Factsheet 13e - February 2005

Printable version (one file)

Static roll thresholds

The stability of goods service vehicles can be greatly affected by the way they're loaded. The load needs to be arranged so that its mass and height are appropriate for the design of the vehicle and type of load.

What a static roll threshold is

The static roll threshold (SRT) of a vehicle is a measure of the likelihood of that vehicle rolling over sideways. Vehicles with a low SRT are more likely to roll over than those with a higher SRT, especially when going around sharp bends and in sudden emergency manoeuvres.

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

If a vehicle doesn't meet (or exceed) the required stability limit, its stability can 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.

Research into heavy vehicle rollovers in New Zealand has shown that goods service vehicles with a GVM over 12 tonnes (class NC) and trailers with a GVM over 10 tonnes (class TD) need to have a SRT value of at least 0.35 g ('g' is the acceleration due to gravity).

What the legal requirements are

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 an 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. (Use the SRT Calculator to check this.)

Why only TD trailers need an SRT certificate

Research into rollovers has shown that TD trailers are the heavy vehicles at most risk of rollover. It is necessary to have these assessed and certified so the appropriate mass and height limits are available to operators and Police.

How to get an SRT Compliance Certificate

To get an SRT Compliance Certificate, you need to have your vehicle assessed by a Land Transport New Zealand-approved SRT certifier or SRT vehicle inspector.

Read a list of SRT certifiers.

How the assessment is carried out

The most common way of assessing the vehicle is with a computer programme called the SRT Calculator. The SRT Calculator is a computer simulation that takes into account the vehicle's loading characteristics (deck height) and the vehicle's mechanical componentry that affects the roll stiffness (suspension type, track width, wheels and tyres).

There are two types of assessment that are carried out: Level 1 and Level 2.

• Read about Level 1 assessments.
• Read about Level 2 assessments.

Operating your trailer safely to prevent rollover

If you operate a TD trailer, you must ensure it is loaded within the mass and height limits endorsed on its Certificate of Loading or on a Static Roll Threshold Compliance Certificate that you carry in its towing vehicle. (The latter must be produced to an enforcement officer on demand.)

While NC trucks don't need this certification, there is still a legal requirement that they are loaded and operated safely to avoid rollover.

If you wish to check the stability performance of your truck (for your own information), you can use the SRT Calculator.

Vehicles that are exempt from SRT requirements

Some vehicles are considered to have a low risk of rollover and are therefore exempt from SRT requirements.

Read a list of vehicles that are exempt from SRT requirements.

Where you can find out more

• Read Land Transport Rule 41001: Vehicle Dimensions and Mass 2002.
• Read Land Transport Rule 61001: Road User 2004 (PDF 200 MB).

Contact Land Transport New Zealand:

• Email us: info@nzta.govt.nz.
• Call our Helpdesk: (freephone) 0800 699 000.
• Write to us: Land Transport New Zealand, PO Box 2840, Wellington.