Four wheel steering

ABSTRACt

Four-wheel steering is a method developed in automobile industry for the effective turning of the vehicle and to increase the maneuverability. Production cars are designed to understeer, and rarely do they oversteer.  In this paper, both front wheel and rear wheels can be steered according to the speed of the other vehicle and space available for turning. Four-wheel steer gives a longer vehicle greater ease while driving at low speed and improves stability, handling and control at higher speed. Four-wheel systems are a serious effort on the part of automotive design engineers to provide near-neutral steering. This system finds application in off-highway vehicles such as forklifts, agricultural and construction equipment mining machinery also in Heavy Motor Vehicles. It is also useful in passenger cars. It improves handling and helps the vehicle make tighter turns. This system is used to minimize the turning radius. In a typical front wheel steering system, the rear wheels do not turn in the direction of the curve and thus curb on the efficiency of the steering. In four-wheel steering the rear wheels turn with the front wheels thus increasing the efficiency of the vehicle.
    
    INTRODUCTIoN

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What is Steering?

Steering is the term applied to the collection of components, linkages, etc. which will allow a machine (ship or Boat or vehicle) to follow the desired course. An exception is the case of rail transport by which rail tracks combined with railroad switches provide steering column, which may contain universal joints, to allow it to deviate somewhat from a straight line.
In normal city cruising driving conditions, the vehicle with higher track width and wheelbase face problems of turning as the space is confined, the same problem is faced in low speed cornering. The turning radius is reduced in the four-wheel steering of the vehicle which is effective in confined space. In this project turning radius is reduced without changing the dimension of the vehicle. In situations like vehicle parking, low speed cornering and driving in city conditions with heavy traffic in tight spaces, driving is very difficult due to vehicle’s larger track width and wheelbase. Hence there is a requirement of a mechanism which result in less turning and less effort that must be put up by the driver. 

 Types of Steering System:

1- CONVENTIONAL STEERING SYSTEM: In this steering system, only the front wheels are steered towards right or left, according to the requirement.
2- FOUR WHEEL STEERING SYSTEM: In this steering system, all four wheels are to be steered according to the steer performed by the driver towards the left or right. Four-wheel steering, 4WS, also called rear-wheel steering or all-wheel steering, provides a means to actively steer the rear wheels during turning maneuvers. It should not be confused with four-wheel drive in which all four wheels of a vehicle are powered. It improves handling and helps the vehicle make tighter turns. If a car could automatically compensate for an under steer /over steer problem, the driver would enjoy nearly neutral steering under varying conditions. A typical 4WS system uses actuators on the rear wheels to modulate toe angles. Honda’s Prelude was the first production car to be fitted with such a system, way back in 1987 and recently it has started to make headlines again, most notably thanks to its inclusion on Porsche’s 991-generation like 911 Turbo, GT3 and GT3 RS, the 918 Spyder, the Nissan GT-R, BMW 7 series models, Audi A8 and Audi Q7. STEERING PRINCIPLES AND COMPONENTS:

1) Ackermann steering mechanism: With perfect Ackermann, at any angle of steering, the center point of all the circles traced by all wheels will lie at a common point. But this may be difficult to arrange in practice with simple linkages. Hence, modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects.
2) Steering ratio: The steering ratio is the number of degrees that the steering wheel must be turned to pivot the front wheels 1 degree. E.g.: steering ratio 18:1 implies that the front wheels will turn by 1 degree when the steering wheel turns 18 degrees. The steering ratios generally used with present day steering gears vary from about 12: 1 for cars to about 35: 1 for heavy vehicles.
3) Turning circle:  The turning circle of a car is the diameter of the circle described by the outside wheels when turning on full lock. There is no hard and fast formula to calculate the turning circle, but an approximate value can be obtained using the formula: Turning circle radius = Track/2 + Wheel base/sin (Average steer angle)
4) Steering geometry: When a car is moving along a curve, all its wheels should roll truly without any lateral slip. This can be achieved if the axis of all four wheels intersects at one point. This point will be the center about which the vehicle will be turning at that instant. The rear wheels rotate along two circles. The centers of both these circles are at O. The front wheels 1 and 2 have different axes. They will rotate along two other circles with the same center point.

5) Turning radius: The turning radius of a vehicle is the radius of the smallest circular turn that the vehicle can make.

 VEHICLE DYNAMICS AND    STEERING

Vehicle dynamics have very importance for a balanced drive of vehicle. It can be say in three terms of steering, which are under-steer, over-steer, and neutral or counter steering.

Under-steer: Under steer is so called because when the slip angle of front wheels is greater than slip angle of rear wheels. 

 Over-steer: Over steer is defined when the slip angle of front wheels lesser than the slip angle of rear wheels. 

 Neutral-steer or Counter-steering: Counter-steering can be defined as when the slip angle of front wheels is equal to slip angle of rear wheels. 

WORKING OF 4 WHEEL STEERING:

 The system is controlled from the remote control consists of
1- 4 Diodes – 4007 diodes
2- Slide switch – to control the forward and backward movement of the wheels.
3- Micro switch – to control the left and right movement of the wheels.
4- Capacitor of 3700 microF which stores the extra energy.
5- Transformer which gives an AC of 24 volt by using the full wave rectifier power supply circuit. Bridge circuit is used of the diodes to convert the AC coming from the transformer into the Full wave DC. Wiper motor transfers the power to the spur gears system which moves the tyres with the help of the rod connected through. Gear motor fixed to the four tyres each help in moving the tyres left and right. The system runs on a speed of 20- 40 kmph. Each tyres rotate on its own axis helping to rotate the whole system to move in the 360 degrees.

Comparison of Four Wheel Steering System with Two Wheel Steering Conventional System:

1. Car more efficient and stable on cornering.
2. Improved steering responsiveness and precision.
3. High speed straight line stability.
4. Notable improvement in rapid, easier, safer lane changing maneuvers.
5. Smaller turning radius and tight space maneuverability at low speed.
6. Relative Wheel Angles and their Control.
7. Risk of hitting an obstacle is greatly reduced.

Advantages:
1. Superior cornering stability.
2. Improved steering responsiveness and precision.
3. High speed straight line stability.
4. Notable improvement in rapid lane changing maneuvers.
5. Smaller turning radius and tight space maneuverability at low speed.
6. Relative wheel angles and their control.

APPLICATIONS:

1. Parallel parking: Due to smaller turning radius the parking and un parking of vehicle is easily performed towards the right or left side.
2. High speed lane changing: In this is less steering sensitive this does require a lot of concentration from driver since he must judge the space and vehicles behind them.
3. Slippery road surfaces: Due to the rear wheel steering operation on low friction surfaces occurs hence vehicle direction easier to control.
4. Narrow Roads: Due to rear wheel steering on narrow roads with tight bends, counter phase steering reduces the turning radius.
5. U-Turns: By minimizing the vehicle’s turning radius and counter phase steering of rear wheels enables U-Turns to be performed on narrow roads.

CONCLUSION 

Powering all the wheels gives maximum performance and since each wheel on the same side of the vehicle travels at the same velocity, independent channels of control is not necessary. Implementing Four-Wheel steering in upcoming car models will provide uncompromised static stability, proper front and rear wheel tracking can be obtained and optimum obstacles climbing capability can be achieved