Heavy Vehicles

What factor of safety should a trailer chassis be designed to?

A building beam carries roughly the same load every day. A trailer chassis doesn't. Every pothole, speed bump and hard brake sends a shock through the frame, loads shift, and rough roads flex the chassis millions of times over its life. As Dr Peter Hart, Chairman of the Australian Road Transport Suppliers Association, puts it: "As a guide the Factor of Safety for truck and trailer parts should exceed 3." In other words, the maximum static working stress should stay below a third of the steel's yield strength.

VSB 6 sets the benchmarks. It requires a design factor of safety of 3 for body mounting components, and a minimum of 5 for tippers and off-highway vehicles, both measured against the yield strength of the material. Those higher factors are there mainly because of fatigue. Chassis rarely fail from one big overload. A small crack starts at a weld, a hole or a sharp change in section, then grows a little with every bump until the rail finally lets go.

But a high factor of safety alone won't save a badly detailed chassis. Holes in highly stressed flanges, poor weld quality, and abrupt section changes, like a sudden step instead of a gradual taper between the turntable and axle group, can crack a frame no matter how big the numbers are. Good chassis design means meeting the required factor of safety and putting the strength where the stresses actually are, backed by hand calculations and FEA.