10 Suspension

10.1 Suspension geometry

The stability and effective handling of a vehicle depends upon the designers' selection of the optimum steering and suspension geometry which particularly includes the wheel camber, castor and kingpin inclination. It is essential for the suspension members to maintain these settings throughout their service life.

Unfortunately, the pivoting and swivelling joints of the suspension system are subject to both wear and damage and therefore must be checked periodically. With the understanding of the principles of the suspension geometry and their measurements it is possible to diagnose and rectify steering and suspension faults. Consideration will be given to the terminology and fundamentals of suspension construction and design.

10.1.1 Suspension terminology

Swivel joints or king pins These are the points about which the steering wheel stub axles pivot.

Pivot centre The point where the swivel ball joint axis or kingpin axis projects and intersects the ground.

Contact patch This is the flattened crown area of a tyre which contacts the ground.

Contact centres This is the tyre contact patch central point which is in contact with the ground.

Track This is the transverse distance between both steering wheel contact centres.

10.1.2 Wheel camber angle (Figs 10.1 and 10.2) Wheel camber is the lateral tilt or sideway inclination of the wheel relative to the vertical (Fig. 10.1). When the top of the wheel leans inwards towards the body the camber is said to be negative, conversely an outward leaning wheel has positive camber.

Road wheels were originally positively cambered to maintain the wheel perpendicular to the early highly cambered roads (Fig. 10.2) and so shaped as to facilitate the drainage of rain water. With modern underground drainage, road camber has been greatly reduced or even eliminated and therefore wheel camber has been reduced to something like % to 1 % degrees.

The axis of rotation of a cambered wheel if projected outwards will intersect the ground at the apex of a cone generated if the wheel was permitted to roll freely for one revolution. The wheel itself then resembles the frustrum of a cone (Fig. 10.1). The path taken by the cambered wheel (frustrum of a cone) if free to roll would be a circle about the apex. Consequently both front wheels will tend to steer outwards in opposite directions as the vehicle moves forwards. In practice, the track rods and ball joints are therefore preloaded as they restrain the wheels from swivelling away from each other when the vehicle is in motion. If both wheels have similar camber angles, their outward pull on the track rods will be equal and therefore balance out. If one wheel is slightly more cambered than the other, due maybe to body roll with independent suspension or because of misalignment, the steering wheels will tend to wander or pull to one side as the vehicle is steered in the straight ahead position.

Fig. 10.1 Wheel camber geometry

Fig. 10.2 Road camber

A negatively cambered wheel leaning towards the radius of a curved track or bend increases its cornering power and reduces the tyre contact patch slip angle for a given cornering force compared to a wheel rolling in an upright position. Conversely, a positively cambered wheel leaning away from the centre of rotation reduces its cornering power and increases the tyre slip angle for a similar cornering force compared to a wheel rolling perpendicular to the ground.

To provide a small amount of understeer, the front wheels are normally made to generate a greater slip angle than the rear wheels by introducing positive wheel camber on the front wheels and maintaining the rear wheels virtually perpendicular to the ground.

When cornering with positive camber angles on both front wheels, the inner and outer wheels will lean inwards and outwards respectively relative to the centre of rotation of the turn. At the same time, body roll transfers weight from the inner wheel to the outer one. As a result the inner wheel will generate less slip angle than the outer wheel because it provides an inward leaning, more effective tyre grip with less vertical load than that of the less effective outward leaning tyre, which supports a greater proportion of the vehicle's weight. The front cambered tyres will generate on average more slip angle than the upright rear wheels and this causes the vehicle to have an understeer cornering tendency.

Steered positive cambered wheels develop slightly more slip angle than uncambered wheels. When they are subjected to sudden crosswinds or irregular road ridging, the tyres do not instantly deviate from their steered path, with the result that a more stable steering is achieved.

With the adoption of wider tyres as standard on some cars, wheel camber has to be kept to a minimum to avoid excessive edge wear on the tyres unless the suspension has been designed to cope with the new generation of low profile wide tread width tyre.

