Front Axle & Steering Geometry

IntroductionPrimary function of the steering system is to achieve angular motion of the front wheels to take a turn.This is done through linkage and steering gear which convert the rotary motion of the steering wheel into angular motion of the front road wheels.

Secondary functions To provide directional stability of the vehicle when going straight ahead.To provide perfect steering condition i.e perfect rolling motion of the road wheels at all times .To facilitate straight a head recovery after completing a turn.To minimize tyre wear.

Requirements of a good steering systemThe steering mechanism should be very accurate and easy to handle.The effort required to steer should be minimal and must not be tiresome to the driver.The steering mechanism should also provide directional stability. This implies that the vehicle should have a tendency to return to its straight ahead position after turning.

Front AxleConventionally front axle is a dead axle. But its true for only heavy vehicles now a days.For four wheel drive vehicles and most of the cars, it is a live axle. In case of a dead axle the front axle beam is usually a drop forging of steel.

Material : steels used for this are 0.4% carbon steel or 0.3% nickel steel.Loads Acting : it has to take bending loads due to weight of the vehicle and also torque loads due to braking of the wheels.Hence it is made of I- section in the central portion, while the ends are made either circular or elliptical.A downward sweep is given to the centre portion to keep a low chassis height.

Stub AxleThe main axle beam is connected to the stub axle by means of king pins and front road wheels are mounted on these stub axles.Stub axle can have any of the four types of shapes:ElliotReversed ElliotLamoineReversed lamoine

Wheel AlignmentPositioning of the steered wheels to achieve the following is termed as wheel alignment:Directional stability during straight a head position.Perfect rolling condition on steering.Recovery after completing the turn.

Toe in & Toe outToe In: in the event if the front wheels of a vehicle are pointing inward when the vehicle is moving on a straight road, they are said to toe in.Toe Out: in the event if the front wheels of a vehicle are pointing outward when the vehicle is moving on a straight road, they are said to toe out.

Set back distance and Thrust angleIt is a condition in which one wheel on an axle is infront of or behind the other wheel in relaton to the chassis frame.A set back of less than 6mm is considered normal tolerance by some manufacturers.

Thrust Angle: it is the angle made by the thrust line with the longitudinal centre line of the vehicle. It should be zero.

If the thrust angle is not zero, the vehicle will dog track and steering wheel will not be centred.

The factors are as follows:Factors pertaining to wheelsSteering geometrySteering linkageSuspension systemFactors influencing the stability and control of vehicle.

Factors Pertaining to wheelsBalance of wheels: if the wheels are not in dynamic balance, the wheel wobble may result, which influences steering control.Inflation of tyres: the steering system is designed for a particular rolling radius. Variation of air pressure from the standard prescribed will change the rolling radius.Brake Adjustment: if the brake is not adjusted properly, i.e if it is draging, it can pull the vehicle to one side while braking.

Steering GeometryCamber: Camber is the tilt of the car wheels from the vertical. Camber is positive if the tilt is outward at the top. It is also called wheel rake.A positive camber causes the wheels to toe out and the negative camber causes it to toe in.However initial positive camber is provided to the wheels so that when the vehicle is loaded, they automatically come to a vertical position.

Camber should not generally exceed 2, however the exact amount of camber is specified taking into account the king pin inclination.Castor: The angle between the king pin centre line (SAI) and the vertical, in the plane of the wheel is called the castor angle.

King Pin InclinationInclination of the king pin from vertical is called the king pin inclination or king pin rake.In modern cars where the king pin has been replaced by the ball joints, this term has also been renamed as steering axis inclination.It is defined as the inclination of the ball joint axis from the vertical. Steering axis is an imaginary line drawn through the lower and upper steering pivot points.

King pin inclination: Inclination of king pin from vertical is called the king pin inclination or king pin rake.Pin replaced by Ball jointsRenamed as Steering Axis Inclination defined as ball joint axis from the vertical.SAI is an imaginary line drawn through the lower and upper steering pivot points.It is non adjustable, since it would change only if the wheel spindle or steering knuckle are bent.Effect: helps the straight ahead recovery, thus providing directional stability. Amount: about 7 to 8 degrees. However exact amount is decided considering the wheel rake value.

Effect: king pin inclination helps the straight ahead recovery, thus providing directional stability. When the vehicle takes a turn, the inclination of king pin causes the vehicle body to move up, in relation to the wheels. As soon as the steering wheel is left after the turn is completed, the weight of the vehicle tends to return the wheels to the straight a head position.Amount : about 7 to 8.

