1.3 VEHICLE STRUCTURE
1.3.3 Unitary/integral construction
Frameless construction
In the previous section dealing with chassis frames, it was stated that the body shell helps to resist the torsional movement of a simple frame, but defects in the construction soon show up because the shell is not designed to withstand these stresses. In the 1930s, the
development of the all-steel body made possible the elimination of a separate frame. A suitably designed body shell was found capable of withstanding the various frame stresses. This frameless or integral arrangement gives a stiff, light construction, which is particularly suitable for mass-produced vehicles, and since the late 1940s and early 1950s nearly all light cars have been built with this construction.
Figure 1.40 shows some of the forces that act on a car body and the general manner in which the various body panels are arranged to form a unitary structure of sufficient strength to resist these forces.
The diagram shows that the weight of the occupants causes a sagging effect which is resisted by the floor and roof panels. Since these two members are widely spaced, thin sheet metal can be used to form a box-like structure that is both strong and light in weight.
Torsional stiffness of the body is achieved by strengthening the scuttle at the front and by using cross-ties, or fitting a ribbed metal panel behind the rear seat squab.
The thickness of the material used depends on the stress taken by the panel. Structural members such as sills, rails and pillars are often about 1.1 mm (0.045 inch) thick, whereas panels such as the roof are
Figure 1.39 A vehicle mounted on a vehicle jig
Figure 1.40 Integral body construction
0.9 mm (0.035 inch) thick. Component attachment points require reinforcement with thicker material, and in some cases a separate sub-frame is used to mount such items as the engine and independent front suspension (IFS) members. This frame is sometimes connected to the body by rubber insulation mountings.
Figure 1.41 Separate sub-frames
Figure 1.42 Comparison of a saloon car with a convertible car
Figure 1.43 Space frame
Since extremely good ductility is essential for the pressing of the panels, a very low carbon (0.1%) steel is required. The low strength (278 MN m–2or 18 tonf in–2)
of this material means that structural members must be stiffened by forming the thin steel sheet into intricate sections, and spot welding into position. Some idea of the number of separate pressings can be gained by the fact that approximately 4000 spot welds are used on a modern car body.
A modified construction is needed when the roof cannot be fully utilized as a compression member. This occurs on convertible (drop-head coupé) models, and in situations where either a sunshine roof, or very thin door pillars are used. In these cases the required strength is achieved by using a strong underbody frame.
In addition, extra stiffness is given to the body-shell parts that are subject to torsion.
A body-shell is normally constructed in one of two ways: it is either made by spot-welding the panels, pillars and pressings together so as to form a strong box, or by building a space frame (Figure 1.43). The latter structure gives a skeleton of high structural strength on to which is attached the steel, aluminium or glass-reinforced plastic (GRP) body panels, doors, roof, etc.
Of the three materials used, steel is the most common for vehicles made in high volume; this is because production costs are lower once the initial outlay on expensive body jigs and robots has been recovered.
To avoid vibration of the panels, which gives an objectionable noise called drumming, a sound-damping material is stuck on the inside of the panels.
Safety in the event of an accident
The safety of a modern car is improved by enclosing the driver and passengers in a rigid cell. At the front and rear of this safety compartment are attached sub-frames; these are designed to concertina on impact as shown in Figure 1.44. The crumple zones of the body are intended to absorb the shock of a collision and, in consequence, reduce the rate of deceleration that is experienced by the occupants.
Before vehicles can be sold, at least one vehicle in a model range must be submitted to an approved centre for an impact test. To pass this severe, destructive test, the level of safety of the occupants must reach a given standard. In addition the doors must remain closed during impact and must be capable of being opened after the test. The inclusion of this test feature shows why special ‘anti-burst’ locks are now in common use.
On modern vehicles safety belts must be provided for the driver and all passengers. Seat belts act as a primary restraint and must be securely anchored to suitable strengthened parts of the body. They reduce the risk of front and rear seat occupants being thrown through the
Figure 1.44 Crumple zones
windscreen, or thrust against the body fittings, when the car slows down faster than its occupants.
Many of today’s vehicles are fitted with supplementary restraint systems (SRSs) which include seat belt pre-tensioners and airbags. If the vehicle system sensors detect a sudden serious impact, the airbag (or airbags) inflate to prevent serious injury to passengers within the vehicle, and the seat belt pre-tensioners help to restrain the passengers within their seats.
