10154Antiroll rubber blocks Fig 10108

A conventional anti-roll bar can be incorporated between the trailing arms to increase the body roll stiffness of the suspension or alternatively built-in anti-roll rubber blocks can be adopted (Fig. 10.108). During equal bump or rebound travel of each wheel the trailing arms swing about their front pivots. However, when the vehicle is cornering, roll causes one arm to rise and the other to fall relative to the chassis frame. Articulation will occur at the rear end of the trailing arm where it is pivoted to the lower spring base and axle member. Under these conditions, the trailing arm assembly adjacent to the outer wheel puts the rubber blocks into compression, whereas in the other trailing arm, a tensile load is applied to the bolt beneath the rubber block. As a result, the total roll stiffness will be increased. The stiffness of these rubber blocks can be varied by adjusting the initial rubber compres-sive preload.

10.15.5 Air spring characteristics (Figs 10.114, 10.115, 10.116 and 10.117)

The bounce frequency of a spring decreases as the sprung weight increases and increases as this weight is reduced. This factor plays an important part in the quality of ride which can be obtained on a heavy goods or passenger vehicle where there could be a fully laden to unladen weight ratio of up to 5:1.

An inherent disadvantage of leaf, coil and solid rubber springs is that the bounce frequency of vibration increases considerably as the sprung spring mass is reduced (Fig. 10.114). Therefore, if a heavy goods vehicle is designed to give the best ride frequency, say 60 cycles per minute fully laden, then as this load is removed, the suspension's bounce frequency could rise to something like 300 cycles per minute when steel or solid rubber springs are used, which would produce a very harsh, uncomfortable ride. Air springs, on the other hand, can operate over a very narrow bounce frequency range with considerable changes in vehicle laden weight, say 60-110 cycles per minute for a rolling lobe air spring (Fig. 10.114). Consequently the quality of ride with air springs is maintained over a wide range of operating conditions.

Fig. 10.114 Effects and comparison of payload on spring frequency for various types of spring media

Fig. 10.116 Effects of static payload on spring air pressure for various spring static heights

Fig. 10.114 Effects and comparison of payload on spring frequency for various types of spring media

Fig. 10.116 Effects of static payload on spring air pressure for various spring static heights ffi 1

Leaf

/

and i /

coil /

/

Air

j_ i-------- j

Spring height (mm]

Fig. 10.117 Relationship of extra air tank volume and spring frequency

Fig. 10.115 Effects of static load on spring height

Fig. 10.117 Relationship of extra air tank volume and spring frequency

Steel springs provide a direct rise in vertical deflection as the spring mass increases, that is, they have a constant spring rate (stiffness) whereas air springs have a rising spring stiffness with increasing load due to their effective working area enlarging as the spring deflects (Fig. 10.115). This stiffening characteristic matches far better the increased resistance necessary to oppose the spring deflection as it approaches the maximum bump position.

To support and maintain the spring mass at constant spring height, the internal spring air pressure must be increased directly with any rise in laden weight. These characteristics are shown in Fig. 10.116 for three different set optimum spring heights.

The spring vibrating frequency will be changed by varying the total volume of air in both extra tank and spring bag (Fig. 10.117). The extra air tank capacity, if installed, is chosen to provide the optimum ride frequency for the vehicle when operating between the unladen and fully laden conditions.

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