Steering System Somnath

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Steering System

STEERING SYSTEM
The function of steering is to steer the front wheel in response to driver command inputs in order to provide overall directional control of the vehicle. The factors kept in mind while designing the steering system were     Simplicity Safety Requiring minimum steer effort Economical

Steering geometry
Ackerman
The Ackerman Steering Principle defines the geometry that is applied to all vehicles (two or four wheel drive) to enable the correct turning angle of the steering wheels to be generated when negotiating a corner or a curve. When a car is travelling around a corner (the red lines represent the path that the wheels follow) the inside wheels of the car follow a smaller diameter circle than the outside wheels. If both the wheels were turned by the same amount, the inside wheel would scrub (effectively sliding sideways) and lessen the effectiveness of the steering. This tire scrubbing, which also creates unwanted heat and wear in the tire, can be eliminated by turning the inside wheel at a greater angle than the outside one.

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The difference in the angles of the inside and outside wheels may be better understood by studying the diagram, where we have marked the inside and outside radius that each of the tires passes through. The Inside Radius (Ri) and the Outside Radius (Ro) are dependent on a number of factors including the car width and the tightness of the corner the car is intended to pass through. Aligning both wheels in the proper direction of travel creates consistent steering without undue wear and heat being generated in either of the tires.

Steering Arm Angles
Creating mis-alignment of the wheels is achieved by a combination of the angle and the length of the steering arms. Below a few diagrams are shown that give examples using parallel and angled steering arms to demonstrate why there is a need for using the Ackerman Steering Principle.

Parallel Steering Arms

The steering arms in the diagram to the left are straight and parallel to the sides of the vehicle, which would create a situation where equal movement of the steering servo would produce equal angular movement of the wheels. As the steering arm pivot point (A) is vertically aligned with the king pin pivot point (B) when the wheel is pointing straight ahead, the same amount of movement to the Left or to the Right moves the steering arm pivot point the same vertical distance forward of its starting point.

Angled Steering Arms

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The steering arms in the image to the left are angled inwards to create a means for the wheel angles to change at a different rate. This is the basis of the Ackerman Steering Principle and creates this unequal angular movement of the wheels. As the steering arms are angled, the pivot point (A) is not vertically aligned and is, in a straight ahead position, part way round the circle. Because of this, a Right movement of the steering arm will cause the pivot point to move a greater distance in the forward direction than a Left movement of the steering arm.

An important point worth noting is that this unequal angular movement is exponential, that is, the more you turn the wheel the greater the angular difference between the wheels - otherwise both the wheels would never point forward when the car is not turning.

Low Lateral Acceleration
At low speeds when the tires have minimal tire shear losses on dry, clean pavement, the true Ackermann steering geometry is beneficial as the tires are in almost a perfect situation of minute slip angle. Parallel or reverse Ackermann in this scenario would push (or under steer) the front of the car away from the desired path. In both situations, the inside tire contributes to this push similarly to a centrifugal force.

High Lateral Acceleration
At high lateral accelerations, true Ackermann becomes disadvantageous as loads on the outside wheel increase and the greater slip angle of the inside tire creates higher tire temperatures and slows down the car due to tire drag. The inside tire has also surpassed the maximum slip angle of grip assuming the outer tire is already at the optimum slip angle. Parallel or reverse setups are

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more advantageous in this situation as both the inside and outside tires still have lateral grip. Reverse Ackermann steering can even be more beneficial than the parallel Ackermann geometry since the outside tire (which currently has more load due to weight transfer) is at the optimum slip angle and the inside is at a lower slip angle with less grip. This in turn allows the inside tire to have grip but less than the outside tire, decreasing the effects of under steer. 100% Ackermann is when both the wheels are travelling in concentric circles while 0% is for travelling in equal circles. Forward Ackermann geometry with 60% Ackermann was chosen for our BAJA vehicle.

Reasons for the choice:
  It creates an additional drag force that helps yaw the car. The second is that the slip angle of maximum lateral force changes with vertical load, so to extract maximum lateral force, the outside wheel needs to be a different amount than the inner.



