HMV Roll Over

When a vehicle rounds a curve it has a lateral force pushing it to the outside of the curve. This is known as the centrifugal force or lateral 'G'.

This lateral 'G' force increases with sharper corners and increased speed.

HMVs have a higher centre of gravity than a motor car, and therefore in most cases a loaded HMV will roll before their tyres lose traction and slide.

Roll Over diagram - (click image for larger view)

Research has shown that HMV roll overs make up the largest proportion of truck crashes in New Zealand putting a large financial burden on operators, insurance companies & government agency's. They also often claim human life.

New Zealand has implemented regulations surrounding static rollover thresholds or SRT's, to reduce incidents of HMV roll overs.

SRT is the measure of the roll over force on a HMV. In New Zealand this threshold has been set at a minimum of 0.35 g.

This means that the lateral force acting on a loaded HMV and its load must exceed 0.35 g to roll the unit.

To ensure their stability, heavy vehicles with a gross vehicle mass (GVM) greater than certain specified limits need to meet the minimum SRT value.

If a vehicle doesn’t meet (or exceed) the required stability limit, its stability may be improved by operating it with a reduced load mass and/or reduced load height, or the vehicle can be modified so it has sufficient roll stiffness to operate safely at maximum carrying capacity.

The maximum height and weight of the load is displayed on the vehicles loading certificate affixed to the vehicle.

Legal requirements:

If you operate a goods service vehicle, there are some important things you have to do to meet the legal requirements in relation to SRT.

• If you operate a trailer that has a gross vehicle mass of more than 10 tonnes (class TD) and a body height or load height exceeding 2.8 m from the ground, you must get a SRT Compliance Certificate and have the information from this endorsed on your vehicle’s Certificate of Loading.
• You are also legally required to load and operate the trailer so it has an SRT of at least 0.35 g.
• If you operate a class NC truck, you are legally required to load and operate it so it has an SRT of at least 0.35 g.

LTNZ provides an online SRT calculator so that operators are able to verify that their loaded unit complies.

Factors that contribute to roll over

Driver Factors:

• Entering a curve at excessive speed
• Failing to anticipate a curves sharpness
• Accelerating through a curve
• Abrupt steering
• Poor load distribution
• Failing to properly restrain a load

Vehicle Factors:

• High centre of gravity
• Low spring stiffness
• Suspension damage
• Tyre damage
• Load shift
• Poor load distribution

Road Factors:

• Advisory speed sign to high
• Negative camber
• Blind corner
• Poor delineation

To prevent roll over HMV crashes it is recommended that;

HMV drivers travel at a minimum of 10 km/h below the sign posted advisory speed on corners when laden, and should travel at the sign posted advisory speed while unladen.