2 GENERALVEHICLE ATTITUDE is taken to mean the geometric condition of all the parts which contribute to the determination of the position of the wheels when moving on the ground, either in a straight line or on bends.This geometric attitude can be checked only in a static state, i.e. when the vehicle is stationary, with the wheels in the straight travel position and in the turning position.
3 VEIHCLE GEOMETRY BASIC CONDITION the vehicle must satisfy certain set conditions of symmetry and perpendicularity in the axes
4 THE CONTROL OF THE GEOMETRIC ATTITUDE OF THE VEHICLE Locate and eliminate any play on the suspension and steering rodsPosition the vehicle on a level surfaceCarefully check the tyre pressuresRespect and take into account the specified load conditionsRespect and take into account the load distributionCheck to see if there is any irregular give on the elastic parts of the suspension or stiffening of the joints.
5 SUSPENSION SYSTEMS THE WORD SUSPENSION IS USED TO DESCRIBE THE TOTALITY OF ELASTIC PARTS THAT CONNECT THE WHEELS TO THE CHASSIS OR UNITIZED BODY SUSPENSION SYSTEMS HAVE THE FOLLOWING FUNCTIONS:To absorb the bumps from uneven road surfaces, to ensure a certain degree of smoothness in the ride for passengers, or for objects being transported, and to avoid subjecting the mechanical parts to excessive wear.To ensure that the tires are in constant contact with the ground to achieve good road-holding and for the correct functioning of the steering and a safe ride.
6 CHARACTERISTIC ANGLES Wheel anglesKingpin anglesWheel camber angleWheel toe-inWheel toe-out on turnsTransverse king pin inclination or, in brief form ""king pin inclinationLongitudinal caster angle or, in brief form "caster angle“6. Longitudinal caster angle or, in brief form "caster angle“7. Wheel toe-in or toe-out
8 CAMBER ANGLE The wheel camber angle is the angle, measured in degrees, between the center line of the wheel and the perpendicular to the ground, looking at the vehicle from the front.If the upper part of the wheel leans towards the outside of the vehicle, the camber is known as POSITIVEIf the upper part of the wheel leans towards the inside of the vehicle, the camber is known as NEGATIVE
9 EFFECTSWith improvements in construction techniques, and with the introduction of independent-arm suspension, the wheel camber angle tends towards a value very close to zero under the most common use or load conditions; it should be taken into account, however, that the camber angles of the wheels will tend to vary as the vehicle is jolted about.When the suspension is in compression, the bump position of the wheel will be higher in relation to the body; with the release, however, the bump position of the wheel will be lower in relation to the bodyDuring these movements the wheel, with its ideal position being perpendicular to the ground, will take on a negative camber angle during compression, and a positive camber angle during releaseThis was one of the many factors that led to independent-arm suspension being preferred to rigid-axle suspension. The benefits of this effect are most apparent on bends, where the compression of the suspension on the outer wheel on the bend, caused by centrifugal force, produces a negative camber on the wheel itself that acts against the overturning of the vehicle; this does not occur with rigid-axle suspension systems.
10 ASCERTAINING THE CAMBER ANGLE OF THE WHEEL Find out and respect the load conditions specified by the manufacturerObserve the angle values and the specified range of tolerancesObserve the tendency of the wheel when subject to jolts, using the following method: rock the vehicle slowly, first downwards then upwards, in order to see the tendency of the wheel towards positive or negative camber.
11 KINGPIN INCLINATIONThe kingpin inclination is the angle, measured in degrees, that forms the line passing through the kingpin and the perpendicular to the ground, looking at the vehicle from the frontA marked increase in this angle created negative effects, especially with the tires at low pressureThe specific need arose, therefore, to reduce the camber angle almost to zero; this was also to achieve regular wear of the tyre. The problem was resolved by inclining the kingpin towards the lower part of the wheel
12 INCLUDED ANGLEThe angle between the kingpin axis and the wheel axis is equal to the algebraic sum of the kingpin inclination angle and the wheel camber angle and is defined as the included angleThe included angle can vary only if there is a deformation between the kingpin axis and the wheel hub axis ("F" and "G" in fig. 29).