Camber Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber. The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. The camber angle taken for Baja car is typically around neg. 1/2 degree as tire develops its maximum cornering force at such a small negative camber angle. This fact is due to the contribution of camber thrust, which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch).

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Caster

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative. Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved. The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by
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imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it. Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.

.  Toe in/Toe out

When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics. For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to
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scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tires, while too much toe-out causes wear at the inboard edges.

In our Baja vehicle we use Toe in of 3-4mm.The reasons for the choice is: Toe in unlike Toe out (which encourages initiation of turn) provides straight line stability. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results. Also when pushed down the road, a non-driven wheel will tend to toe itself out especially in rear-drive cars. Thus, toe in helps in providing stability while going down the road. The basic types of steering systems used are described below-

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1. RACK ANS PINION STEERING
As the name implies, rack-and-pinion steering consists of two major components -- a rack and a pinion. The rack -- also known as a steering rack -- is a long piece of metal that is flat on at least one side. The flat side contains teeth running the length of the rack. The teeth are cut perpendicular to the edges of the rack, meaning they run side by side from one end of the rack to the other. The other major component, the pinion -- more correctly, the pinion shaft -- is a round rod that also has teeth on it, although these teeth run parallel to the length of the shaft, not lengthwise as on the rack. The pinion shaft comes into the rack at a ninety-degree angle, held in place by a collar, and the teeth on the pinion mesh with the teeth on the rack. The pinion is connected directly to the steering column, so when the steering wheel is turned to the left, for instance, the pinion rotates counter-clockwise (from the driver's perspective). Simply put, the rotary motion of the pinion is changed to transverse motion by the rack. The rack moves to the right, making the wheels go left. The car turns left.

Rack and pinion steering mechanisms have the following advantages—
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Advantages: • Simple construction • Economical and uncomplicated to manufacture • Easy to operate due to good degree of efficiency • Contact between steering rack and pinion is free of play and even internal damping is maintained • Tie rods can be joined directly to the steering rack • Minimal steering elasticity compliance • compact (the reason why this type of steering is fitted in all European and Japanese front -wheel drive vehicles) • The idler arm (including bearing) and the intermediate rod are no longer needed • Easy to limit steering rack travel and therefore the steering angle

2. PITMAN ARM TYPE
Pitman arm mechanisms have a steering 'box' where the shaft from the steering wheel comes in and a lever arm comes out - the pitman arm. This pitman arm is linked to the track rod or centre link, which is supported by idler arms. The tie rods connect to the track rod. There are a large number of variations of the actual mechanical linkage from direct-link where the pitman arm is connected directly to the track rod, to compound linkages where it is connected to one end of the steering system or the track rod via other rods.

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Most of the steering box mechanisms that drive the pitman arm have a 'dead spot' in the centre of the steering where you can turn the steering wheel a slight amount before the front wheels start to turn. This slack can normally be adjusted with a screw mechanism but it can't ever be eliminated. The traditional advantage of these systems is that they give bigger mechanical advantage and thus work well on heavier vehicles. With the advent of power steering, that has become a moot point and the steering system design is now more to do with mechanical design, price and weight. The following are the four basic types of steering box used in pitman arm systems.

a)Worm and sector

In this type of steering box, the end of the shaft from the steering wheel has a worm gear attached to it. It meshes directly with a sector gear (so called because it's a section of a full gear wheel). When the steering wheel is turned, the shaft turns the worm gear, and the sector gear pivots around its axis as its teeth are moved along the worm gear. The sector gear is mounted on the cross shaft which passes through the steering box and out the bottom where it is splined, and the the pitman arm is attached to the splines. When the sector gear turns, it turns the cross shaft,
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which turns the pitman arm, giving the output motion that is fed into the mechanical linkage on the track rod. The following diagram shows the active components that are present inside the worm and sector steering box. The box itself is sealed and filled with grease.

Worm and roller

The worm and roller steering box is similar in design to the worm and sector box. The difference here is that instead of having a sector gear that meshes with the worm gear, there is a roller instead. The roller is mounted on a roller bearing shaft and is held captive on the end of the cross shaft. As the worm gear turns, the roller is forced to move along it but because it is held captive on the cross shaft, it twists the cross shaft. Typically in these designs, the worm gear is actually an hourglass shape so that it is wider at the ends. Without the hourglass shape, the roller might disengage from it at the extents of its travel.

