Page 547 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 6. 2 Standard sedan (saloon) – baseline load paths

For example, sometimes the rear seat bulkhead is
omitted to allow a ‘split rear seat’ feature (see Fig. 16.2-18).
In this case there is no ability to react edge forces Q 1
and Q 6 around the rear seat back, so that the shear forces
Q 1 at the bottom of the rear window and on the rear of the
floor must be zero. Hence (for force equilibrium) the edge
forces Q 1 at the other ends of these surfaces become zero. Ring beam
This will ‘propagate’ all the way round the compartment in opening
so that all forces Q 1 (roof, windshield, front bulkhead,
floor) must also be zero. The complementary forces on
all of these structural surfaces will also become zero.
The only compartment edge forces remaining will be
those which can be balanced by ‘external’ torques. Thus Fig. 16.2-19 Ring frame to remedy faux sedan problem.
in the front bulkhead the torque T is balanced by the
couple Q 2 B from forces Q 2 at the A-pillar. The couple
Q 7 B from edge forces Q 7 on the floor are balanced by the This is possible, in some cases, by using a ring frame in
couples P FT S F and P RT S R . place of the missing panel (Fig. 16.2-19). This restores
It is assumed here that a load path is available to shear integrity to the surface in question, whilst main-
transmit the reaction torque T (from the rear suspen- taining a substantial opening. For such a ring frame, care
sion) to the sideframe as edge forces Q R on the C-pillars must be taken to ensure an effective path for local
via the outer rear fender and/or via floor members. (The bending moments all round the frame especially at the
rear seat bulkhead is no longer available for this.) corners. For example, in the rear seat bulkhead case,
parcel shelf, side-wall beams and floor cross-member
The moments on the sideframe due to forces Q x1 Q x2
Q 2 Q R and Q 7 (see Fig. 16.2-18) are all in the same di- (which could be under the floor) must all have high
rection. These moments can only be balanced by mo- stiffness for bending about axes normal to the plane of
ments from forces R NF and R NR . These cause reactions the frame, and they must all be well connected for
R NF , R NR normal to the plane of the floor. This causes the bending at the corners (e.g. gusseted joints). C-pillars
floor to twist out-of-plane. The SSS assumptions are not and parcel shelves are often not stiff in the required
satisfied in this case. Similar twisting of other surfaces direction, and so require special design attention.
(e.g. front bulkhead/parcel shelf) will occur. If a larger opening is required (e.g. hatchback, station
In practice, such a ‘faux sedan’ structure is much more wagon, opening rear screen with split seat) then a ring
flexible (and less weight efficient) in torsion than a ‘closed frame, running the full height of the sideframe (through
box’ sedan. The sideframes tend to act as ‘levers’ to twist the C-pillars, across the roof and across the floor), is
the cowl/dash (engine bulkhead/parcel shelf) assembly. a possibility. This works better if the ring frame is as
High local stresses and large strains are experienced by planar as possible, and if the corner joints are good in
the remaining loaded members (in the floor etc.) leading bending. Even so, the result will not be as weight efficient
to: (a) poor fatigue life and (b) damage to the paint as a true shear panel.
system with resulting early corrosion. An alternative to the ring beam is a triangulated bay in
The same ‘faux sedan’ problem will be encountered if the opening (see Fig. 16.2-13).
any one (or more) of the compartment structural sur- 16.2.3.7.2.2 Provision of ‘closed torque box’ in
faces are missing or of reduced structural integrity (e.g.
roof, front bulkhead or floor missing, or poorly connected part of structure
to adjacent components). Similarly loss of integrity of There are several areas in the integral car body that are
sideframe ‘ring beam’ members: e.g. poor quality joints, ‘box like’. Some of these are:
‘panoramic’ A-pillars, pillarless sedans (to some extent), (a) The cowl/footwell assembly (the region enclosed by
corroded rockers, etc. – especially if the weakening is at the parcel shelf, the engine bulkhead, the lower
the high bending moment corners of the ring frames. A-pillars and the floor).
(b) The engine compartment and/or the rear luggage
16.2.3.7.2 Remedies for the faux sedan compartment.
16.2.3.7.2.1 Replacement of missing shear panel (c) The region under the rear seat, where there is often
with ring frame a step in the floor.
The ideal remedy for the faux sedan is to modify it so as If any of these can be converted into a ‘closed box’
to restore the ‘closed box’ type structure with its weight structure by the addition of shear panels or ring frames,
efficient shear panel load paths. and can be well connected to the sideframes, then this

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