STEERING SYSTEMSteering system: It is the system which provides directional change in the performance of an automobile. This system converts rotary movement of the steering wheel into angular movement of the front wheels. It multiplies driver’s effort by mechanical advantage, enabling him to turn the wheels easily.Steering system requirements and functionsFor proper and smooth operation and performance of the system, the steering system of any vehicle should fulfill the following requirements:

1. It should multiply the turning effort applied on the steering wheel by the driver.2. It should be to a certain extent irreversible. In other words, the shocks of the road

surface encountered by the wheels should not be transmitted to the driver’s hands.3. The mechanism should have self rightening effect i.e., when the driver releases the

steering wheel after negotiating the turn, the wheel should try to achieve straight ahead position.

Functions of the steering system are as follows:a) It helps in swinging the wheels to the left or right.b) It helps in turning the vehicle at the will of the driver.c) It provides directional stability.d) It helps in controlling wear and tear of tyres.e) It helps in achieving the self-rightening effect.f) It converts the rotary movement of the steering wheel into an angular turn of

the front wheels.g) It multiplies the effort of the driver by leverage in order to make it fairly

easy to turn the wheels.h) It absorbs a major part of the road shocks thereby preventing them to get

transmitted to the hands of the driver. The complete steering system which performs the above functions, can be divided into two portions, namely, steering gear provided at the end of the steering column and the linkage between the steering gear and the wheels.Types of steering system:Steering system is of the following types(a) Fifth wheel steering system,(b) Side pivot steering system.Fifth wheel steering system:

It is single pivot steering system in which the front axle along with the wheels, moves to right or left. The movement to the whole axle and wheel assembly is affected by means of a steering and a wheel which is placed between chassis frame and axle. The fifth wheel acts as a turntable. The axle assembly is connected with the frame by means of a pin which serves as a pivot around which the axle assembly moves. The fifth wheel contains a ring gear mounted at its rim and is moved by

means of a steering. Movement of the steering wheel tends the front axle and wheel assembly to move away.Side pivot steering mechanism:There are two types of steering gear mechanisms:1. Davis steering gear mechanism2. Ackermann steering gear mechanismThe main difference between the two steering gear mechanisms is that the Davis steering has sliding pairs, whereas the Ackermann steering has only turning pairs. The sliding pair has more friction than the turning pair; therefore the Davis steering gear will wear out earlier and become inaccurate after certain time. The Ackermann steering gear is not mathematically accurate except in three positions, contrary to the Davis steering gear which is mathematically correct in all positions. However, the Ackermann steering gear is preferred to the Davis steering gear.Davis Steering Gear:

The Davis gear mechanism consists of a cross link KL sliding parallel to another link AB and is connected to the stub axles of the two front wheels by means of two similar bell crank levers ACK and DBK pivoted at A and B respectively. The cross link KL slides in slides in the bearing and carries pins at its end K and L. The slide blocks are pivoted on these pins and move with the turning of bell crank levers as the steering wheel is When the vehicle is running straight, the gear said to in its mid-position. The short arms AK and BL are inclined an angle 90+α to their stub axles AC and BD. The correct steering depends upon a suitab1e selection of cross-arm angle α, and is given by

tan α = b / 2lWhere b=AB=distance between the pivots of front axles.l=wheel base.The range of b / l is 0.4 to 0.5 hence angle α lies between 11.3 and 14.10.Ackermann Steering GearThe Ackermann steering gear mechanism consists of a cross link KL connected to the short axles AC and BD of the two front wheels through the short arms AK and BL, forming bell crank levers CAK and DBL respectively. When the vehicle is running straight, the cross link KL is parallel to AB, the short arm AK and BL both make angle α to the horizontal axis of chassis. In order to satisfy the fundamental equation for correct steering, the links AK and KL are suitably proportioned and angle α is suitably selected.For correct steeringcot φ – cot θ = b / l

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The angles φ and θ are shown in the Figure. The value of b / l is between 0.4 and 0.5, generally 0.455. the value of cot φ – cot θ corresponds to the positions when the steering is correct. In fact there are three values of angle θ which give correct steering of the vehicle: first while it is turning to right, second while it is turning to left and third while it is running straight.Fundamental condition for true rolling and correct steering angle