Worm and nut or recirculating ball

This is by far the most common type of steering box for pitman arm systems. In a recirculating ball steering box, the worm drive has many more turns on it with a finer pitch. A box or nut is clamped over the worm drive that contains dozens of ball bearings. These loop around the worm drive and then out into a recirculating channel within the nut where they are fed back into the

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worm drive again. Hence recirculating. As the steering wheel is turned, the worm drive turns and forces the ball bearings to press against the channel inside the nut. This forces the nut to move along the worm drive. The nut itself has a couple of gear teeth cast into the outside of it and these mesh with the teeth on a sector gear which is attached to the cross shaft just like in the worm and sector mechanism. This system has much less free play or slack in it than the other designs, hence why it's used the most. The example below shows a recirculating ball mechanism with the nut shown in cutaway so you can see the ball bearings and the recirculation channel.

Cam and lever

Cam and lever steering boxes are very similar to worm and sector steering boxes. The worm drive is known as a cam and has a much shallower pitch and the sector gear is replaced with two studs that sit in the cam channels. As the worm gear is turned, the studs slide along the cam channels which forces the cross shaft to rotate, turning the pitman arm. One of the design features of this style is that it turns the cross shaft 90° to the normal so it exits through the side of the steering box instead of the bottom. This can result in a very compact design when necessary.

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3. RECIRCULATING BALL TYPE STEERING MECHANISM
The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm. The steering wheel connects to a threaded rod, similar to a bolt that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel -- without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.

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THE STEERING MECHANISM MOST SUITABLE FOR OUR VEHICLE
We are planning on implementing the rack and pinion mechanism because it is comparatively easier to manufacture and the most cost effective. Apart from this, it has the least number of parts hence requires less maintenance. An additional advantage is that a variable ratio steering can be designed which has been detailed below.

STEERING RATIOS
The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio.

Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response -- you don't have to turn the steering wheel as much to get the wheels to turn a given distance -- which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.

VARIABLE STEERING RATIOS
Some cars have variable-ratio steering, which uses a rack-and-pinion gear set that has a different tooth pitch (number of teeth per inch) in the centre than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the centre), and also reduces effort near the wheels turning limits. Advantages of variable ratio steering— 1. In high speed driving, increased lateral acceleration as a result of high downforce and higher normal loads acting on the tyres result in increased loads on the steering rack. By employing a variable ratio a reduction in steering torque can be achieved.

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2. Variable ratio can also be used to tune the yaw gain or sensitivity of the vehicle to steering inputs. A high yaw gain makes the vehicle feel very nervous in high speed corners where the smallest steering wheel input results in what feels like an excessive response. 3. In electric power steering systems variable ratios allow the designer to balance the power requirements of the system. 4. Variable ratio rack and pinion systems eliminate the compromises that constant ratio systems have by allowing the designer to utilise a best fit approach for tuning the vehicle response over a wide range of driving conditions.

The steering ratio opted for our BAJA vehicle is 12:1 in support of the following reasons1. Moderate steering effort-The steering effort increases with reduction in steering ratio. The above ratio selected offers moderate steering effort required during cornering. 2. Lesser no. of lock to lock turns- With the above steering ratio, the no. of lock to lock turns of steering wheel is reduced. During cornering, the driver has to turn the steering wheel in lesser amount. Since variable steering ratios offer a much greater advantage over constant steering ratio we are trying to look for manufacturers who can supply readymade rack and pinion ratios with desired ratios. Nevo Developments and Sona Koyo steering systems,Gurgaon have been contacted with regards to the requirement. A response is awaited.

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POWER STEERING

Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist. Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a vehicle is stopped or moving slowly. Also, power steering provides some feedback of forces acting on the front wheels to give an ongoing sense of how the wheels are interacting with the road; this is typically called "rοad feel". Representative power steering systems for cars augment steering effort via an actuator, a hydraulic cylinder, which is part of a servo system. These systems have a direct mechanical connection between the steering wheel and the linkage that steers the wheels. This means that power-steering system failure (to augment effort) still permits the vehicle to be steered using manual effort alone.

