently, the car will respond eagerly to the steering. It will also be apparent that it requires less initial movement of the steering wheel.

The term ‘neutral steer’ means that the slip angles are the same at the front and the back.

Cars which have front-wheel drive, and also high performance cars with rear- drive, are often sensitive to the manner in which the driver operates the accelerator, because this will affect the slip angles of the tyres.

Although understeer and oversteer are most easily explained in relation to cornering, their influence on a car’s behaviour when holding a straight course is equally important.

A car that is being driven along a straight road is often subjected to sideways forces—for example, a gust of wind or a change in road camber. The response of an understeering car is stable—that is, self-correcting—but an oversteering car is unstable and requires continual correction by the driver.

The higher the speed, the less time there is for correction by steering, so that beyond a certain speed (depending upon the road conditions and the skill of the driver), all directional control may be lost.