11 Integral body construction

The integral or unitary body structure of a car can be considered to be made in the form of three box compartments; the middle and largest compartment stretching between the front and rear road wheel axles provides the passenger space, the extended front box built over and ahead of the front road wheels enclosing the engine and transmission units and the rear box behind the back axle providing boot space for luggage.

These box compartments are constructed in the form of a framework of ties (tensile) and struts (compressive), pieces (Fig. l.l(a & b)) made from rolled sheet steel pressed into various shapes such as rectangular, triangular, trapezium, top-hat or a combination of these to form closed box thin gauge sections. These sections are designed to resist direct tensile and compressive or bending and torsional loads, depending upon the positioning of the members within the structure.

Structure Bending Bulkhead

(b) Transverse body loading

Fig. 1.1 (a and b) Structural tensile and compressive loading of car body

(b) Transverse body loading

Fig. 1.1 (a and b) Structural tensile and compressive loading of car body

1.1.1 Description and function of body components (Fig. 1.2)

The major individual components comprising the body shell will now be described separately under the following subheadings:

1 Window and door pillars

2 Windscreen and rear window rails

3 Cantrails

4 Roof structure

5 Upper quarter panel or window

6 Floor seat and boot pans

7 Central tunnel

8 Sills

9 Bulkhead

10 Scuttle

11 Front longitudinals

12 Front valance

13 Rear valance

14 Toe board

15 Heel board

Windowscreen and door pillars are identified by a letter coding; the front windscreen to door pillars are referred to as A post, the centre side door pillars as BC post and the rear door to quarter panel as D post. These are illustrated in Fig. 1.2.

These pillars form the part of the body structure which supports the roof. The short form A pillar and rear D pillar enclose the windscreen and quarter windows and provide the glazing side channels, whilst the centre BC pillar extends the full height of the passenger compartment from roof to floor and supports the rear side door hinges. The front and rear pillars act as struts (compressive members) which transfer a proportion of the bending effect, due to underbody sag of the wheelbase, to each end of the cantrails which thereby become reactive struts, opposing horizontal bending of the passenger compartment at floor level. The central BC pillar however acts as ties (tensile members), transferring some degree of support from the mid-span of the cantrails to the floor structure.

Windscreen and rear window rails (Fig. 1.2(2)) These box-section rails span the front window pillars and rear pillars or quarter panels depending upon design, so that they contribute to the resistance opposing transverse sag between the wheel track by acting as compressive members. The other function is to support the front and rear ends of the roof panel. The undersides of the rails also include the glazing channels.

Cantrails (Fig. 1.2(4)) Cantrails are the horizontal members which interconnect the top ends of the vertical A and BC or BC and D door pillars (posts). These rails form the side members which make up the rectangular roof framework and as such are subjected to compressive loads. Therefore, they are formed in various box-sections which offer the greatest compressive resistance with the minimum of weight and blend in with the roofing. A drip rail (Fig. 1.2(4)) is positioned in between the overlapping roof panel and the cantrails, the joins being secured by spot welds.

Roof structure (Fig. 1.2) The roof is constructed basically from four channel sections which form the outer rim of the slightly dished roof panel. The rectangular outer roof frame acts as the com-pressive load bearing members. Torsional rigidity to resist twist is maximized by welding the four corners of the channel-sections together. The slight curvature of the roof panel stiffens it, thus preventing winkling and the collapse of the unsupported centre region of the roof panel. With large cars, additional cross-rail members may be used to provide more roof support and to prevent the roof crushing in should the car roll over.

Upper quarter panel or window (Fig. 1.2(6)) This is the vertical side panel or window which occupies the space between the rear side door and the rear window. Originally the quarter panel formed an important part of the roof support, but improved pillar design and the desire to maximize visibility has either replaced them with quarter windows or reduced their width, and in some car models they have been completely eliminated.