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ELECTRIC POWER STEERING
A steering sensor is located on the input shaft where it is bolted to the gearbox housing. The sensor performs two different functions: Firstly as a torque sensor, it converts steering torque input and direction into voltage signals for the ECU to monitor and convert into a binary code, and secondly as a rotation sensor, which converts the rotation speed and direction into voltage signals for the ECU to monitor and convert into a binary code. An interfaced ECU circuit that shares the same housing converts the signals from the torque and rotation sensors into signals that the ECU can process and provide an active output. The microprocessor control unit analyzes inputs from the steering sensor as well as the vehicle’s speed sensor. The sensor inputs are then compared to determine how much power assist is required according to the ‘forces capability map data’ stored in the ECU’s memory. This map data is pre-programmed by the manufacturer. The ECU then emits the appropriate command to the ‘power unit or current controller’, which supplies the electric motor with the necessary current to activate. The motor then pushes the rack either to the right or left. Direction of rack movement is dependent on which way the voltage flows; reversing the current flow reverses directional rotation of the motor. Increasing current to the motor increases the amount of power assist. The electric power assistance system has three operating modes: 1. In normal control mode left or right power assist is provided in response to input from the torque and rotation sensor’s inputs. 2. The return control mode is used to assist steering return after completing a turn. 3. The damper control mode changes the vehicle speed to improve road feel and dampen kickback. If the steering wheel is turned and held in the full-lock position and steering assist reaches maximum, the control unit reduces current to the electric motor to prevent an overload situation that might damage the motor. The control unit is also designed to protect the motor against voltage surges from a faulty alternator or charging problem.
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The electronic steering control unit is capable of self-diagnosing faults by monitoring the system’s inputs, outputs, and the driving current of the electric motor. If a problem occurs, the control unit turns the system off by actuating a fail-safe relay in the power unit. This eliminates all power assist, causing the system to revert back to manual steering. An in-dash EPS warning light is also illuminated to alert the driver.

TYPES OF EPS
1. COLUMN ASSIST TYPE
-The power assist unit, controller and torque sensor are attached to the steering column. -This system is compact and easy to mount on the vehicle. -This power assist system can be applied to fixed steering columns, tilt-type steering columns and other column types.
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Steering System 2. PINION ASSIST TYPE
-The power assist unit is attached to the steering gear's pinion shaft. -The power assist unit is outside the vehicle's passenger compartment, allowing assist torque to be increased greatly without raising interior noise. -Combined with a variable-ratio steering gear, this system can suffice with a compact motor and offer superior handling characteristics.

3. RACK ASSIST TYPE
-The power assist unit is attached to the steering gear rack. -The power assist unit can be located freely on the rack, allowing great flexibility in layout design. -The power assist unit's high reduction gear ratio enables very low inertia and superior driving feeling.

4. DIRECT DRIVE TYPE
-The steering gear rack and power assist unit form a single unit. -The steering system is compact and fits easily in the engine compartment layout. -The direct provision of assistance to the rack enables low friction and inertia, and in turn ideal steering feeling.

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VEHICLE DYNAMICS
UNDERSTEER
Understeer is so called because the car steers less than what is wanted. Understeer can be brought on by all manner of chassis, suspension and speed issues but essentially it means that the car is losing grip on the front wheels. Typically it happens as brakes are applied and the weight is transferred to the front of the car. At this point the mechanical grip of the front tyres can simply be overpowered and they start to lose grip (for example on a wet or greasy road surface). The end result is that the car will start to take the corner very wide. In racing, that normally involves going off the outside of the corner into a catch area or on to the grass. In normal driving, it means crashing at the outside of the corner. Getting out of understeer can involve letting off the throttle in front-wheel-drive vehicles (to try to give the tyres chance to grip) or getting on the throttle in rear-wheel-drive vehicles (to try to bring the back end around).

OVERSTEER
Oversteer is the opposite of understeer. With oversteer, the car goes where it's pointed far too efficiently and ends up diving into the corner much more quickly than expected. Oversteer is brought on by the car losing grip on the rear wheels as the weight is transferred off them under braking, resulting in the rear kicking out in the corner. Without counter-steering, the end result in racing is that the car will spin and end up going off the inside of the corner backwards. In normal driving, it means spinning the car and ending up pointing back the way it came.

