114Body subframes

Front or rear subframes may be provided to brace the longitudinal side members so that independent suspension on each side of the car receives adequate support for the lower transverse swing arms (wishbone members). Subframes restrain the two halves of the suspension from splaying outwards or the

Fig. 1.4 Stress and strain imposed on beam when subjected to bending

longitudinal side members from lozenging as alternative road wheels experience impacts when travelling over the irregularities of a normal road surface.

It is usual to make the top side of the subframe the cradle for the engine or engine and transmission mounting points so that the main body structure itself does not have to be reinforced. This particularly applies where the engine, gearbox and final drive form an integral unit because any torque reaction at the mounting points will be transferred to the subframe and will multiply in proportion to the overall gear reduction. This may be approximately four times as great as that for the front mounted engine with rear wheel drive and will become prominent in the lower gears.

One advantage claimed by using separate subframes attached to the body underframe through the media of rubber mounts is that transmitted vibrations and noise originating from the tyres and road are isolated from the main body shell and therefore do not damage the body structure and are not relayed to the occupants sitting inside.

Cars which have longitudinally positioned engines mounted in the front driven by the rear wheels commonly adopt beam cross-member subframes at the front to stiffen and support the hinged transverse suspension arms (Fig. 1.6(a)). Saloon cars employing independent rear suspension sometimes prefer to use a similar subframe at the rear which provides the pivot points for the semi-trailing arms because this type of suspension requires greater support than most other arrangements (Fig. 1.6(a)).

Fig. 1.5 Bending resistance for various sheet sections

When the engine, gearbox and final drive are combined into a single unit, as with the front longitudinally positioned engine driving the front wheels where there is a large weight concentration, a subframe gives extra support to the body longitudinal side members by utilising a horseshoe shaped frame (Fig. 1.6(b)). This layout provides a platform for the entire mounting points for both the swing arm and anti-roll bar which between them make up the lower part of the suspension.

(c) Rectangular subframe

Fig. 1.6 (a-c) Body subframe and underfloor structure

Front wheel drive transversely positioned engines with their large mounting point reactions often use a rectangular subframe to spread out both the power and transmission unit's weight and their dynamic reaction forces (Fig. 1.6(c)). This configuration provides substantial torsional rigidity between both halves of the independent suspension without relying too much on the main body structure for support.

Soundproofing the interior of the passenger compartment (Fig. 1.7)

Interior noise originating outside the passenger compartment can be greatly reduced by applying layers ofmaterials having suitable acoustic properties over floor, seat and boot pans, central tunnel, bulkhead, dash panel, toeboard, side panels, inside of doors, and the underside of both roof and bonnet etc. (Fig. 1.7).

Acoustic materials are generally designed for one of three functions:

a) Insulation from noise — This may be created by forming a non-conducting noise barrier between the source of the noises (which may come from the engine, transmission, suspension tyres etc.) and the passenger compartment.

b) Absorption of vibrations — This is the transference of excited vibrations in the body shell to a media which will dissipate their resultant energies and so eliminate or at least greatly reduce the noise.

c) Damping of vibrations — When certain vibrations cannot be eliminated, they may be exposed to some form of material which in some way

Fig. 1.7 Car body sound generation and its dissipation

modifies the magnitude of frequencies of the vibrations so that they are less audible to the passengers.

The installation of acoustic materials cannot completely eliminate boom, drumming, droning and other noises caused by resonance, but merely reduces the overall noise level.

Insulation Because engines are generally mounted close to the passenger compartment of cars or the cabs of trucks, effective insulation is important. In this case, the function of the material is to reduce the magnitude of vibrations transmitted through the panel and floor walls. To reduce the transmission of noise, a thin steel body panel should be combined with a flexible material of large mass, based on PVC, bitumen or mineral wool. If the insulation material is held some distance from the structural panel, the transmissibility at frequencies above 400 Hz is further reduced. For this type of application the loaded PVC material is bonded to a spacing layer of polyurethane foam or felt, usually about 7 mm thick. At frequencies below 400 Hz, the use of thicker spacing layers or heavier materials can also improve insulation.

Absorption For absorption, urethane foam or lightweight bonded fibre materials can be used. In some cases a vinyl sheet is bonded to the foam to form a roof lining. The required thickness of the absorbent material is determined by the frequencies involved. The minimum useful thickness of polyurethane foam is 13 mm which is effective with vibration frequencies above 1000 Hz.

Damping To damp resonance, pads are bonded to certain panels of many cars and truck cabs. They are particularly suitable for external panels whose resonance cannot be eliminated by structural alterations. Bituminous sheets designed for this purpose are fused to the panels when the paint is baked on the car. Where extremely high damping or light weight is necessary, a PVC base material, which has three times the damping capacity of bituminous pads, can be used but this material is rather difficult to attach to the panelling.

1.1.5 Collision safety (Fig. 1.8) Car safety may broadly be divided into two kinds: Firstly the active safety, which is concerned with the car's road-holding stability while being driven, steered or braked and secondly the passive safety,

Fig. 1.8 Collision body safety

which depends upon body style and design structure to protect the occupants of the car from serious injury in the event of a collision.

Car bodies can be considered to be made in three parts (Fig. 1.8); a central cell for the passengers of the welded bodywork integral with a rigid platform, acting as a floor pan, and chassis with various box-section cross- and side-members. This type of structure provides a reinforced rigid crush-proof construction to resist deformation on impact and to give the interior a high degree of protection. The extension of the engine and boot compartments at the front and rear of the central passenger cell are designed to form zones which collapse and crumble progressively over the short duration of a collision impact. Therefore, the kinetic energy due to the car's initial speed will be absorbed fore and aft primarily by strain and plastic energy within the crumble zones with very little impact energy actually being dissipated by the central body cell.

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