Internal body trim, fittings and controls must all conform to safety standards, and changes during recent years in the design of parts such as steering wheels, control knobs, and even seat construction, have materially reduced the risk of human injury.
Extra protection for the occupants during roll-over of a drop-head model car is given by incorporating in the body structure a strong tubular bar to take the place of the metal roof panel. This tube, set across the car, can be either fixed rigidly or made to move to its protection position automatically when roll-over of the car is sensed.
Figure 1.45 Impact testing
Body shape
Body shape is dictated by a number of factors. The shape must appeal to the buyer, and should have a good performance in relation to the ease with which it passes through the air.
The aerodynamic shape of cars is expressed as a drag coefficient. The lower the value of this coefficient, the easier the car slips through the air, as a result of which fuel economy is improved. Today fuel cost and environmental impact are important, so greater attention is paid to the air resistance of a car.
Manufacturers now use wind tunnels during the design process to ensure that the optimum aerodynamic shape is achieved.
Air resistance
Under normal conditions the power required to drive a vehicle through the air consumes most of the engine energy, so ways of reducing this energy drain need to be sought.
Air resistance is given by the expression:
Air resistance = Drag coefficient × Area × Velocity2 This expression shows that the air resistance increases with the square of the velocity of the vehicle relative to the air, so the resistance becomes very great when the vehicle speed is high (Figure 1.46). Compared with the resistance at 50 km/h, the resistances at speeds of 100 km/h and 150 km/h are four and nine times as great respectively.
Wind tunnel tests enable the air resistance of a vehicle to be measured. Since the cross-sectional area of the vehicle and its velocity relative to the air are known, it is possible to calculate the aerodynamic drag constant (Cd). When a vehicle is designed with the aid of computer software, this calculation is used to obtain the optimum aerodynamics before a dummy vehicle is placed in a wind tunnel.
A low Cdis obtained when the body is streamlined to enable it to pass through the air with the minimum
disturbance. Since much of the resistance is caused by the low-pressure region at the rear of the vehicle, the aim is to return the air to this region with the minimum of turbulence after it has flowed over the body.
Resistance is directly proportional to the cross-sectional area, so a low and sleek sports-type car performs well in this respect.
Various refinements are made to the body to reduce air drag. These include the recessing of protruding items such as door handles, and the shaping of the body below the front bumper to form an air dam (Figure 1.47a). Although it can be seen that protruding items are shaped to reduce air resistance, much of the under body is also streamlined to ensure a smooth airflow.
Many vehicles now have plastic shields fitted to their undersides, particularly around the engine and transmission areas. Many of these small components may seem insignificant but reduce the Cdvalue and can dramatically improve the stability of the vehicle at high speeds.
Airflow control devices are sometimes fitted by car manufacturers (or owners) to the rear of the vehicle.
According to their shape and position, these devices Figure 1.46 Force required to overcome air resistance
Figure 1.47 Air dam and spoiler
either smooth out the air flow to reduce the disturbance, or act as a spoiler to deflect the air upwards so as to increase the adhesive force acting on the rear wheels (Figure 1.47b). Although these arrangements are beneficial on racing cars and high speed sports cars, on many production cars they may be regarded just as ‘image creation’ embellishments.
Older generation vehicles were often fitted with bumpers made from metal that was chromium-plated to provide an attractive finish to the component. Although referred to as a bumper, they offered limited protection if the vehicle was involved in an accident. Bumpers fitted to the front and rear of the vehicle of a modern car are
made from a plastic material, which is easily moulded into a shape that provides a low resistance to the air flow, and also enhances the appearance of the vehicle.
These bumpers often form the majority of the front and rear of the vehicle and therefore little (or in some cases no) further body panels are required to form the front or rear of the vehicle. Such bumpers are also referred to as ‘pedestrian friendly’, since if a car collides with a pedestrian at a very low speed, the injuries sustained by the pedestrian will hopefully be very minor.
Paintwork
When bare metal is exposed to the atmosphere, it soon corrodes. In the case of steel, the surface becomes pitted and the strength of structural parts rapidly decreases as the rust ‘eats’ into the sound metal.
One way of reducing corrosion is to coat the surfaces with paint; this acts as a barrier between the metal and oxygen in the atmosphere.
Figure 1.48 Moulded bumpers
Figure 1.49 Underbody aerodynamics
Figure 1.50 Indication of air flow over a car in a wind tunnel