CHASSIS AND BODY

CHASSIS AND BODY

CHASSIS AND BODY

CLASSIFICATION OF CHASSIS

CLASSIFICATION OF CHASSIS

CLASSIFICATION OF CHASSIS

FRAME

FRAME

TYPES OF FRAME

TYPES OF FRAME: INTEGRAL AND CHASSISSLESS FRAME Integral and chassisless construction The terms integral and chassisless construction are often confused, but the difference is simple. Integral construction is that in which a chassis frame is welded to, or integrated with, the body. It was the first stage in the evolution of the chassisless form of construction, in which no chassis frame can be discerned. The first two quantity-produced vehicles in the latter category were almost certainly the Saab 92 and the Austin A30, the design of which was very fully described in Automobile Engineer, October and December 1952, and March and April 1954.

TYPES OF FRAME: CHASSISLESS FRAME The first two of these four references describe the vehicle itself, while the second pair elaborate on methods, developed by the author, for use in its structural design. The details of chassisless construction are much too numerous, varied and complex to be described here. In principle, however, its advantages stem from the facts that beams formed by the body panels may be something like 50 cm deep, whereas a chassis frame for a car is only about 8 to 13 cm deep, and the area enclosed by a complete body is similarly vastly bigger than that enclosed by the cross-section of a frame side or transverse member. Since the strength and stiffness of a beam are proportional respectively to the square and cube of its depth, while both the torsional stress and stiffness of a box section are proportional to the area enclosed by it, it follows that the strength and stiffness of a body shell are potentially much greater than of a chassis frame.

CHASSISLESS FRAME The Austin A30 was almost certainly the first car of truly chassisless construction to go into quantity production anywhere in the world. Virtually all the panels of 0.914 mm thick steel, the principal exceptions being some 1.299, 1.626 and 2.032 mm brackets carrying the front and rear suspension, the 1.626 mm front apron and two inverted channel sections on each side of and parallel to the engine large load provided that it is stabilised – supported against buckling or other forms of distortion

TYPES OF FRAME BACK BONE TYPE FRAME: Backbone-type frames have also been used. The advantages of the backbone frame include high torsional stiffness at low cost, and light weight. A disadvantage is the length of the outrigger arms needed to carry the body sides. These arms tend to introduce torsional vibration problems because of their bending flexibility.

SUB-FRAMES Sub-frames are employed for one or more of three basic reasons. The first is to isolate the high frequency vibrations of, for example, an engine or a suspension assembly, from the remainder of the structure. In this case, rubber or other resilient mountings are interposed between the sub-frame and main structure. Secondly, a sub-frame can isolate an inherently stiff sub-assembly such as the engine or gearbox from the effects of the flexing of the chassis frame. This is done generally by interposing a three-point mounting system between the sub-frame and main frame, one of the mountings being on the longitudinal axis about which the main frame twists, and the others one on each side. Thirdly, a sub-frame may be used to carry, for instance, the front and rear suspension sub-assemblies, where to utilise the front and rear ends of the body structure for this purpose would increase unacceptably its complexity or cost, or introduce difficulties in either manufacture or servicing, or both. A good example of such sub-frame usage is the BL Mini, the front and rear sub-frame assemblies of which have been used by some kit car manufacturers because of the ease with which the engine and front suspension, on its subframe, can be bolted to the front, and the rear suspension, similarly on its sub-frame, bolted to the back of a different body designed to receive them.

MATERIAL OF FRAME & LOADS ON FRAME Mild steel – easily pressed and welded – used to be the invariable choice for all frames, but modern heavy commercial and even some light vehicles frequently have frames of carbon manganese steel with a yield stress of about 3620 kg/cm2. With the introduction of independent front suspension, chassis frames were called upon to take much higher torsional loading. This was because, whereas the centres of semi-elliptic leaf springs on a beam axle have to be well inboard of the front wheels to leave a clearance for steering them, the effective spring base – distance between spring centres – with independent front suspension is approximately equal to the track. In these circumstances, when a wheel on one side only rises over a bump, the upward thrust it exerts on the frame has a much greater leverage about the longitudinal axis of the car The transverse members most heavily loaded in torsion are of course those that support the independent front suspension. This is partly because of brake-torsion reaction which is applied by the rearward thrust of the road on the tyre contact patch and transmitted through the brake disc or the drum brake backplate to the stub axle, and thence through the suspension links to the frame

LOADS ON FRAME . Additionally, an entirely different torsional loading arises in this transverse member as a result of single-wheel bumps – when the wheel on only one side rises. Such a bump, lifting one side of the front end of the frame, leaves the far side and the rear end down in their original positions, thus causing the side members to tend to twist the front transverse member and, incidentally, all the others. Hence, heavy gusseting is needed between the transverse and side members. Sudden local changes in stiffness at or near the junctions between the transverse and side members have to be avoided, otherwise trouble due to fatigue failures will be experienced.

CROSS-SECTIONS OF FRAME Tubular sections of any shape – round, oval, triangular, square, rectangular, etc – are inherently very rigid torsionally. Such sections therefore began to be used for both longitudinal and transverse members on car frames. A selection of sections that have been used is illustrated in Fig.