Chassis Layout and Frames Questions and Answers

Answer: A

A damaged cell can enter thermal runaway, so the pack must be shielded by very strong sill structures and cross members that prevent any intrusion into the cell area. Manufacturers publish detailed emergency response guides for exactly this reason. Deliberately loading the pack would be the worst possible strategy.

Answer: B

A damaged cell can overheat and ignite, and the heat released can propagate to neighbouring cells in a self sustaining chain that is very difficult to extinguish. Pack design incorporates barriers and venting to limit propagation. Reignition hours or days after an incident is a recognised hazard that emergency services must plan for.

Answer: C

The front cross member spans the rails ahead of or beneath the engine and carries the front mountings, transmitting the weight and the reaction torque into the frame. A separate gearbox cross member supports the rear of the power unit. The X member, where fitted, sits between them and is included principally to improve the torsional stiffness of the frame rather than to carry any component. The rear cross member carries the towing attachment and closes the frame at the back.

Answer: D

Diagonal members convert a twisting load into tension and compression along the diagonals, which the frame resists far more effectively than it resists torsion in the rails. The arrangement was common in the nineteen thirties before boxed sections became usual. It also raises the frame in the centre, which conflicts with a low floor.

Answer: A

The side rails sweep upward over the rear axle so that the floor between the axles can be set low while still leaving clearance for the axle to move through its travel. A dropped centre section achieves the same result between the axles. The two features are usually combined in a modern commercial vehicle frame.

Answer: B

Tipping concentrates the whole payload onto the rear hinge and the ram mounting, so the frame must be reinforced at those points and a subframe is usually fitted to spread the load. Body builders follow the chassis manufacturer's guidance closely. Excessive frame stiffness can actually be harmful, since a stiff body on a stiff frame concentrates rather than distributes the loads. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Manufacturers publish detailed guidelines specifying permitted drilling and welding, subframe requirements, weight distribution limits and the routing of services. Ignoring them invalidates the warranty and can compromise the structure. Type approval of the completed vehicle is a separate legal process that the body builder must also satisfy.

Answer: D

The flanges carry the bending stresses and any notch or heat affected zone there creates a stress raiser that can initiate a fatigue crack. Attachments must be made to the web instead, using bolts through existing or carefully positioned holes. This restriction follows directly from the bending stress distribution in a beam.

Answer: A

A frame experiences millions of load cycles over its life, and a notch concentrates the stress sufficiently to initiate a crack that then propagates slowly until failure. The initial static strength may be entirely adequate. Recognising fatigue rather than static overload as the governing failure mode is the reason for the restriction.

Answer: B

Body alignment is checked by measuring between defined reference points, usually on a jig or with a laser measuring system, and comparing the diagonals and datum dimensions with the manufacturer's published figures. Discrepancies of a few millimetres matter for the suspension geometry. Ride height and visual inspection are useful preliminary checks but cannot detect the small displacements that affect handling and tyre wear, and overall length is far too coarse a measure to reveal anything useful.