10.1.3 Swivel or kingpin inclination

Swivel pin or kingpin inclination is the lateral inward tilt (inclination) from the top between the upper and lower swivel ball joints or the kingpin to the vertical (Fig. 10.3). If the swivel ball or pin axis is vertical (perpendicular) to the ground, its contact centre on the ground would be offset to the centre of the tyre contact patch (Fig. 10.4). The offset between the pivot centre and contact patch centre is equal to the radius (known as the scrub radius) of a semicircular path followed by the rolling wheels when being turned about their pivots. When turning the steering the offset scrub produces a torque T created by the product of the offset radius r and the opposing horizontal ground reaction force F (i.e. T = Fr (Nm)). A large pivot to wheel contact centre offset requires a big input torque to overcome the opposing ground reaction, therefore the steering will tend to be heavy. No offset (zero offset radius) (Fig. 10.5) prevents the tread rolling and instead causes it to scrub as the wheel is steered so that at low speed the steering also has a heavy response. A compromise is usually made by offsetting the pivot and contact wheel centres to roughly 10-25% of the tread width for a standard sized tyre. This small offset permits the pivot axis to remain within the contact patch, thereby enabling a rolling movement to still take place when the wheels are pivoted so that tyre scruff and creep (slippage) are minimized. One other

Fig. 10.3 Swivel (king) pin inclination

Fig. 10.4 Swivel (king) pin vertical axis offset

Fig. 10.5 Dished wheel centre point steering

Fig. 10.4 Swivel (king) pin vertical axis offset

Fig. 10.5 Dished wheel centre point steering effect of a large pivot to contact centre offset is when one of the wheels hits an obstacle like a bump or pothole in the road; a large opposing twisting force would be created momentarily which would be relayed back to the driver's steering wheel in a twitching fashion.

To reduce or even eliminate pivot to wheel centre offset, the whole stub axle, hub bearing assembly and disc or drum would have to be positioned within the centre region of the wheel rim and also extend, and therefore protrude, beyond the wheel rim flange (Fig. 10.5). A dished wheel arrangement of this type is known as centre point steering because both pivot centre and contact patch centres coincide in the middle of the wheel.

The alternative and realistic way of reducing the pivot to contact patch centre offset is to laterally incline the axis of the swivel joints so that the whole hub assembly and disc or drum is positioned inside the wheel and only the upper swivel joint may protrude outside the wheel rim.

The consequences of tilting the swivel pin axis is the proportional lowering of the stub axle axis in the horizontal plane as the wheel assembly swivels about its pivot points relative to the straight ahead position (Fig. 10.6(a and b)). Because the road wheels are already supported at ground level, the reverse happens, that is, both upper and lower wishbone arms or axle beam which supports the vehicle body are slightly raised. This unstable state produces a downward vehicle weight component which tends to return both steered wheel assemblies to a more stable straight ahead position. In other words, the pivot inclination produces a self-centring action which is independent of vehicle speed or traction but is dependent upon the weight concentration on the swivel joints and their inclination. A very large swivel ball or pin inclination produces an excessively strong self-centring effect which tends to kick back on turns so that the swivel ball or pin inclination angle is usually set between 5 and 15°. A typical and popular value would be something like 8 or 12°.

The combination of both camber and swivel joint inclination is known as the included angle and the intersection of both of these axes at one point at ground level classifies this geometry as centre point steering (Fig. 10.7). In practice, these centre lines projected through the ball joints or pins and through the centre of the wheel are made to meet at some point below ground level. Thus an offset exists between the projected lines at ground level, which produces a small twisting movement when the wheels are steered. As a result, the wheels tend to roll about a circular path with the offset as its radius, rather than twist about its swivel centre with a continuous slip-grip action which occurs when there is no offset as with the centre point steering geometry.

