Preventing heavy vehicle rollovers

Each year, across Western Australia, Main Roads sees a significant number of heavy vehicle rollovers. Apart from the significant risk to human and animal life, rollovers can cause damage to goods, vehicles and road infrastructure. Although there is no single solution to prevent rollovers, an understanding of what causes them can help to significantly reduce the risk.

One of the main challenges for heavy vehicle drivers is coping with loads of various sizes and weight, which affect the vehicle’s centre of gravity. In some cases, such as when carrying containers, drivers can’t see what’s in the container, or how it’s been loaded.

To reduce the risk, a driver needs to understand: • vehicle dynamics and what can cause a rollover • use of correct driving techniques to manage the forces at work • vehicle speeds appropriate for driving and road conditions; and • effects of speed and road conditions on different loads.

Speed

The faster the vehicle is going and the tighter the turn, the more likely it is that a rollover could occur, often before the driver has even realised the vehicle has commenced rolling over and had an opportunity to brake.

This effect of speed is highlighted in figure 1 below.

The arrow in the left-most illustration represents the overturning force acting on a truck in a 30 kilometre per hour (km/h) corner. If the same truck is driven through the same corner at 60 km/h, the overturning forces will be four times higher, represented by the arrow in the middle illustration. If the truck is driven through the corner at 90 km/h, the overturning forces will be nine times higher than at 30

Figure 1

km/h, as shown in the third illustration.

Speed therefore, has a dramatic impact on vehicle stability and controllability, and these impacts are amplified through trailers in a multi-trailer combination.

Centre of Gravity

Simply put, the higher the centre of gravity, the more unstable the truck will be. The lower the centre of gravity, the m ore stable the truck will be. The payload centre of gravity height has a direct impact on the vehicle’s Static Rollover Threshold.

Static Rollover Threshold (SRT)

SRT is the side force required to cause a vehicle to roll over. It is the basic measure of rollover stability, expressed in units of ‘g’. High values of SRT (i.e. greater than or equal to 0.35g) imply better resistance to rollover (i.e. a greater side force is required to cause a rollover).

Rollover stability is very sensitive to the centre of gravity of the vehicle and it increases by decreasing centre of gravity

4 x 9 x

height. In a multi-combination vehicle, rollover stability could be improved by reducing the payload centre of gravity height and mass of the rearmost trailer.

Bulk liquid transport tankers with partially filled compartments allow the liquid to move from side to side, producing a “slosh” load condition. Slosh is a potential safety concern, as it negatively impacts on the vehicle’s centre of gravity, particularly when cornering. In this situation, properly designed longitudinal baffles in tankers can reduce the risk of rollover crash.

It is a requirement under the WA Heavy Vehicle Accreditation Scheme for operators to have processes in place to minimise rollover risk. In this regard, transport operators are encouraged to use the SRT Calculator available under Assessment Tools at www.mainroads.wa.gov.au to assess the rollover risk of their particular vehicle configuration and load. The SRT calculator will enable operators to either adjust the load accordingly or ensure there are suitable controls in place to mitigate any risks.

Braking

The ability of a vehicle to remain stable, controllable and keep within its lane during heavy braking is a key safety consideration in all road transport tasks. Rollover or loss of control (such as when a jack-knife occurs) presents high safety risks to the driver and other road users.

If a load is not centred across its width, the stability will be reduced when cornering. Additionally, if the load is not properly distributed along the length of the trailer (i.e. if there is more weight on some wheels than others), then each wheel will not brake with the same force, which can cause wheels to lock up.

With most rollovers involving multitrailer vehicle combinations, the rear trailer has commenced rolling over before the driver realises and has an opportunity to apply the brakes. Electronic Braking Systems (EBS) with Rollover Stability System (RSS) can prevent this from occurring as the systems will sense the vehicle is at risk of rolling over and automatically control the braking system to stabilise the vehicle. If you would like a copy of Main Roads’ free Heavy Vehicle Rollover presentation DVD, contact Heavy Vehicle Services on 138 486.