02-05-2014, 03:04 PM
FOUR WHEEL STEERING
FOUR WHEEL.DOC (Size: 62.5 KB / Downloads: 14)
ABSTRACT
The front wheels steering system conventional vehicle, which fails to keep the vehicle stable high speeds and prove futile for heavy vehicles like tracks and trailers, especially in tight space and corners. But if the rear wheels will be enchanced. This is the evolution Four-wheel steering (4WS). In 4Ws, rear wheel are steered in combination with the conventional front steering system. This system helps in achieving superior cornering stability, improved steering responsiveness and preceision, high –speed straight-line stability and a smaller turning radius and tight-space maneverability at low vehicle speed range.
Introduction
The concept of steering all four wheel, of a passenger car is an outstanding feature of modern automobile technology that has to origin in the early history of the motor vehicle and has military connections. Four wheel steering is a system that allows the rear wheels to turn for maneuvering, rather than just follow the front wheels. 4WS isn't new, you might remember it as an option on Honda Preludes in the late 1980's and early 1990's.
History:
During the 1st world war an omnibus with counter steer was designed by a French automobile pioneer Eugene Brille of Schneider Company. It’s aim was to improve maneuverability of the vehicle and there by simplify and save time in the operation of loading and unloading.
In 1923 a vehicle knows as Holeverter car was designed by French motor work in London. In this type of vehicle the steering could be set so that the rear wheel could be steered in either the same phase or counter phase of the front wheels. These system were used by both British and German armoured vehicles.
Apart from the limited use of 4WS in industrial and military vehicles, interests were shown in applying the concept to traditional load vehicles from mid 1960’s.
NEED FOR 4WS:-
In conventional front-steered vehicle, the tyres are subjected to forces of grip, momentum, and steering input when working a movement other than straight-ahead driving. These forces compete with each other during steering maneuvers. With a front-steered vehicle, the rear end is always trying to catch up to the directional changes of the front wheels. This causes the vehicle to sway. As a normal part of operating a vehicle, the driver learns to adjust to these forces without thinking about them.
Principle:
The idea behind Four-wheel steering is that a vehicle requires less driver input for any steering maneurer if all four wheels are steering the vehicle. As with two-wheel-steer vehicles, tire grip holds the four wheels on the road. However when the driver turns the wheels slightly, all four wheels react to the steering input, causing slip angles to form at all four wheels. The entire vehicles moves in are direction rather than the rear half attempting to catch up to the front. There is also less sway when the wheels are turned back to a straight-ahead position. The vehicle responds more quickly to steering input because rear wheel lay is eliminated.
4WS implies that when the front is steered at certain angles, that rear wheel is steered at certain angles. The rear wheels are adjusted to various positions to provide a better method of controlling the car when turning. The rear wheels can be steered in the same direction of the front ones (parallel steer) or in the opposite direction (counter steer). This type of steering is based on the speed the vehicle is travelling and the angle to which the steering wheel is turned.
In a 4WS system, the phase and ratio of the rear wheel steering to the front wheels varies according to the vehicle speed. Initially if the vehicle is moving at a speed more than 35 kmph the rear wheels are turned in the same direction (phase) as the front wheels, but not at the same rate. The rear wheels are steered at a much slower rate. When the front wheels are steered through about 9 degrees the rear wheels would have been steered through a maximum of 2 degrees. They are not allowed to turn further in the same direction. (Figures….)
Hydraulic 4 WS
The hydraulic operated four-wheel-steering system shown in figure3 is a simple design, both in components and operation. The rear wheels turn only in the same direction as the front wheel. The also turn no more than 1½ degrees. The system only activates at speeds above 30 mph (50km/h) and doesnot operated when the vehicle mores in reverse.
A two-way hydraulic cylinder mounted on the rear stub turns the wheels. Fluid for this cylinder is supplied by a rear steering pump that is driven by the differential. The pump operates only when the front wheels are turning. A common reserve tank in the engine compartment supplies the rear steering pump with fluid.
When the steering wheel is turned, the front rack and pinion unit. This forces fluid into the front power cylinder and the front wheels turn in the direction steered. The fluid pressure varies with the turning of the steering wheel. The faster and farther the steering wheel is turned, the greater the fluid pressure. The fluid under the same pressure operates a hydraulic control valve that regulates flow of fluid from rear steering pump to the power steering cylinder for the rear wheels.