The perfect steering is achieved when all the four wheels are rolling perfectly under all conditions of running. While taking turns, the condition of perfect rolling is satisfied if the axes of the front wheels when produced meet the rear wheel axis at one point. Then this point is the instantaneous centre of the vehicle. It is seen that the inside wheel is required to turn though a greater angle than the outer wheel. The larger the steering angle, the smaller is the turning circle. There is, however, a maximum to which we can go as regards the steering angle. It has been found that steering angle (of the inner wheel) can have a maximum value of about 440. The extreme positions on either side are called ‘lock’ positions. The diameter of the smallest circle which the outer front wheel of the car can

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traverse and obtained when the wheels arc at their extreme positions is known as the turning circle. Referring to Fig, for correct steering,

Equation represents the basic condition for the steering mechanism for perfect rolling of all wheels.Components of Steering system:

Steering wheelSteering is effected by the steering wheel. The steering wheels of commercial vehicles have a metal armature comprised of a screw machined hub with metal spokes and rim. The hub, spokes and rim are all fabricated into one piece by welding. The armature serves as the load bearing structure of the wheel. The armature is surrounded by a moulded rubber or plastic material. Rubber wheels are painted, and plastic wheels utilize impregnated colours.The steering wheel is of large diameter. This helps to convert the available driver rim pull into maximum input torque. However, the size of the wheel is limited by the following: (1) The comfort of the driver when using the steering wheel. (2) The space available for the steering in the interior of the cabin. (3) The ease of performing maneuvers requiring more than an eighth of a turn of the steering wheel.The diameter of the steering wheel lies between 42 and 45 cm the case of motor cars, whereas it is 50 to 55 cm in the case of commercial vehicles.The rim of the steering wheel is elliptical in cross section with the finger indentations on the undersurface. The section of the rim is so designed and dimensioned to provide the driver a good grip both with and without heavy gloves.The lower portion of the steering wheel hub fits into the upper flange of the steering column. Within the steering column, the steering shaft exists and is free to rotate. On the inside diameter of the steering wheel hub, female serrations are provided. On the upper end of the steering shaft matching male serrations and a locking taper are provided. The

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steering wheel is mated to the upper end of the steering shaft by means of the locking taper and the serrations. The steering wheel is held on the steering column and fixed to the steering shaft by a nut. The nut mates with the male threads on the upper end of the steering shaft.Modern steering wheels have spring spokes. These spokes are generally so arranged to give an unrestricted view of the instrument panel.Some cars have a telescoping steering wheel. This wheel can be moved out of or into the veering column to suit the driver’s convenience.In some designs, the hub of the steering wheel also houses part of the turn signal, horn, and vehicle hazard flasher mechanism.Steering column:Steering column positions the steering wheel in the driver’s cabin in relation to the driver’s seat and pedal controls. The steering column is made either as fixed or adjustable.In the fixed type, the location of the steering wheel cannot be changed. The steering wheel position is then decided taking into account the range of seat position and driver size. On the other hand, if the steering column is made adjustable, then the steering wheel movement can take care of the optimum wheel to driver relationship in all seat positions.The fixed steering column is attached to the cabin by brackets on the instrument panel and firewall. In some cases, a bracket on the toe board is used for structural integrity. The fixed steering column is usually tubular in construction. It has a stamped flange welded on to the upper end.A bearing in the upper part of the steering column serves for centering the steering shaft. The type of bearing, bush or ball bearing or roller bearing provided for the shaft affects the steering effort. Usually no bearing is placed in the lower end of the steering column, if the steering shaft has adequate support at the steering gear. However, when the lower end of the steering shaft is terminated by a double cardan or constant velocity joint, a bearing is provided at the lower end of the steering column. This bearing becomes essential for the proper functioning of the joint.The adjustable steering column can be subdivided into adjustment by rotation and adjustment by sliding. The former type is called tilt steering wheel column assembly while the latter type is called collapsible steering column assembly.Tilt steering wheel column assembly

In this unit, the steering wheel is pivoted about a point in the steering column. The wheel can now be moved in a circular arc, forward and up or rearward and down. The tilting wheel permits the driver to change the steering wheel angle to the horizontal to suit his build. He can also change the position of the steering wheel to the horizontal during a long drive to suit his driving posture.The tilting and telescoping steering column can be seen in figure.All the above devices, wherein the position of the steering wheel can be altered with respect to the driver have locking mechanisms, which lock the steering wheel into the position selected.