10.1.4 Castor angle (Figs 10.8 and 10.9) The inclination of the swivel ball joint axis or kingpin axis in the fore and aft direction, so that the tyre contact centre is either behind or in front of the imaginary pivot centre produced to the ground, is known as the castor angle (Fig. 10.8(b and c)). Positive castor angle is established when the wheel contact centre trails behind the pivot point at

Fig. 10.6(a and b) Swivel and kingpin inclination self-straightening tendency
Fig. 10.7 Camber and swivel pin inclination centre point steering

ground level (Fig. 10.8(b)). Negative castor angle exists if the wheel contact centre leads the pivot axis intersection at ground level (Fig. 10.8(c)).

If the pivot centre and wheel contact patch centre coincide the castor is nil (Fig. 10.8(a)). Under these conditions the steered wheels become unstable as they tend to twitch from side to side when the vehicle travels along a straight path.

A rear wheel drive vehicle has the front wheel steer pivot axis inclined backward to produce positive castor (Fig. 10.9(a)). As the vehicle is propelled from the rear (the front wheels are pushed by the driving thrust transmitted by the rear drive wheels), it causes the front wheels to swing around their pivot axis until the tyre contact centre trails directly behind. This action takes place because the drag force of the front tyres on the road causes both tyres to move until they are in a position where no out of balance force exists, that is, positioned directly to the rear of the pivot swivel balls or pin axis.

With front wheel drive vehicles the situation is different because the driving torque is transmitted through the steered front wheels (Fig. 10.9(b)). By inclining the pivot axis forwards, a negative castor is produced and instead of the pivot axis being pushed by the rear wheel drive thrust, traction is now transmitted through the front wheels so that the pivot axis is pulled forwards. The swivel balls or pin mounting swing to the rear of the contact patch centre, due to the vehicle rolling resistance acting through the rear wheels, opposing any forward motion.

The effects of castor angle can be seen in Fig. 10.9(a and b), when the steering is partially turned on one lock. The trail or lead distance between the pivot centre and contact patch centre rotates as the steered wheels are turned so that the forward driving force Fd and the equal but opposite ground reaction FR are still parallel but are now offset by a distance x. Therefore a couple (twisting movement) M is generated of magnitude M = Fx, where F = Fd = Fr. With the vehicle in motion, the couple M will continuously try to reduce itself to zero by eliminating the offset x. In other words, the driving and reaction forces Fd and FR are at all

Fig. 10.8 Castor angle steering geometry

(a) Rear wheel drive castor (b) Front wheel drive castor angle self-righting torque effect angle self-righting torque effect

Fig. 10.9(a and b) Illustration of steered wheel castor self-straightening tendency times tending to align themselves with the wheels rolling when the steering has been turned to one lock. As a result the trailing or leading offset x produces a self-righting effect to the steered wheels. The greater the angle the wheels have been steered, the larger the pivot centre to contact patch centre offset x and the greater the castor self-centring action will be. The self-righting action which tends to straighten out the steering after it has been turned from the straight position, increases with both wheel traction and vehicle speed.

10.1.5 Swivel joint positive and negative offset (Figs 10.10-10.15)

When one of the front wheels slips during a brake application, the inertia of the moving mass will tend to swing the vehicle about the effective wheel which is bringing about the retardation because

Fig. 10.10 Swivel pin inclination positive offset

there is very little opposing resistance from the wheel on the opposite side (Fig. 10.12).

If the offset of the swivel ball joints is on the inside of the tyre contact patch the swivel inclination is known as positive offset (Fig. 10.10). When the wheels are braked the positive offset distance and the inertia force of the vehicle produce a turning movement which makes the wheels pivot about the contact patch centre in an outward direction at the front (Fig. 10.10). If the offside (right) wheel moves onto a slippery patch, the vehicle will not only veer to the left, due to the retarding effect of the good braked wheel preventing the vehicle moving forward, but the near side (left) wheel will also turn and steer to the left (Fig. 10.13). Therefore the positive offset compounds the natural tendency for the vehicle to swerve towards the left if the right hand wheel skids instead of continuing on a stable straight ahead path.