Floor seat and boot pans (Fig. 1.3) These constitute the pressed rolled steel sheeting shape to enclose the bottom of both the passenger and luggage compartments. The horizontal spread-out pressing between the bulkhead and the heel board is called the floor pan, whilst the raised platform over the rear suspension and wheel arches is known as the seat or arch pan. This in turn joins onto a lower steel pressing which supports luggage and is referred to as the boot pan.

To increase the local stiffness of these platform panels or pans and their resistance to transmitted vibrations such as drumming and droning, many narrow channels are swaged (pressed) into the steel sheet, because a sectional end-view would show a

Fig. 1.2 Load bearing body box-section members semi-corrugated profile (or ribs). These channels provide rows of shallow walls which are both bent and stretched perpendicular to the original flat sheet. In turn they are spaced and held together by the semicircular drawn out channel bottoms. Provided these swages are designed to lay the correct way and are not too long, and the metal is not excessively stretched, they will raise the rigidity

Bulk head

Bulk head

{a) Small central tunnst, sills, front valance, rear wtieel arches and all round spring towers
Www Civil Technology Section Pics Com
4b) Lar^e central tunnel, sills, front valance and spring tower with rear ¡x»* section reinforced seat pan
Www Civil Technology Section Pics Com

(c| Longitudinal front members, sills, rear seet pen with bulkhead and spring towers Fig. 1.3 (a-c) Platform chassis of these panels so that they are equivalent to a sheet which may be several times thicker.

Central tunnel (Fig. l.3(a and b)) This is the curved or rectangular hump positioned longitudinally along the middle of the floor pan. Originally it was a necessary evil to provide transmission space for the gearbox and propeller shaft for rear wheel drive, front-mounted engine cars, but since the chassis has been replaced by the integral box-section shell, it has been retained with front wheel drive, front-mounted engines as it contributes considerably to the bending rigidity of the floor structure. Its secondary function is now to house the exhaust pipe system and the hand brake cable assembly.

Sills (Figs l.2(9) and l.3(a, b and c)) These members form the lower horizontal sides of the car body which spans between the front and rear road-wheel wings or arches. To prevent body sag between the wheelbase of the car and lateral bending of the structure, the outer edges of the floor pan are given support by the side sills. These sills are made in the form of either single or double box-sections (Fig. l .2(9)). To resist the heavier vertical bending loads they are of relatively deep section.

Open-top cars, such as convertibles, which do not receive structural support from the roof members, usually have extra deep sills to compensate for the increased burden imposed on the underframe.

Bulkhead (Figs l.2(l) and l.3(a and b)) This is the upright partition separating the passenger and engine compartments. Its upper half may form part of the dash panel which was originally used to display the driver's instruments. Some body manufacturers refer to the whole partition between engine and passenger compartments as the dash panel. If there is a double partition, the panel next to the engine is generally known as the bulkhead and that on the passenger side the dash board or panel. The scuttle and valance on each side are usually joined onto the box-section of the bulkhead. This braces the vertical structure to withstand torsional distortion and to provide platform bending resistance support. Sometimes a bulkhead is constructed between the rear wheel arches or towers to reinforce the seat pan over the rear axle (Fig. l.3(c)).

Scuttle (Fig. l.3(a and b)) This can be considered as the panel formed under the front wings which spans between the rear end of the valance, where it meets the bulkhead, and the door pillar and wing. The lower edge of the scuttle will merge with the floor pan so that in some cases it may form part of the toe board on the passenger compartment side. Usually these panels form inclined sides to the bulkhead, and with the horizontal ledge which spans the full width of the bulkhead, brace the bulkhead wall so that it offers increased rigidity to the structure. The combined bulkhead dash panel and scuttle will thereby have both upright and torsional rigidity.

Front longitudinals (Figs l.2(l0) and l.3(a and b)) These members are usually upswept box-section members, extending parallel and forward from the bulkhead at floor level. Their purpose is to withstand the engine mount reaction and to support the front suspension or subframe. A common feature of these members is their ability to support vertical loads in conjunction with the valances. However, in the event of a head-on collision, they are designed to collapse and crumble within the engine compartment so that the passenger shell is safeguarded and is not pushed rearwards by any great extent.

Front valance (Figs l.2 and l.3(a and b)) These panels project upwards from the front longitudinal members and at the rear join onto the wall of the bulkhead. The purpose of these panels is to transfer the upward reaction of the longitudinal members which support the front suspension to the bulkhead. Simultaneously, the longitudinals are prevented from bending sideways because the valance panels are shaped to slope up and outwards towards the top. The panelling is usually bent over near the edges to form a horizontal flanged upper, thus presenting considerable lateral resistance. Furthermore, the valances are sometimes stepped and wrapped around towards the rear where they meet and are joined to the bulkhead so that additional lengthwise and transverse stiffness is obtained.

If coil spring suspension is incorporated, the valance forms part of a semi-circular tower which houses and provides the load reaction of the spring so that the merging of these shapes compounds the rigidity for both horizontal lengthwise and lateral bending of the forward engine and transmission compartment body structure. Where necessary, double layers of sheet are used in parts of the spring housing and at the rear of the valance where they are attached to the bulkhead to relieve some of the concentrated loads.

Rear valance (Fig. 1.2(7)) This is generally considered as part of the box-section, forming the front half of the rear wheel arch frame and the panel immediately behind which merges with the heel board and seatpan panels. These side inner-side panels position the edges of the seat pan to its designed side profile and thus stiffen the underfloor structure above the rear axle and suspension. When rear independent coil spring suspension is adopted, the valance or wheel arch extends upwards to form a spring tower housing and, because it forms a semi-vertical structure, greatly contributes to the stiffness of the underbody shell between the floor and boot pans.

Toe board The toe board is considered to form the lower regions of the scuttle and dash panel near where they merge with the floor pan. It is this panelling on the passenger compartment side where occupants can place their feet when the car is rapidly retarded.

Heel board (Fig. 1.3(b and c)) The heel board is the upright, but normally shallow, panel spanning beneath and across the front of the rear seats. Its purpose is to provide leg height for the passengers and to form a raised step for the seat pan so that the rear axle has sufficient relative movement clearance.

1.1.2 Platform chassis (Fig. 1.3(a c)) Most modern car bodies are designed to obtain their rigidity mainly from the platform chassis and to rely less on the upper framework of window and door pillars, quarter panels, windscreen rails and contrails which are becoming progressively slender as the desire for better visibility is encouraged.

The majority of the lengthwise (wheelbase) bending stiffness to resist sagging is derived from both the central tunnel and the side sill box-sections (Fig. 1.3(a and b)). If further strengthening is necessary, longitudinal box-section members may be positioned parallel to, but slightly inwards from, the sills (Fig. 1.3(c)). These lengthwise members may span only part of the wheelbase, or the full length, which is greatly influenced by the design of road wheel suspension chosen for the car, the depth of both central tunnel and side sills, which are built into the platform, and if there are subframes attached fore and aft of the wheelbase (Fig. 1.6 (a and b)).

Torsional rigidity of the platform is usually derived at the front by the bulkhead, dash pan and scuttle (Fig. 1.3(a and b)) at the rear by the heel board, seat pan, wheel arches (Fig. 1.3(a, b and c)), and if independent rear suspension is adopted, by the coil spring towers (Fig. 1.3(a and c)). Between the wheelbase, the floor pan is normally provided with box-section cross-members to stiffen and prevent the platform sagging where the passenger seats are positioned.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


  • jouni
    What is a section member in an integral body?
    3 years ago
    What is integral construction of a vehicle?
    2 years ago
  • robert villalobos
    What the function of roof structure of a car body?
    2 years ago
  • Geneva
    What is integral body construction?
    2 years ago
  • ross
    What is horizontal member in car?
    2 years ago
  • Wilma
    What is integral body type vehicle?
    2 years ago
  • bobbi
    What is intergral in a car?
    2 years ago
    What is intergral in motor vehicle body construction?
    2 years ago

Post a comment