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Steering System BUMP STEER

It is defined as the tendency of a wheel to steer as it moves upwards into jounce. It is typically measured in degrees per meter or degrees per foot. Bump steer in many stock vehicles is usually noticed by lowering the ride height, changing the suspension geometry. This is mainly due to the tie rod not moving in the same arc motion as the control arm (upper or lower depending on suspension type). The examples below will further explain bump steer.

Example #1: Bump Steer Scenario

The picture displays the FRONT RIGHT section of a typical Formula SAE chassis. As can be seen, there are two A-arms and a stationary steering rack (silver bar) with a tie rod. This will represent the situation of a driver with no steering input (zero degrees of steering angle). The tie rod as well as the A-arms are connected to the upright which hold the wheel hub, brake rotor, caliper, wheel, and tire.

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Initially the vehicle has no toe in or out and is traveling in a straight line. All seems well as the driver holds the wheel steady with no input over the smooth pavement road.

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Suddenly the driver hits a pothole compressing the front suspension. As can be seen, the front tire immediately toes in making the vehicle less predictable and unstable. The steering can also feel a bit light and loose under bump steer. The same issue can occur with hard braking which would compress the front suspension due to forward weight transfer. As seen below, due to the high difference in angle and length of the tie rod, the arc motion is completely off when compared to the upper A-arm. As the upper A-arm loses its lateral (left to right) displacement under jounce, the tie rod gains lateral distance pushing the upright out toeing in the front suspension.

Example #2: Scenario without Bump Steer

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CALCULATIONS:Steering ratio- 12:1 Track- 60 inches Wheel base- 58 inches Maximum steering angleInner front wheel- 40 deg. +/- 2 deg. Outer front wheel- 28 deg. +/- 2 deg. Lock to lock- 2.26 turns. Ackermann %=60% Turning circle radius- using the formula listed above we get it close to 11.2 feet. Rack travel- 2.67 inches per turn Lateral rack movement- 6.675 inches.

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Contents
Steering geometry .........................................................................................................................................................1 Ackerman ..................................................................................................................................................................1 Steering Arm Angles ..................................................................................................................................................2 Parallel Steering Arms ...............................................................................................................................................2 Angled Steering Arms ................................................................................................................................................2 Low Lateral Acceleration ...........................................................................................................................................3 High Lateral Acceleration ..........................................................................................................................................3 Reasons for the choice: .............................................................................................................................................4 1. RACK ANS PINION STEERING .....................................................................................................................................8 2. PITMAN ARM TYPE ....................................................................................................................................................9 a)Worm and sector .................................................................................................................................................10 Worm and roller ......................................................................................................................................................11 Worm and nut or recirculating ball .........................................................................................................................11 Cam and lever .........................................................................................................................................................12 3. RECIRCULATING BALL TYPE STEERING MECHANISM ...............................................................................................13 THE STEERING MECHANISM MOST SUITABLE FOR OUR VEHICLE ...............................................................................14 STEERING RATIOS ........................................................................................................................................................14 VARIABLE STEERING RATIOS ........................................................................................................................................14 POWER STEERING ........................................................................................................................................................16 ELECTRIC POWER STEERING ........................................................................................................................................17 TYPES OF EPS ...............................................................................................................................................................18 1. COLUMN ASSIST TYPE .........................................................................................................................................18 2. PINION ASSIST TYPE.............................................................................................................................................19 3. RACK ASSIST TYPE ................................................................................................................................................19 4. DIRECT DRIVE TYPE .............................................................................................................................................19 VEHICLE DYNAMICS .....................................................................................................................................................20 UNDERSTEER ...........................................................................................................................................................20 OVERSTEER ..............................................................................................................................................................20 BUMP STEER ............................................................................................................................................................21 Example #1: Bump Steer Scenario ..........................................................................................................................21 Example #2: Scenario without Bump Steer .............................................................................................................23 CALCULATIONS:- ..........................................................................................................................................................24

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Dept. of Mechanical Engineering

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