13 EFFECTSThe king pin inclination angle, amongst other things, creates the phenomenon of the return of the wheels to the straight position after a steering operationIt tends to maintain this position after an impact with an obstacle attempts to alter their trajectory.This natural effect, due to the inclination of the kingpin, derives from the fact that the wheel, when turning about this oblique axis, forms an inverted cone, as shown inTherefore, each perturbation force (Fp) on the straight line direction of the wheels will meet with an equal counteracting force, due to the weight of the vehicle, acting in a transverse direction along the axis of the oblique kingpin, and thus contributing enormously to conserving the stability of the drive of the vehicle
14 CASTER ANGLEThe caster angle is the angle, measured in degrees, formed between the axis of the kingpin and the perpendicular to the ground, looking at the vehicle from the sideIt has been established by convention that, if the extension of the kingpin axis falls in front of the point where the wheel stands on the ground, the caster angle is defined as POSITIVEif it falls behind this point the caster angle is defined as NEGATIVEThe caster angle is zero when the kingpin is perfectly vertical
15 STABILITY PHENOMENAThis is created in on the basis of the distance "B" between the point at which the kingpin axis extension falls "1" (in relation to the direction of travel) and the point of contact between the wheel and the groundIn fact, in the case of the positive caster angle, the wheel is pulled, as it is the line of application of the force applied to the axis that passes through point 1 in front of the wheel, without taking the direction of travel into considerationHowever, in the case of a negative caster angle, the wheel is pushed, as it is the line of application of the force applied to the axis passing through point 1 behind the wheel, without taking the direction of travel into consideration
16 TILT STATE PHENOMENA WITH THE WHEELS IN THE TURNING POSITION When the wheel turns about a kingpin with a positive caster angle:If it is in the outer position on a bend it will take on a negative camber that increases with the strength of the steering action, and will thus act against the overturning of the vehicleIf, on the other hand, it is in the inner position on the bend it will take on a positive camber which follows and assists the turn.
17 OTHER FACTORS INFLUENCING THE CASTER ANGLE Low-pressure tires, in modern vehicles, contribute to reducing the caster angle given to the kingpin during constructionIn fact, when under the influence of active or braking thrust, the tires deform and tend to increase the longitudinal offset by shifting their point of contact with the ground
18 WHEEL TOE-IN AND TOE-OUT Wheel toe-in is the angle formed by the center line of the wheel and the longitudinal axis of the vehicle, looking at the vehicle from above.The sum of the toe-in values for each single wheel (α+β) gives the total toe-in valueWhen the extensions of the wheel center lines tend to meet in front of the direction of travel of the vehicle, this is known as toe-outIf, however, the lines tend to meet behind the direction of travel of the vehicle, this is known as toe-out
19 MEASUREMENTSThe toe-out value is rarely given in degrees by the manufacturer, and it is more common to give a value expressed in mm which measures the difference in mm between circles in the wheels; the two measurements are taken in front of and behind the wheel hub, at a distance from the ground halfway up the wheel.When the toe-in/out is measured in degrees (i.e. as an angle value), to go back to the mm value, the diameter of the wheel circle must be taken into account, with the value being inserted at intervals.The wheel toe-in value set by the manufacturer is such that, when travelling under average vehicle use conditions, it goes to a value near to 0E.
20 EFFECT OF LOAD ON TOE-IN/OUT DYNAMIC EFFECTS ON TOE-IN/OUT 1. When measuring the toe-in/out, it is advised to refer to the manufacturers' specifications and check whether the toe-in/out tendency is positive or negative by moving the vehicle up and down in a vertical direction; after this, apply the specified data and tolerances according to the average use and load conditions.DYNAMIC EFFECTS ON TOE-IN/OUTThe front wheels or rear non-drive wheels have a toe-in position when the vehicle is stationaryThe front wheels or rear drive wheels have a toe-out position when the vehicle is stationary
21 WHEEL DRIFTIt is evident from fig. 55 that, after running for 1 km, the free wheel would be in position B, but the reaction of the other wheel forces it go to point A; thus, with every turn, the wheel makes a sideways movement which, based on a distance of a kilometer has a value equal to the segment A'B, which increases in accordance with the size of the toe-in value.This sideways movement is defined as WHEEL DRIFTIt has been established by convention that wheel drift measurement is in meters per kilometer, which means how many meters he wheel drifts on the basis of a kilometer of travel.
