105 Rubber spring bump or limiting stops

10.5.1 Bump stop function (Figs 10.40 and 10.42)

Suspension bump and body roll control depends upon the stiffness of both the springs and anti-roll bar over the normal operating conditions, but if the suspension deflection approaches maximum bump or roll the bump stop (Fig. 10.40(a, b, c and d)) becomes active and either suddenly or progressively provides additional resistance to the full deflection of the wheel and axle relative to the body (Fig. 10.42). The bump stop considerably stiffens the resisting spring rate near the limit of its vertical movement to prevent shock impact and damage to the suspension components. The stop also isolates the sprung and unsprung members of the suspension under full deflection conditions so that none of the noise or vibrations are transmitted through to the body structure. In essence the bump stop enables an anti-roll bar to be used which has a slightly lower spring rate, therefore permitting a more cushioned ride for a moderate degree of body roll.

10.5.2 Bump stop construction (Fig. 10.40(a-d)) Bump stops may be considered as limiting springs as they have elastic properties in compression similar to other kinds of spring materials. Solid and hollow spring stops are moulded without reinforcement from natural rubber compound containing additives to increase the ozone resistance. The deflection characteristics for a given size of rubber stop spring are influenced by the hardness of the rubber, this being controlled to a large extent by the proportion of sulphur and carbon black which is mixed into the rubber compound. The most common rubber compound hardness used for a rubber spring stop is quoted as a shore hardness of 65; other hardness ranging from 45 to 75 may be selected to match a particular operating requirement. A solid cylindrical rubber block permits only 20% deflection when loaded in compression, whereas hollow rubber spring stops have a maximum deflection of 50-75% of their free height. The actual amount of deflection for a given spring stop height and response to load will depend upon a number of factors such as the rubber spring stop size, outer profile, wall thickness, shape of inner cavities, hardness of rubber compound and the number of convolution folds.

Bump rubber spring stops may be solid and conical in shape or they may be hollow and cylindrical or rectangular shaped with a bellow profile (Fig. 10.40(a, b, c and d)) having either a single, double or triple fold (known as convolutions). The actual profile of the rubber bump stop selected will depend upon the following:

1 How early in the deflection or load operating range of the suspension the rubber begins to compress and become active.

2 Over what movement and weight change the bump stop is expected to contribute to the sudden or progressive stiffening of the suspension so that it responds to any excessive payload, impact load and body roll.

10.5.3 Bump stop characteristics (Figs 10.41 and 10.42)

The characteristics of single, double and triple convolution rubber spring stops, all using a similar rubber hardness, are shown in Fig. 10.41. It can be seen that the initial deflection for a given load is large but towards maximum deflection there is very little compression for a large increase in load. The relation between load and deflection for bump is not quite the same on the release rebound so that the two curves form what is known as a hysteresis loop. The area of this loop is a measure of the energy absorbed and the internal damping within

Double Rubbers Triple Springs
Fig. 10.40(a-d) Suspension bump stop limiter arrangements
Fig. 10.41 Characteristics of hollow rubber single, double and triple convolute progressive bump stops

TS 300

TS 300



/ Double


Bump----y /

Rebound .XJ / / If . i i


25 50 75 100 Bump stop deflection |mm)

25 50 75 100 Bump stop deflection |mm)

Fig. 10.42 Combined characteristics of suspension spring and rubber bump stops the rubber in one cycle of compression and expansion of the rubber spring stop. For hollow rubber spring stops they always end in a point; this means for any load change there will be some spring deflection.

Fig. 10.42 shows how the bump spring stop deviates from the main spring load-deflection curve at about two-thirds maximum deflection and that the resultant stiffness (steepness of curve) of the steel spring, be it leaf, coil or torsion bar, and that of the bump spring stops considerably increases towards full load.

10.6 Axle location

10.6.1 Torque arms (Figs 10.28(c) and 10.44) Torque arms, sometimes known as radius arms or rods, are mounted longitudinally on a vehicle between the chassis/body structure and axle or unsprung suspension member. Its purpose is to permit the axle to move up and down relative to the sprung chassis/body and to maintain axle alignment as the torque arm pivots about its pin, ball or conical rubber joint. Sometimes the upper torque rods are inclined diagonally to the vehicle's lengthwise axis to provide lateral axle stability (Figs 10.28(c) and 10.44). These arms form the link between the unsprung suspension members and the sprung chassis/body frame and are therefore able to transmit both driving and braking forces and to absorb the resulting torque reactions.

Panhard rods, also known as transverse control rods or arms, are positioned across and between both rear wheels approximately parallel to the axle (Fig. 10.28(b)). One end of the rod is anchored to one side of the axle span while the other end is anchored to the body structure; both attachments use either pin or ball type rubber joints. A Panhard rod restrains the body from moving sideways as the vehicle is subjected to lateral forces caused by sidewinds, inclined roads and centrifugal forces when cornering. When the body is lowered, raised or tilted relative to the axle, the Panhard rod is able to maintain an approximate transverse axle alignment (Fig. 10.28(b)) relative to the chassis/body thus relieving the suspension springs from side loads.

10.6.3 Transverse located Watt linkage

A Watt linkage (Fig. 10.43) was the original mechanism adopted by James Watt to drive his beam steam engine. This linkage is comprised of two link rods pivoting on the body structure at their outer ends and joined together at their inner ends by a coupler or equalizing arm which is pivoted at its centre to the middle of the rear axle. When in mid-position the link rods are parallel whereas the equalizing arm is perpendicular to both (Fig. 10.43(b)).

If vertical movement of the body occurs either towards bump (Fig. 10.43(c)) or rebound (Fig. 10.43(a)) the end of the link rods will deviate an equal amount away from the central pivot point of the coupling arm. Thus the left hand upper link rod

Fig. 10.43(a-c) Transversely located Watt linkage

will tend to pull towards the left and the right hand lower link rod will apply an equal pull towards the right. The net result will be to force the equalizing arm to rotate anticlockwise to accommodate the inclination to the horizontal of both link rods. If the left hand link rod were made the lower link and the right hand rod the upper link, then the direction of tilt for the equalizing arm would now become clockwise.

For moderate changes in the inclination of the link rods, the body will move in a vertical straight line, thus maintaining a relatively accurate body to axle lateral alignment. Excessive up and down movement of the body will cause the pivot centre to describe a curve resembling a rough figure eight, a configuration of this description being known as a lemniscoid (Fig. 10.43(b)).

Under body roll conditions when cornering, the whole body relative to the axle and wheels will be restrained to rotate about the equalizing arm pivot centre at mid-axle height; this point therefore becomes the roll centre for the rear end of the body.

A similar Watt linkage arrangement can be employed longitudinally on either side of the wheels to locate the axle in the fore and aft direction.

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