104 Antiroll bars and roll stiffness Fig 1035

10.4.1 Anti-roll bar function

A torsion anti-roll bar is incorporated into the suspension of a vehicle to enable low rate soft springs to be used which provides a more comfortable ride under normal driving conditions. The torsion bar does not contribute to the suspension spring stiffness (the suspension's resistance to vertical deflection) as its unsprung weight is increased or when the driven vehicle is subjected to dynamic shock loads caused possibly by gaps or ridges where concrete sections of the road are joined together. However, the anti-roll bar does become effective if one wheel is raised higher than the other (Fig. 10.35) as the vehicle passes over a hump in the road or the body commences to roll while cornering. Under these conditions, the suspension spring stiffness (total spring rate) increases in direct proportion to the relative difference in deflection of each pair of wheels when subjected to the bump and rebound of individual wheels or body roll when the vehicle is moving on a circular path.

10.4.2 Anti-roll bar construction (Fig. 10.36) Generally the anti-roll bar is formed from a medium carbon steel solid circular sectioned rod which is positioned transversely and parallel to the track (Fig. 10.36) so that it nearly spans the distance between the road wheels (Fig. 10.35). The bar is bent at both ends in right angles to form cracked arms. These arms can then be actuated by short link rods attached to the unsprung portion of the suspension such as the axle beam or transverse wishbone arms for independent suspension. The main transverse span of the rod is supported by rubber bearings positioned on the inside of the cranked arms at each end. These bush bearings are either mounted directly onto the body structure when incorporated

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Fig. 10.35 Transverse double wishbone coll spring Independent suspension with anti-roll bar

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Fig. 10.35 Transverse double wishbone coll spring Independent suspension with anti-roll bar

Fig. 10.36 Transverse double wishbone torsion bar independent suspension with anti-roll bar
Fig. 10.37 Relationship of body roll and suspension spring and anti-roll bar stiffness

on cars (Fig. 10.35) or indirectly for commercial vehicles (Fig. 10.39) on short vertical arms which provide a swing attachment to the chassis.

10.4.3 Anti-roll bar operating principle

When a pair of road wheels supported on an axle travel over a bumpy road one or other wheel will lift and fall as they follow the contour of the road surface. If the springs were relatively hard, that is they have a high spring rate, then the upthrust caused by the bumps would be transmitted to the body which would then lift on the side being disturbed. Thus the continuous vertical deflection of either wheel when the vehicle moves forward would tend to make the body sway from side to side producing a very uncomfortable ride. On the other hand if softer springs were used for the suspension, the small road surface irregularities would be adequately absorbed by the springs and dampers, but when cornering there would be insufficient spring stiffness to resist the overturning moment; this would therefore permit excessive body roll which could not be tolerated. Incorporating an anti-roll bar with relatively soft suspension springs mostly overcomes the difficulties discussed and therefore greatly improves the vehicle's ride. This is possible because the soft springs improve the suspension's response on good straight roadways (Fig. 10.37), with the benefits of the anti-roll bar automatically increasing the suspension roll stiffness when the vehicle is cornering.

10.4.4 Anti-roll bar action caused by the body rolling (Fig. 10.39(a and b))

When cornering, the centrifugal force acting through the centre of gravity of the sprung body

-

Spring and

anti-roll bar

Spring

r 1

0odv roll (degl

0odv roll (degl

Fig. 10.38 Relationship of body roll and the understeer tendency with and without an anti-roll bar produces an overturning moment created by its offset to the body's roll centre which will therefore tend to make the body roll (Fig. 10.39(a and b)). The rolling body will tilt the transverse span of the roll bar with it so that the cranked arms on the outside wheel to the turn will be depressed downward, whereas the cranked arm on the opposite end near the inside wheel to the turn will tend to rise. The consequence of this misalignment of the anti-roll bar arms is that the two cranked arms will rotate in opposite directions to each other and so transmit a torque from the inside wheel which is subjected to less load to the outside wheel which is now more heavily loaded. The effect of the tor-sional wind-up in the bar is that it tries to rotate the outside wheel cranked arm and since the arm is attached to the axle or indirectly to the wishbone arm it cannot move. The alternative is for the roll bar and the rubber bearing mount near the outside wheel to lift in proportion to the degree of twisting torque. It therefore counteracts some of the downward push due to the increase in weight to the outside wheel and as a result stiffens the roll resistance of the springing on the outside wheel as a whole. Consequently a larger slip angle is generated on the front outside wheel relative to the rear wheel, and as a result, the vehicle will develop a small initial but progressive understeer tendency approximately proportional to the amount the body rolls (Fig. 10.38).

10.4.5 Anti-roll bar action caused by single wheel lift (Fig. 10.39(c and d))

If one of a pair of axle wheels lifts as it climbs over a bump (Fig. 10.39(c)) in the road, then the vertical

Fig. 10.39 (a-d) Anti-roll bar action

deflection of the wheel and spring raises and rotates the anti-roll bar's cranked arm on that side so that the transverse span of the bar is twisted. The bar is therefore subjected to a torque which is proportional to its angle of rotation.

This twisting torque is transferred to the opposite cranked arm which then applies a downward force onto the axle and wheel. However, because the wheel cannot sink into the ground, the reaction occurs on the rubber bearing mount arm which therefore tends to lift up the side of the chassis on the opposite side to the vertically deflected wheel. As a result, both sides of the chassis (body) will have been raised, thereby enabling the vehicle's body to remain upright instead of tilting to one side. Similarly, if the opposite wheel hits an obstacle in the road (Fig. 10.39(d)), the torsional wind-up of the bar transfers an upward thrust to the other side, which again tends to lift the chassis on the undisturbed wheel side and so maintains the sprung chassis and body on an even keel (Fig. 10.39(c)).

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