Static roll thresholds
The information in this factsheet describes the legal requirements in the Land Transport Rule: Vehicle Dimensions and Mass 2002 that apply to heavy vehicle stability, and explains what a 'static roll threshold (SRT)' is, and which vehicles must be certified for SRT.
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.
In New Zealand, prior to 2002, one in 11 log trucks were rolling over each year. The Rule introduced a minimum stability requirement for heavy vehicles and heavy vehicle combinations. This has been in place for several years, and rollover among log trucks is now much less common.
What is a static roll threshold?
The static roll threshold (SRT) of a vehicle is a measure of the potential for that vehicle to roll 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. That means the lower your SRT the less stable your vehicle is.
To improve their stability, heavy vehicles with a gross vehicle mass (GVM) greater than certain specified limits must meet or exceed 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.
Which vehicles must meet SRT?
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 0.35g ('g' is the acceleration due to gravity). Vehicles operating with a High-Productivity Motor Vehicle (HPMV) permit also need to meet SRT.
What are the 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 metres 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 0.35g.
- If you operate a class NC truck, you are legally required to load and operate it so it has an SRT of at least 0.35g. (Use the SRT Calculator to check this.)
Why do only TD trailers need an SRT certificate?
Research into heavy vehicle rollover has shown that class TD trailers are the heavy vehicles at most risk of rollover. These are the heaviest trailers, with GVM over 10 tonnes. It is necessary to have these assessed and certified so the appropriate mass and height limits are available to operators and police.
How do I get an SRT Compliance Certificate
To get an SRT Compliance Certificate, you need to have your vehicle assessed by an SRT certifier or SRT vehicle inspector approved by the NZ Transport Agency (NZTA).
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.
How do I operate a 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.
Can I check my own SRT?
Yes, but only for your own information. You can use the you can use the SRT Calculator on the NZTA website.
What vehicles are exempt from SRT requirements
Some vehicles are considered to have a low risk of rollover and are therefore exempt from SRT requirements. High-productivity motor vehicles are not exempt from SRT requirements; please refer to Factsheet 13g, High productivity motor vehicles.
Read a list of vehicles that are exempt from SRT requirements.
Where you can find out more
- Factsheets 13, 13a, 13b, 13c, 13d, 13f, and 13g.
- Factsheet 53, Overdimension vehicles and loads.
- Guide to safe loading and towing for light vehicles.
- Land Transport Rule 41001: Vehicle Dimensions and Mass 2002 and its subsequent amendments.
Contact details
- Email us: info@nzta.govt.nz.
- Call our contact centre 0800 699 000.
- Write to us: NZ Transport Agency, Private Bag 6995, Wellington 6141.
