Car Parts and Accessories

The Benefits of Power Steering Assistance

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.

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