22 IRREGULAR TYRE WEARThe wear caused on a tyre with excessive toe-in or toe-out, has certain typical characteristics:A wheel with excessive toe-in tends to drag along the road from the inside towards the outside, so that after a short run it produces a transverse wear pattern on the profile of the tread that is both visible and touch-detectableOn the other hand, a wheel with excessive toe-in tends to drag from the outside towards the inside, thus producing a clearly visible wear pattern in the opposite direction on the tread
23 STEERING GEOMETRY WHEN THE WHEELS ARE TURNED, ANOTHER VERY IMPORTANT CONDITION IS CREATED THAT IS DIRECTLY LINKED TO THE RADIUS OF THE CURVE BEING NEGOTIATEDTo understand this condition, it is better to start by considering a wheel that is travelling at a very low speed without interference; An indispensable condition, in order to prevent the wheel form being subject to sideways drag, which would be very damaging for the tread, is that, whilst following a curved trajectory, the wheel must be in a position that is perpendicular to the radius of the curve itselfWhen a whole axis is turned, the wheels, although they are travelling on two different circumferences, must keep in a perpendicular position to the radii of these circumferences so that they turn around the same center of rotation
24 STEERING GEOMETRYWhen a whole vehicle is turned, which means two axes simultaneously, the same conditions must be respected, and the rear axis must turn about the same rotation centerHowever, for obvious reasons of stability and under-chassis space restrictions, this cannot be done on motor vehicles; in fact, motor vehicles are steered by means of the joints created by the kingpins; the wheel hubs, in this case, behave as if there were two separate axes (Fig.62)
25 TOE-IN ON BENDSThe steering geometry is defined as the toe-out position taken by the during a turn; it is expressed by the two values, in degrees, through which the wheels turn (considering one fixed value on the turn of 20 grade, set by convention).The ideal condition is the following:The wheels, when travelling in a straight line, must have a toe-out value near to 0E, and as soon as they start to turn the toe-out value must increase progressively, becoming more accentuated with the increase in the angle of the turn.
26 STEERING PARALLELOGRAMS This toe-out on the wheels, proportional to the radius of the turn, is generated by the two steering track rods attached to the kingpin, with well-defined lengths and positions.If, however, the two steering track rods form a symmetrical jointed steering parallelogram with the other track rods, these needs will not be satisfied (fig. 65).In fact, it can be seen in fig. 65 that the steering track rods are parallel to the longitudinal axis of the vehicle, and that they remain parallel during the turn; the same thing happens to the wheels, which are unable to take on the necessary toe-out position.
27 STEERING PARALLELOGRAMS The required toe-out on the turn can only be achieved if the two steering track rods form an asymmetrical parallelogram with the other track rods, which means that the steering track rods converge towards the rear part of the vehicle
28 INSTANTANEOUS ROTATION CENTER In the asymmetric steering parallelogram, when a turn is made, as the steering rods move, the two steering track rods follow two arcs of different circumferencesThe asymmetric joints steering parallelogram, a turning difference between the inner wheel and outer wheel on the turn is achieved, thus producing a progressive toe-out movement in relation to the width of the bend that is being negotiated.
29 DIRECTION CENTERING It is taken to mean the perfect symmetrical condition of the directional parts in relation to the longitudinal axis of the vehicleThe direction of the vehicle is centered when:the steering track rods converge towards the center of the rear axisthe axes passing through the rear wheels (regardless of whether they are toe-in) are symmetrical in relation to the rear wheelsthe steering track rod, steering bolt and steering return are in the mid positionthe adjustable track rods are of the same lengththe steering wheel spokes are in a symmetrical position
30 AXIS DEVIATION THESE DEVIATIONS CAN BE GROUPED INTO THREE CLASS TYPES. The front axis is oblique in relation to the longitudinal axis of the vehicle and has a value of α (Fig.72)these will put themselves into a symmetrical position in relation to the rear axis, with projections A and A equal, as in fig. 71; naturally, this is assuming that the wheels are linked by the steering track rod system.
31 AXIS DEVIATIONThe rear axis is oblique in relation to the longitudinal axis to a value of α (fig. 73).the front wheels put themselves into a symmetrical position with their projections to the rear wheels A and A equal, which means that the track rods, the steering center and the steering wheel center are also regular.In this case, the rear wheels undergo a negative and positive drift respectively, thus causing irregular wear.
32 AXIS DEVIATION3. There is a transverse, non oblique deviation S between the two axes (fig. 74).The front wheels go to a symmetrical angle (%) that is no longer in relation to the rear wheels, but to their axis, which means that, on the road, the front wheels are in the condition of least resistance.
33 PARTICULAR DYNAMIC EFFECTS Tyre drift is the result of lateral elastic deformation of the tyre under the effect of a perturbation force; the drift can also be created if the vehicle is travelling in a straight line, but less so than on bends.The angle of drift of the tyreis the angle formed on the horizontal plane between the theoretical directional axis of the tyre and the effective axis:it increases with the increase of weight on the wheelsit decreases with the increase in tyre pressureit decreases with the increase in tyre cross section areait can vary to a greater or lesser degree according to the geometrical attitude of the wheels
34 PARTICULAR DYNAMIC EFFECTS -INFLUENCE OF DRIFT ON STRAIGHT LINE TRAVEL- Let us assume that a perturbation force Fp acts on a vehicle, tending towards the right:As a consequence of the perturbation force Fp, if the drift Da is greater than Dp, the vehicle will tend to start to steer towards the right, taking it from position 1 to position 2, but as a result of this a centrifugal force Fp is created that counteracts and cancels out Fp. The vehicle, which in this case has counteracted the deviation caused by the drift, is known as STABLE.If, on the other hand, as a result of the same perturbation force Fp, the drift Dp is greater than Da, the vehicle will tend to start to steer towards the left, taking it from position 1 to position 2. The centrifugal force Fc created at that moment will add itself to the perturbation force Fp. The vehicle, which tends to increase the deviation caused by the drift, is termed as UNSTABLE.
35 PARTICULAR DYNAMIC EFFECTS -INFLUENCE OF DRIFT DURING STEERING- On a turn, the centrifugal force represented by Fc applied to the center of gravity G of the vehicle, produces lateral elastic deformations on the tires which, more often than not, differ from one to the other; these deformations are directly related to:the weight distribution on the axes (as this varies point G of the application of force Fc)the acceleration or deceleration of the engine, wherever this is applied (front wheel drive or rear wheel drive).
36 PARTICULAR DYNAMIC EFFECTS -INFLUENCE OF DRIFT DURING STEERING- If the rear tires are subject to the greater drift, the vehicle will attempt to go through a curve of a smaller radius than that set by the turned wheels. The vehicle in this case is said to be UNDERSTEERING, in that it attempts to enter into the bend.If, on the other hand, it is the front tires that are subject to a greater drift, the trajectory followed by the vehicle will be larger than that set by the turned wheels; the vehicle tends to go out of the curve and is said to be UNDERSTEERING.In order to correct these drifts, or to make them positive, the manufacturer has attempted to redistribute the weight, to establish which tires and tyre pressures are to be used and to adapt the geometric attitude of the vehicle to these demands so that it is correctly balanced for the acceleration and deceleration thrusts that the vehicle is subject to and the drive position, without detracting from the needs examined in previous chapters.
37 PARTICULAR DYNAMIC EFFECTS -INFLUENCE OF DRIFT DURING STEERING- All the characteristic angles of the vehicle influence the drift and thus the dynamic behavior of the vehicle itselfA single example will be given of the dynamic correction of a vehicle on a bend, achieved by shifting the instantaneous center of curvature C.
38 PARTICULAR DYNAMIC EFFECTS -PRIMARY AND SECONDARY DRIFT- PRIMARY DRIFT - this is when the sideways elastic deformation of the tyre is generated by the structural asymmetry of the breaker belt in relation to the latitudinal plane of the wheel; in fact, then cords that make up the breaker belt of the tyre are not perpendicular to the tread layer but are positioned obliquely; the primary drift will be to the left or right depending on the orientation of the cords that make up the outer belt nearest to the tread, and this tendency is inverted if the direction of rotation of the wheel is inverted.SECONDARY DRIFT - this is when the sideways elastic deformation of the tyre is generated by an asymmetry in the rigidity of the sides due to a factory imperfection in the tyre.If these two possible phenomena are accentuated, they can affect the straight line travel of the vehicle with the annoying interference known as "PULL" to one side which obliges the driver to make continuous corrections to the travel direction with the steering wheel.