Combined angle & scrub RadiusCombined angle or included angle is the angle formed in the vertical plane between the wheel centre line and the king pin centre line (steering axis) It is equal to camber plus king pin inclination.The forward tractive force acts at the point on the road where the steering axis or the king pin axis meets when projected, the road resistance acts at the wheel contact point on the road.The distance between the two points is called scrub radius.

Effects: the point of intersection of the wheel and the king pin centre lines Above the ground (negative scrub radius) causes to toe in.Just at the ground (zero scrub radius) no effect.Below the ground ( positive scrub radius) causes to toe out.Combined angle may be 9-10 sand the scrub radius

CastorThe angle between the king pin centreline (or steering axis) and the vertical, in the plane of the wheel is called the castor angle.If the the king pin centre line meets the ground at a point a head of the vertical wheel center line, it is called positive castor while if it is behind the vertical wheel centre line, it is called negative castor.

CastorEffect: in rear wheel drive vehicles, the steering axis pulls the front tyres, whereas the tyre drag on account of the vehicle is on the vertical line at the centre of the footprint. Since in positive castor steering axis would meet the ground ahead of the centre of tyre print, the later would always follow the former.Thus positive castor on the car wheels provides directional stability, i.e straight line trackings is improved.How ever positive castor increases the effort required to turn the steering wheel.

CastorIn case of negative castor steering would be unstable i.e there would be poor directional control, as the centre of the tyre print leads the steering axis.Extremely negative castor would cause wheel shimmy and consequently cupped wear of front tyres.About 3 of castor gives good results.

Steering System componentsThe manual steering system consists of Steering wheelSteering columnSteering shaftSteering gearSteering linkages and wheels.Rotation of steering wheel is transmitted via the steering gear to the arms or rods of the steering linkage.The steering linkage turn the steerable wheels.

Steering linkagePit man arm or drop armBall jointsDrag linkSteering armSpindleTie rodKing pin assembly

Pit man Arm: Converts the out put torque from the steering gear into a force to the drag link.Attached to secondary shaft of steering gear by a split joint.End of pit man which connects the drag link has a tapered hole in it.Ball joint: used on both ends of drag link and tie rod.These take care of angular displacement and rotational movement of drag link & tie rod, which are caused by the front wheel rotation and suspension articulation.

Left spindle & king pin: the torque from the steering arm rotates the left spindle, wheel and tyre about the king pin.Left tie rod arm: converts the torque available to turn the right wheel into a force in the tie rod.Tie rod: The tie rod is a tubular member which connects the left and right tie rod arms. Tie rod ends have female threads and these are in mating with male threads of ball joint shafts.Right tie rod arm, spindle and king pin: it is mirror of left. Converts the force from the tie rod into a moment to turn through the knuckle arm, right spindle wheel and the tyre about the king pin.

Steering Stops: used to limit the angular deflections of the front wheels. Purpose of these stops is to avoid rubbing of tyres against the frame which would cause wear and tear of tyres.Placed in path of motion of the steering arm or drop arm.Placed in path of motion of steering knuckle.Steering Wheel: made of armature comprised of a screw machined hub with metal spokes and rim.Armature surrounded by a moulded rubber or plastic material. Wheel of large dia helps to convert the available driver rim pull into max input torque.

Rack-and-pinion steering is quickly becoming the most common type of steering on cars, small trucks and SUVs. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.

Rack-and-pinion Steering

The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. The rack-and-pinion gearset does two things: It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels. It provides a gear reduction, making it easier to turn the wheels. On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right).

The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn or (turning angles of the stub axle.) 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.The steering ratios generally used with the present day steering gears vary from about 12:1 for cars to about 35:1 for heavy vehicles.

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.

When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.

Power rack and Pinion

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.

Recirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system.

Recirculating-ball Steering

The recirculating-ball steering gear contains a worm gear. we 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 (see diagram above). 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.

Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is provided by supplying higher-pressure fluid to one side of the block.

There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism.In addition it consists of a pump and rotary valvePower Steering

PUMP

PumpThe hydraulic power for the steering is provided by a rotary-vane pump (see diagram in previous slide). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed.

The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds.The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped.

Rotary Valve A power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called the rotary valve.

The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has.

As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel.When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

The Future of Power SteeringSince the power-steering pump on most cars today runs constantly, pumping fluid all the time, it wastes horsepower. This wasted power translates into wasted fuel.One of the coolest ideas on the drawing board is the "steer-by-wire" or "drive-by-wire" system. These systems would completely eliminate the mechanical connection between the steering wheel and the steering, replacing it with a purely electronic control system.

General Motors has introduced a concept car, the Hy-wire, that features this type of driving system. One of the most exciting things about the drive-by-wire system in the GM Hy-wire is that you can fine-tune vehicle handling without changing anything in the car's mechanical components -- all it takes to adjust the steering is some new computer software.