Arranging for the swivel ball joint inclination centre line to intersect the ground on the outside of the contact patch centre produces what is known as negative offset (Fig. 10.11). With negative offset the

Fig. 10.11 Swivel pin inclination negative offset
Fig. 10.12 Directional stability when one wheel skids whilst being braked

Fig. 10.13 Directional stability with positive offset when one wheel skids whilst being braked

Fig. 10.14 Directional stability with negative offset when one wheel skids whilst being braked

Fig. 10.13 Directional stability with positive offset when one wheel skids whilst being braked momentum of the vehicle will produce a turning moment that makes the wheels swivel inwards at the front about the contact patch centre (Fig. 10.11) because the swivel ball joints and stub axle assembly are being pulled forwards and around the patch centre caused by the negative offset distance. The consequence of negative offset is that the effective braked wheel twists in the opposite direction to that to which the vehicle tends to veer (Fig. 10.14) and so counteracts the swerving tendency, enabling the vehicle to remain in a stable straight ahead direction.

In both positive and negative offset layouts, the skidding wheel turns in the same direction as the initial swerving tendency, but since it is not contributing greatly to the tyre to ground grip, its influence on directional stability is small.

The effect of negative offset is ideal for a split line braking system where if one brake line should fail, the front brake on the opposite side will still operate as normal (Fig. 10.14). The tendency for the car to veer to the side of the braked wheel is partially corrected by the wheel being turned due to the negative offset in the opposite direction (inwards), away from the direction in which the car wants to swerve.

Fig. 10.14 Directional stability with negative offset when one wheel skids whilst being braked

When cornering, the sideways distortion of the tyre walls will misalign the wheel centre to that of the tread centre so that the swivel ball joint inclination offset will alter. The outer front wheel which supports the increase in weight due to body roll reduces positive offset (Fig. 10.15(a)), while negative offset becomes larger (Fig. 10.15(b)) and therefore makes it easier for the car to be steered when negotiating a bend in the road.

10.1.6 MacPherson strut friction and spring offset (Figs 10.16 and 10.17) The MacPherson strut suffers from stickiness in the sliding motion of the strut, particularly under light load with an extended strut since the cylinder rod bearing and the damper piston will be closer together. Because the alignment of the strut depends upon these two sliding members, extending and reducing their distance will increase the side loading under these conditions.

The problem of reducing friction between the inner and outer sliding members is largely overcome in two ways:

when cornering

when cornering

Swivel pin inclination offset change

Fig. 10.16 Concentric coil spring and swivel pin axes permit bending moment reaction

Fig. 10.17 Coil spring to swivel pin axis offset counteracts bending moment

Fig. 10.16 Concentric coil spring and swivel pin axes permit bending moment reaction

Fig. 10.17 Coil spring to swivel pin axis offset counteracts bending moment

(a) By reducing the friction, particularly with any initial movement, using a condition which is known as stiction. This is achieved by facing the bearing surfaces with impregnated poly-tetra-fluorethytene (PTFE) which gives the rubbing pairs an exceptionally low coefficient of friction.

(b) By eliminating the bending moment on the strut under normal straight ahead driving although there will be a bending moment under cornering conditions.

The tendency for the strut to bend arises because the wheel is offset sideways from the strut, causing the stub axle to act as a cantilever from the base of the strut to the wheel it supports, with the result the strut bends in a curve when extended or under heavy loads (Fig. 10.16).

A simple solution which is commonly applied to reduce the bending moment on the strut is to angle the axis of the coil spring relative to the swivel joint axis causing the spring to apply a bending moment in the opposite sense to the vehicle load bending moment (Fig. 10.17). Under normal conditions this coil spring axis tilt is sufficient to neutralize the bending moment caused by the inclined strut and the stub axle offset, but the forces involved while cornering produce much larger bending moments which are absorbed by the rigidity of the strut alone.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment