Page 218 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 8 .1 Types of suspension and drive

Fig. 8.1-24 The rear axle on a Ford Escort Express delivery vehicle. Single leaf springs carry the axle and support the body well
at four points. The shock absorbers (fitted vertically) are located close to the wheel, made possible by slim wheel-carriers/hub
units. The additional elastomer springs sit over the axle tube and act on the side members of the body when at full bump.

the body of passenger cars can be supported in two This measure can, of course, have an adverse effect
places at the back: at the level of the rear seat and under when the vehicle is travelling on bad roads, but it does
the boot (Fig. 8.1-27). This reduces the stress on the prevent the standard passenger car’s tendency to
rear end of the car body when the boot is heavily laden, oversteer when cornering. Even driven rigid axles ex-
and also the stress on the lorry frame under full load hibit – more or less irrespective of the type of suspen-
(Fig. 8.1-20). sion – a tendency towards the load alteration (torque
The longitudinal leaf springs can be fitted inclined, steering) effect, but not to the same extent as semi-
with the advantage that during cornering the rigid rear trailing link suspensions. Details can be found in Section
axle (viewed from above) is at a small angle to the vehicle 10.1.12.2.
longitudinal axis (Fig. 8.1-28). To be precise, the side of On front-wheel-drive vehicles, the wheels of the
the wheel base on the outside of the bend shortens trailing axle can take on a negative camber. This improves
somewhat, while the side on the inside of the bend the lateral grip somewhat, but does not promote perfect
lengthens by the same amount. The rear axle steers into tyre wear. This is also possible on the compound crank
the bend and, in other words, it is forced to self-steer suspension (a suspension-type halfway between a rigid
towards ’roll-understeering’ (Fig. 8.1-29).

Fig. 8.1-25 On rigid axles the axle body absorbs the bending
moments which arise as a result of lateral forces. Only the force
F T occurs between the suspension and the body, and its size
corresponds to the lateral forces F Y,W,r,o and F Y,W,r,i .On
a horizontal Panhard rod, the distance h Ro,r is also the height of Fig. 8.1-26 Longitudinal leaf springs can absorb both forces in
the body roll centre. The higher this is above ground, the greater all directions and the drive-off, braking and lateral force
the wheel force change 6F z,wr . moment.

220