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In one design, the steering wheel and the column can be swung to one side. This makes the driver to get into or out of the car easily. The tilt steering column has an interlock to the transmission operating lever. This interlock is a safety feature. This mechanism locks the transmission system, until the steering column is reset in the driving position. It also prevents the steering column from being accidentally moved when the car is in motion.Collapsible steering column assemblyThis unit is a safety device. In the front end crash, the driver will be thrown forward and on to the steering wheel. The steering column will now collapse in its length due to this impact and absorb the possibility of the driver getting injured. The collapsible steering column is therefore called the energy absorbing steering column. The Collapsible steering column assembly permits to maintain the optimum steering wheel in all positions of adjustment.There are two types of collapsible steering column assembly - the Japanese lantern type and the tube and ball type.In the Japanese lantern type, the flexible portion of the steering column folds up on impact. The tube and ball type has a stationary outer tube and a sliding inner tube. Grooves are there in these tubes; these grooves form ball races for ball bearings. The outer tube is attached to the fireball and instrument panel. The inner tube supports the steering shaft and the steering wheel. On impact, the inner tube is forced into the outer tube. Now the balls plough furrows in the tubes to permit the relative motion. The movement of the steering column absorbs the shock. The ball and tube type is claimed to give a more uniform collapse rate than the Japanese lantern type.Steering shaftThe steering shaft assembly performs two important functions:1. It transmits the driver’s turning effort or torque from the steering wheel to the steering gear.2. It absorbs the angular and/or length changes in the relationship between the steering wheel (chassis mounted) and steering gear (cab mounted) for the following operating conditions: cab to chassis movement during driving, length change for adjustable columns and cab on tilt cab models.On the non-adjustable column installations, the axial length displacement of the steering shaft is usually achieved through the use of displacement characteristics of a flexible coupling, pot joint or splined section in the shaft with cardan or constant velocity joint.Steering gearThe heart of the steering system is the steering gear. This unit is also called steering mechanism. This unit is ordinarily fixed to the bottom of the steering column. This unit is located between the steering shaft and the steerable stub axles which carry the road wheels. The input shaft of the steering gear is operated by the steering shaft. The steering gear performs two functions:1. It converts the rotary motion of the steering wheel into linear motion of the steering linkage which moves the front wheels.2. It introduces a leverage between the steering wheel and the stub axles. This leverage reduces the effort that has to be applied by the driver to the steering wheel in order to overcome the frictional forces opposing the turning of the stub axles and the road wheels.In order to have the above leverage, the steering wheel has to be turned through larger angles than the stub axles. In the case of automotive vehicles, normally the road wheels are deflectable to about 500 on each side of the straight ahead position. The extreme wheel positions are called full lock positions of the wheels. To effect this extent of turning of the road wheels, the steering wheel may have to be turned through from 4 to 9 or 10 times that angle. This relationship is called steering ratio. The term steering ratio is the ratio of the

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number of degrees of movement at the hand wheel (steering wheel) which will produce one degree of movement of the front wheels.The amount of leverage provided by the steering gear depends upon several factors. The most important among them are the weight of the vehicle and the type of tyre used. The typical gear ratios are 14:1 or higher. In heavy duty vehicles this ratio is sometime as high as 30:1 to 35: 1. The greater the ratio, steering gear ratio, the easier the steering wheel turns. Trucks are provided with higher leverage than cars.The steering gear incorporates another important feature called back locking. The steering gear is so constructed that it is easy to turn the vehicle by the steering wheel but it is difficult to turn the steering wheel by turning the front wheels. This irreversible character of the steering gear prevents the bumps and shocks experienced by the wheel at the road surface from being transmitted to the steering wheel, but still give the driver the feel of the road.The steering gear is mounted to the vehicle frame by bolts in the mounting pad of the steering gear.Steering linkage The steering linkage consist of pitman arm, ball joints, drag link, steering arm, spindle, tie rod and king pin assembly. Pitman arm – It is also called the drop arm. It converts the output torque from the steering gear into a force to the drag link. It is attached to the sector shaft of the steering gear by a split joint. In this construction either full serrations or partial spline is used to transmit the torque from the sector shaft to the pitman arm. The split arm is tightened around the sector shaft by the clamping bolt to mate the male and female serrations or splines. The end of the pitman arm which connects with the drag link has a taper hole in it. The ball stud on the drag link is fitted into this hole.Ball joints - are used on both ends of the drag link and the tie rod. These take care of the angular displacement and rotational movement of the drag link and the tie rod, which are caused by the front wheel rotation and suspension articulation.Drag link - connects the pitman arm and the steering arm. In some cases, it is a one piece forging with a ball joint socket formed in the end.Steering arm - is usually a forged component and is attached to the steering knuckle. It converts the drag link force into a turning moment about the left king pin. The steering arm is attached to the spindle by a keyway, a locking taper and a nut. The arm extends either to the front or rear of the spindle, depending upon the package constraints and then bends to locate the steering arm ball joint at the correct geometric location. The end of the steering arm which connects with the drag link has a tapered hole in it to accept the ball stud on the drag link.Left spindle and king pin - The torque from the steering arm rotates the left spindle, wheel and tyre about the king pin.Left tie rod arm - The left tie rod arm is attached to the spindle in the same manner as the steering arm, that is, key, taper and clamping nut. This converts the torque available to turn the right wheel into a force in the tie rod. The tie rod of this link has a tapered hole to accept the tie rod ball stud.Tie rod - The tie rod is a tubular member which connects the left and right tie rod arms. As such it transmits the force between these two components. The tie rod ends have female threads. The ball joint shafts have mating male threads. The threaded connections can be held together firmly by the locking clamps after the proper length has been set. The length of the tie rod has to be adjusted so that the front axle toe in will be to the specified amount.Right tie rod arm, spindle and kingpin - The right tie rod arm is a mirror image of the left. This converts the force from the tie rod into a moment to turn through the knuckle arm, the right spindle wheel and the tyre about the king pin. The right spindle and the king pin

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assembly is similar to the assembly on the left side except that it has no steering arm attached to it.Steering stops - In order to limit the angular deflections of the front wheels, stops must be provided. The purpose of these stops is to avoid rubbing of tyres against the frame or against the fenders which would cause undue wear and tear of the tyres. These steering stops can be provided at two different places. First, they may be placed in the path of motion of the steering arm or drop arm. Secondly, they may be placed in the path of motion of the steering knuckle.Steering gears:If the steering wheel is connected directly to the steering linkage it would require a great effort to move the front wheels. Therefore, to assist the driver, a reduction system is used having a movement ratio between 10:1 to 24:1 the actual value depending upon the type and weight of the vehicle. But the power steering reduces the ratio on an average by 20 percent. The low gear ratios produce fast steering, while the high ratios produce slow steering. When the mechanical advantage of the linkage between cross shaft and stub axles is considered then this ratio is increased from 15 to 20 percent and is called “overall steering ratio”. The steering gear is a device for converting the rotary motion of the steering wheel into straight line motion of the linkage with a mechanical advantage.The steering gears are enclosed in a box, called the steering gear box. There are many different designs of steering gear box. They are as follows:

a) Worm and wheel steering gearb) Worm and sector steering gearc) Cam and lever / peg steering geard) Cam and roller or worm and roller steering geare) Worm and nut or screw and nut steering gearf) Recirculating ball steering gearg) Rack and pinion steering gear.

Worm and wheel steering gear:

The system consists of worm wheel which is carried in bearings in a cast iron case. The case is made in halves. The outer end of the worm wheel is fixed to a drop arm which is having ball end to connect the side rod. The side rod is connected to the steering arm which is fixed to the stub axles. The worm which is keyed on to a steering shaft have a mesh with the worm wheel. The steering wheel is mounted at the upper end of the steering shaft. When driver rotates the steering

wheel then drop arm moves either backward or forward direction. This motion results in motion of the stub axles.Worm and sector steering gear: This is the modified form of steering wheel type, in which the wheel is being replaced with sector of wheel. In actual case, the worm wheel is

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not essential as it is having only partial rotation. Hence in this type only a sector of wheel is used instead of worm wheel.Cam and lever/ peg steering gear:

The example of the cam steering gear is shown in the simplified sketch. In this system helical groove is formed at the bottom end of the steering wheel shaft. The helical groove engages the projected pin of the drop arm spindle lever. The drop-arm is made rigid with the lever (sometimes referred as peg) by a splined spindle. The to and

fro motion is obtained at the drop-arm when the steering wheel shaft is turned. This motion results the turning of the stub axles. The end play of the steering wheel shaft can be adjusted by putting a suitable washer at the lock nut. The meshing of the projected pin in helical groove is also adjusted by a screw provided at the end of the lever spindle. In the recent models, the projected pin is made in the form of a roller. The projected pin may be one or two in number, accordingly they are referred as cam and single lever or double lever steering gear mechanism.Warm and roller steering gear

The type of steering gear is shown in the sketch where a two toothed roller is fastened to the cross-shaft so that it meshes with the threads of the warm gear. The worm gear is formed on the bottom end of the steering wheel shaft. Worm is fastened between the two ball bearings in the casing. Play of the bearings can be adjusted by an adjuster provided at the end of the casing. The outer end of the cross-shaft is formed in the spindle to fix the drop arm. When the worm gear is turned by the steering wheel shaft, it causes the roller to move in an arc so as to rotate the cross-shaft and at the same time turn

on the roller pin connecting it to the cross-shaft. The casing of the system is fixed with the column and generally bolted to the frame. Similarly there can be other design of steering gears which may use one or- three-tooth rollers.

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Screw and nut type steering gear or worm and nut type steering gear

In this system a screw or worm is formed at the lower end of the steering shaft and the upper end is fixed to the steering wheel. The nut consists of integral trunnions which pivot in the holes of the arms of the fork. The fork is connected to the drop arm by a splined shaft. The upper end of the steering shaft is supported in the steering column by a ball

and socket joint so that the shaft may swung slightly. The swing of the shaft is essential because the trunnions of the nut move in arc when the nut moves along the axis of the shaft. Some times instead of ball and socket joint an ordinary journal bearing supported in a rubber bush is used at the upper end of the steering shaft because the rubber accommodates the rocking of the steering shaft. This mechanism is very cheap and reduces the number of the bearings required.Recirculating ball type Steering Gear

It consists of a worm at the end of steering rod. A nut is mounted on the worm with two sets of balls is the grooves of the worm, in between the nut and, the worm. The balls reduce the friction during the movement of the nut on the worm. The nut has a number of with on the outside, which mesh with the teeth on a worm wheel sector, on which is further mounted the drop arm, which steers the road wheels through the link rod and the steering arms.

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When the steering wheel is turned, the balls in the worm roll in the grooves and cause the nut to travel along the length of the worm. The balls, which are in 2 sets, are recirculated through the guides, as shown in the figure. The movement of the nut causes the wheel sector to turn at an angle and actuate the link rod through the drop arm, resulting in the desired steering of the wheels.The end play of the worm can be adjusted by means of the adjuster nut provided. To compensate for the wear of the teeth on the nut and the worm, the two have to be brought nearer bodily. To achieve this, the teeth on the nut are made tapered in the plane perpendicular to the plane of Figure. A screw is also provided by means of which the drop arm, aid hence, the wheel sector can be positioned along its axis. When the wheel sector has to be moved bodily closer to the nut to eliminate backlash due to wear, the screw is turned which slides the wheel sector in a direction in which the tapered teeth on the nut are narrower, till the required adjustment is achieved.Rack and pinion steering mechanism

It is very simple and common type mechanism, the system is shown in simplified sketch. This type is very well suitable in an independent suspension system. The system consists of a rack housed in a tubular casing. The casing is supported on the frame near its ends. The ends of the rack are connected to the track rods with the help of ball and socket joints. The pinion shaft is carried in the plain bearings housed in casing. The pinion is meshed with the rack and the clearance is adjusted with the adjusting screw. When the pinion is given rotary motion with the steering wheel, then the rack slides in either sides. This sliding motion of the rack is used through the track rods to turn the wheels in desired side.POWER STEERINGPrinciples of the Power steeringPower steering has two types of device for steering effort one type is a hydraulic device utilizing engine power. The other type utilizes an electric motor. For the former, the engine is used to drive a pump. For the latter, an independent electric motor in the front luggage compartment is used the pump. Both develop fluid pressure, and this pressure acts on a piston within the power cylinder so that the pinion assists the rack effort. The amount of this assistance depends on the extent of pressure acting on the piston. Therefore, if more steering force is required, the pressure must be raised. The variation in the fluid pressure is accomplished by a control valve which is linked to the steering main shaft.Neutral (Straight-ahead) position:Fluid from the pump is sent to the control valve. If the control valve is in the neutral position, all the fluid will flow pass through the control valve into the relief port and back

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to the pump. At this time, hardly any pressure is created and because the pressure on the cylinder piston is equal on both sides, the piston will not move in either direction.While turning:When the steering main shaft is turned in either direction, the control valve also moves, closing one of the fluid passages. The other passage then opens wider, causing a change in fluid flow volume and, at the same time, pressure is created. Consequently, a pressure difference occurs between both sides of the piston and the piston moves in the direction of the lower pressure so that the fluid in that cylinder is forced back to the pump through the control valve.

Straight ahead condition While taking turnThere are two kinds of power steering currently in use a) Integral power steering and b) Linkage booster power steeringIntegral power steering

The figure shows the arrangement of integral power steering when the vehicle moves straight ahead on the road. In this system the oil pump is driven by a belt from the engine

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crank shaft pulley. The system consists of solid cylinder on which two grooves have been cut, known as “valve spool”, which slides closely within the hole in the valve housing. The housing has three internal grooves the central groove is connected to the pump and two at ends are connected to the reservoir. The two additional openings from the internal collars are connected to the two sides of the cylinder as shown in the Figure. When the valve spool is in the position shown in the Figure, then the pump delivers the oil in the central part of the housing and then delivers back to the reservoir by the passages shown by the arrows. In this position there will be no oil pressure in the cylinder and there is no tendency for the piston to slide in any direction. Thus there is no steering action and vehicle moves straight ahead.

Similarly refer the above figure when the valve spool is moved towards right side then the direct return line from the pump to reservoir is closed. The oil now flows into the cylinder by the right side passage and pushes the piston to slide left ward as shown by the arrow in the Figure. The oil on the left side of the piston is discharged to the reservoir thro the valve housing under this position. This outward move of the piston rod results to turn the vehicle tow left side on the road. Similarly the vehicle can be turned to right side by reversing the steering operation.

Linkage-booster power steeringIn this type power assistance is applied directly to the steering linkage. The power cylinder consists of piston and the piston rod is extended out on the right and is fixed to the frame member. The relay rod is linked with the cylinder housing.In the neutral position the spool valve is held in the centre position by the centering springs. In this position oil from pump flows to both sides of the piston in the power cylinder at equal pressure and then there will not be displacement in the power cylinder thus there will no steering action. In this position the vehicle moves straight ahead on the road.

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Again when the steering wheel is turned anticlockwise as shown, then the ball of the pitman arm shifts the valve spool towards right side. Due to this shifting, the oil from pump flows in the valve section of the unit, through the ports. Then the oil through feed line flows into the right hand side of the power cylinder. The high pressure oil inside the cylinder, forces it to move to the right which results to turn the stub axles to the left side. For the right side of the vehicle this operation of the system is reversed to it.Reversible and Irreversible SteeringWhen deflection of the steered wheels due to road surface is transmitted through the steering linkage and steering gear box to the steering wheel, the system is said to be reversible, if every small imperfection of the road surface causes the steering wheel to rotate, the driver would find much tiring and frustrating. Such reversibility is not desired. Some degree of reversibility is desired so that the wheels will find to straighten up after negotiating a bend. Some degree of irreversibility is desired to stop shocks sustained by the road wheels. Such a steering system is known as semi-reversible. When the steered wheels do not transfer any deflection to the steering wheel, the steering system is said to be irreversible. It would not tend to straighten out after negotiating a turn, and would not easily follow the course of a gutted road without undue stress on the mechanism. Therefore, in most of the passenger cars semi-reversible steering gears are used.Under-steering and Over-steeringWhile taking a turn, the wheels are not always pointing in direction in which the vehicle is moving, due to the distortion of tyre tread. The angle between the wheel inclination and the path taken by the wheel is known as slip angle. When the angle is greater at the rear than at

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the front, the vehicle tends to oversteer, that is to turn into the curve more than the driver intended.

When the slip angle is smaller at the rear than at the front, the vehicle tends to understeer. Of course, the understeer is opposite to oversteer and is preferred because correction by the driver involves rotating the steering wheel a little more in the direction of the turn. It is to be noted that the slip angle is affected by the road camber, side winds, tyre inflation and variations in the load on either the front or rear axle.

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