Modern Cars Steering Operations
THE CASTOR AND CAMBER
Steering and brakes are the major parts of any vehicle when it comes to seeing it moving on the road. Besides safety, other factors such as power consumption, convenience, and other important factors are the reasons manufacturers pay serious attention to the steering and braking system of any of their car brand.
When a supermarket trolley is pushed, the castoring wheels automatically trail so that the trolley travels in a straight line. The same effect is desirable on a car so that when it emerges from a corner the steering automatically centers itself.
The difference between the castor trolley and a car in terms of wheel movement is that the trolley castor steering pivot is ahead of the wheel and as soon as the trolley moves, the wheel cannot help but follow.
On a car, the steering pivot is near the wheel center, but a castoring effect can be achieved if the steering pivot is tilted backward.
If an imaginary line drawn through the steering pivot to the road meets the ground ahead of the center of the tire contact point, the small amount of resultant castor is sufficient to provide steering that automatically self-centers itself.
When looked at from the front, the front wheels on most cars lean slightly inwards or outwards. This is known as wheel camber.
Most cars have a slight positive camber, where the wheels at the top are further apart than at the bottom. Wheels that are closer together at the top are said to have negative camber.
Positive camber is used to relieve the stress on the steering linkage. Unlike a bicycle, which has its steering pivot above the center line of the tire, a car has its steering pivot alongside the road wheel.
The heavy loading that puts pressure on the pivot can be reduced if the wheel, or pivot axis, or both, are angled so that an imaginary line through the pivot meets the ground at the center of the tire contact patch. That is known as center-point steering and gives a light action.
Absolute center-point steering is not used on many cars now because it has been found that small amount of offset which the pivot meets the ground inside the center of the tread contact area reduces feedback and the amount of high-speed judder that the road wheels can transmit to the steering wheel, particularly if they are out of balance.
The small amount of offset which is sometimes referred to as the scrub radius means that each wheel would try to turn outwards, but as both wheels have the same offset, they cancel each other out under normal driving conditions.
If one tire suddenly deflates, the offset built into the opposite wheel tends to keep the car on a straight course rather than allow it to slew dangerously to one side.
POWER-ASSISTED STEERING SYSTEM
Power assistance makes low-speed maneuvering considerably easier, particularly on large, heavy cars, and reduces the amount of kickback to the steering wheel in the event of a tire burst. The main components consist of a pump, which feeds hydraulic fluid to a piston in a cylinder, and a control valve.
Hydraulic fluid or light oil is supplied under pressure by the engine-driven pump to the control valve which is operated by the steering shaft.
Movement of the steering column to the left causes the valve to feed high-pressure fluid to one side of the cylinder. This moves the piston, which assists the linkage to move the road wheels to the left.
When the steering wheel is turned to the right, fluid pressurizes the other side of the piston, moving it in the opposite direction. On some rack-and –pinion systems, the piston is incorporated in the rack unit.
HOW THE CORNERING WORKS
Since tires are soft and springy, it means that the car may not necessarily follow the exact line of the front wheels. The difference between the direction in which the front wheels point and the actual path taken by the car is called the slip angle.
At low speeds on gentle curves, the car follows almost exactly the direction of the steered wheels and slip angles are small; fast cornering results in large slip angles, and if the cornering speed is increased sufficiently, a break-away point is reached and the tire slides sideways.
A car understeers if the slip angles of the front tires are greater than those of the rear tires. The car tries to run wide on a bend, and the driver has to correct it by applying extra steering lock.
On an over steering car, the slip angles are greater at the rear. The car responds quickly to the steering, and in some instances will respond more sharply than the driver intends, swinging the rear of the car outwards.
When this happens, the steering needs to be turned in the opposite direction to correct the tail-out attitude.
Most current cars have a small amount of built-in-under steer. This gives predictable cornering and better straight-line stability in crosswinds.
A car that tends to over-steer usually needs constant steering corrections. This applies even when traveling in a straight line, in order to keep the car on a stable course.
