Factsheet 13e – February 2005

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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 metres (m) from the ground, you must get an SRT Compliance Certificate (see below) 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. (You can use the SRT Calculator on our website to check this: http://www.landtransport.govt.nz/srt-calculator/)

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 an NZ Transport Agency (NZTA) approved SRT certifier or SRT vehicle inspector.

You can read a list of SRT certifiers on our website: http://www.landtransport.govt.nz/srt-calculator/srt-certifiers.html.

How the assessment is carried out

The most common way of assessing the vehicle is with a computer programme called the SRT Calculator. (This is on our website: http://www.landtransport.govt.nz/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.

Level 1 assessment

The Level 1 assessment is a basic assessment where typical data for flat, sloping or step-deck trailers is used to assess their rollover performance. Level 1 assessment caters for the following types of trailer:

• full trailers (including pole trailers) with stanchions or relatively flat decks
• semi-trailers (flat decks or step-decks, such as in low loaders or B-trains)
• simple trailers with relatively flat decks.

Two typical load scenarios – mixed freight or uniform density – are used:

• A load of mixed freight has the centre of gravity equivalent to 40 percent of the load height (taken from the base of the load).
• A load of uniform density (eg gravel, logs and grain) has the centre of gravity halfway up the load. Construction equipment and machinery may also be assessed this way if its centre of gravity does not exceed half its load height above deck.

The SRT inspector or certifier assesses the vehicle's rollover performance at the maximum legal mass limits and maximum allowable load heights. (The mass limits are assessed at maximum potential axle set limits or a lower limit controlled by trailer:truck mass ratio, if applicable.)

Overweight loads: If you carry overweight loads under an overweight permit, you may elect to increase the mass limit to a vehicle axle index (VAI) of 1.1. Unless the vehicle has a body that restricts the load height, the assessment will be made at 4.25 m.

Trailer meets requirement

If your trailer, at maximum potential load mass and height, meets or exceeds the 0.35 g SRT requirement, an SRT certificate will be issued. The Certificate of Loading will be endorsed with the maximum load and height values.

Trailer doesn't meet requirement

If your trailer doesn't meet the 0.35 g SRT requirement at maximum load and height, a reduced mass/reduced height certificate will be issued. This will specify the reduced gross mass or reduced load height that you can operate the trailer with to meet the 0.35 g SRT. (It may also include a table of allowable gross mass/load height combinations.) The Certificate of Loading will be endorsed accordingly. Alternatively, you may elect to have a more detailed analysis done by undergoing a Level 2 assessment.

Level 2 assessment

The Level 2 assessment requires more detailed data about the vehicle's mechanical characteristics, such as the actual stiffness values of the suspension, and load characteristics.

This should give a more accurate assessment of the trailer's SRT, which could mean that a reduction of mass or load height is less likely.

A Level 2 assessment is also needed when the load's centre of gravity cannot easily be assessed (eg for irregular or complex shaped hoppers or body shapes of irregular cross section). Non-uniform loads, such as construction machinery and equipment, will need Level 2 certification if they have not been certified as uniform density under Level 1. In these cases, a Level 2 SRT Certifying Engineer must assess the vehicle.

Trailer meets requirement

If your trailer, at maximum potential load mass and height, meets or exceeds the 0.35 g SRT requirement, an SRT certificate will be issued. (The Certificate of Loading will be endorsed with the maximum load and height values.)

Trailer doesn't meet requirement

If your trailer doesn't meet the 0.35 g SRT requirement, a reduced mass/reduced height certificate will be issued. This will specify the reduced gross mass or reduced load height that you can operate the trailer with to meet the 0.35 g SRT. (It may also include a table of allowable gross mass/load height combinations.) The Certificate of Loading will be endorsed with these maximum load and height values.

If your trailer doesn't meet the required 0.35 g SRT, you may wish to have it modified so it does. Talk to your engineer about the best options.

You can read a list of workshops that modify trailers on our website.

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 on our website.

Vehicles that are exempt from SRT requirements

The following vehicles are considered to have a low risk of rollover and are therefore exempt from SRT requirements:

• Rigid vehicles fitted with a turntable coupling to tow a semi-trailer, provided they don't have a deck or a body for carrying a load (these are usually called 'tractor units').
• Vehicles (including their loads) that have dimensions greater than standard or have axle weights greater than a vehicle axle index of 1.1, provided such vehicles comply with the relevant legislation and the conditions of any permits they need to operate outside the standard dimensions and axle weights.
• Construction vehicles being used on roads that are designated 'road construction zones' under the Heavy Motor Vehicle Regulations.
• Construction vehicles being used on roads designated 'road works zones' by a road controlling authority.
• Special vehicles that are not usually used on roads but that have been authorised to cross a particular road by a road controlling authority.
• Motor scrapers and similar vehicles that are designed for transporting bulk materials and only move along roads without a load.
• Vehicles with tipping load bodies, but only when they are in the process of discharging their load by tipping up their body and moving at a speed of up to 10 km/h. When the load body isn't tipped, these vehicles need to comply with SRT requirements.
• Vehicles engaged in a vehicle recovery service that are designed to tow or transport disabled heavy vehicles.
• Vintage or veteran vehicles first registered before 1 January 1940.

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).

These are both on our website. You can get printed copies from bookshops that sell legislation.

Contact the NZTA:

• Email us: info@nzta.govt.nz.
• Call our contact centre: 0800 699 000.
• Write to us: NZ Transport Agency, Private Bag 6995, Wellington 6141.