SRT Calculator User Guide

Understanding SRT Results

The Static Rollover Threshold (SRT) is the main output of this calculator. It represents the level of lateral acceleration (side force) at which the vehicle is predicted to roll over during steady cornering, expressed in multiples of g (gravitational acceleration).

SRT Value

The SRT value is shown as a decimal, for example 0.382. This means the vehicle would be predicted to roll over at a lateral acceleration of 0.382 × gravity (≈ 3.74 m/s²). In practice this relates directly to safe cornering speeds under ideal conditions.

A higher SRT value indicates better rollover stability. For example, an SRT of 0.40 means the vehicle can sustain a lateral acceleration of 0.40g (≈ 3.92 m/s²) before rollover is predicted.

• Vehicles carrying general freight must typically meet a minimum of 0.35 g.
• Vehicles carrying dangerous goods must meet a higher threshold of 0.40 g.

If your result is below these thresholds, the vehicle may not be permitted to operate on all networks without modifications or load restrictions.

Tilt Angle (φ)

The tilt angle shown is the equivalent static tilt test angle at which rollover would occur. This is a more intuitive measure for operators: for example, a tilt angle of 20.9° means the vehicle could be tilted sideways in a static position by ~21° before reaching its rollover limit.

Safe Speed Curves

The calculator also provides a set of safe speeds for typical curve radii (e.g. 25 m roundabout, 60 m rural curve, 100 m regional connector). These speeds are derived from the SRT value and represent the maximum recommended speed before rollover risk significantly increases. It is important to note that these speeds are theoretical static rollover limits based on the calculated SRT and do not account for other critical factors such as road conditions, vehicle dynamics, or driver behavior. In particular, they do not consider the effects of sudden steering inputs, load shifts, or uneven road surfaces, which can significantly affect rollover risk. Therefore, while these speeds provide a useful reference, they should be applied with caution and always within the context of safe driving practices and conditions.

Each speed result is labelled against a representative road type or curve radius.

Graphs

The graphs tab provide further insight into how SRT changes with load height, payload and centre of gravity height:

• CoG Height vs Payload to meet SRT target: The line indicates the maximum allowable payload and CoG height combinations that meet the target SRT (green shading below the line is also allowable combinations). The CoG height is measured from ground to the payload centre of gravity (centre of mass) and is dependent on the deck and load heights as well as the payload distribution - mixed freight vs uniform density loads for example. As the payload increases (more mass), the maximum allowable CoG height to meet the target SRT decreases. Conversely, a lower CoG height allows for a higher payload while still meeting the target SRT. If the target SRT is not met then either the payload mass must be reduced or the CoG height lowered (for example by redistributing the load or using a lower deck height).
• Load Height vs Payload to meet SRT target: The line indicates the maximum allowable payload and load height combinations that meet the target SRT (green shading below the line is also allowable combinations). The load height is measured from ground to the overall top of the cargo (approximated for step and slope decks). As the payload mass increases, the maximum allowable load height to meet the target SRT decreases. Conversely, a lower load height allows for a higher payload while still meeting the target SRT. If the target SRT is not met then either the payload mass must be reduced or the load height lowered (for example by redistributing the load or using a lower deck height).
• SRT vs Payload: This graph shows the effect of increasing payload on SRT. Generally, adding payload increases rollover risk and SRT. The current vehicle configuration and SRT is marked with a dot and the green shading indicates the target SRT is met. Practically, this means that as you load more weight onto the vehicle, the SRT value tends to decrease, indicating a higher risk of rollover.
• SRT vs CoG Height: This graph shows how the vehicle's payload centre of gravity height affects SRT for a fixed payload (mass). A higher CoG reduces rollover stability and the SRT result. The current vehicle configuration and SRT is marked with a dot and the green shading indicates the target SRT is met. Practically, this means that as the centre of gravity of the load is raised (for example, by stacking cargo higher), the SRT value tends to decrease, indicating a higher risk of rollover.

These graphs are useful for testing “what-if” scenarios, such as loading configurations for this vehicle, to see whether the target SRT is met.

Warnings and Assumptions

The calculator uses simplified assumptions about suspension, tyres, and payload distribution. It cannot account for dynamic effects such as sudden steering, uneven surfaces, or load shift. Further, the results are sensitive to input accuracy - incorrect axle weights, tyre sizes, or suspension types and load details can significantly affect results. Results should be used as a guide and verified with engineering data where available.

Disclaimer

The Static Rollover Threshold (SRT) Calculator is provided as an engineering guide only. It is designed to assist operators, engineers, and regulators in understanding vehicle rollover stability but is not a substitute for professional assessment or regulatory certification.

No registration is required to use this tool and no personal information is collected. The tool and its outputs are supplied “as is”, without warranty of any kind, either express or implied. Results may contain errors or omissions and should always be interpreted using appropriate engineering judgment.

The authors and maintainers accept no liability for any use of the calculator or reliance placed on its results. Users remain responsible for ensuring that vehicles meet all applicable design, certification, and compliance requirements. Actual rollover risk is influenced by many factors, including vehicle dynamics, driver behaviour, load securement, and road conditions, which are not captured by this tool.