Data Input required for SRT calculation.

Front: Refers to the front axle set.

Rear: Refers to the rear axle set.

(Note: Semi trailers and simple trailers have no front axle set)

Tyre Data:

Axle: Axle number (from front) for which data is required.
Tyre Size: Size of tyres fitted on the axle. Select from drop down list.
Tyre Configuration: Single, Single Large, Single Mega or Dual. Select from drop down list.

Load Categories:

Mixed Freight: assumes that the payload consists of a mixture of items which have been sensibly loaded so that 70% of the load mass is in the bottom half of the load space and 30% of the mass is in the top half. This would include container trucks, curtainsiders and van body vehicles.
Uniform Density: applies to vehicles which can be loaded from the load bed to the load height with a uniform density load. It also assumes that payload Cg is midway between the load bed and the load height. This category includes logs , stock, liquid tankers, tipper bodies, chip liners etc. Car carriers can be certified using this load type as this will underestimate their SRT and be conservative.
Other: is for loads not covered by the previous two categories. In this case the user needs to calculate the height of the payload Cg. This category includes large single item loads such as heavy machinery and stockfeed hopper vehicles with triangular crossection tanks.

Axle Load Data:

Gross Mass: Gross mass carried by the axle group in kgs.
Tare Mass: Unladen vehicle mass carried by the axle group in kgs.
Payload Mass: Payload mass carried by the axle group in kgs.


  1. Only gross mass and tare mass are entered. The payload mass is calculated automatically from these two values.
  2. The gross mass and tare mass values are the values for the axle group. For semi-trailers both these values will differ from the total values for the vehicle, while for pole trailers, the gross mass will differ. For the first semi-trailer in a B-train combination, the values should include the imposed weights from the second trailer.

Vehicle Geometry:

Load Bed Height from the ground: Height of the load bed from the ground in metres.
Load Height from the ground: Max height of the load from the ground in metres.
Payload Cg Height from the ground: Height of the payload centre of gravity from the ground in metres.


  1. Where a vehicle does not have a constant load bed height the value should be the weighted average of the load bed heights using the the proportion of payload mass carried by each section as the weighting function.
  2. Load Bed Height, Load Height and Payload Cg Height are all determined in the laden condition. That is, the compression of the suspension and the tyres due to the load is included either by using manufacturer supplied data for spring deflections or by direct measurement. Note, however, that an unladen vehicle (including the vehicle body) must also stay within the maximum legal height limit of 4.25m.

Suspension Data:

Suspension type: Select from Generic Air - low roll stiffness type, Generic Air - high roll stiffness type, Generic Steel or User Defined. Fill in remaining fields using manufacturer-supplied data if user defined option selected.
Suspension Brand: Brand and Model ID of suspension used.
Total Roll Stiffness / axle: Roll stiffness per axle in Newton metres per radian.
Spring Stiffness / spring: Spring stiffness per spring in Newtons per metre.
Suspension Track Width: Spacing between spring centres in metres.
Axle lash: For a steel spring as the load changes from compression to tension there is a small range of axle displacement for which there is minimal resistance. This is called the lash (mm). With air suspensions there is minimal resistance in tension until the dampers or retaining straps prevent further movement. Thus the lash is large, typically 300mm or more.
Roll Center Height: Measured from axle centre in metres. Positive is above the axle and negative below.


  1. There are two types of Generic Air suspension, the low roll stiffness type and the high roll stiffness type. The high roll stiffness type occurs where the axle acts as an anti-roll bar. For this to occur the suspension needs to have a beam axle and the trailing arms need to be rigidly clamped or welded to the beam axle. If there is no beam axle (independent suspensions) or the trailing arms are bushed onto the axle rather than rigidly attached, the suspension is a low roll stiffness type. If in doubt assume low roll stiffness type.
  2. As the axle lash and suspension track width can be measured relatively simply, the user can enter actual measured values in place of the defaults for generic suspensions.

If the Generic Suspension Options are Selected the Following Values are Used:

Suspension Name and Model Number
Suspension spring stiffness (N/m)
Suspension track width (m)
Total roll stiffness per axle (Nm/radian)
Suspension lash (mm)
Roll centre height from axle (m)
Generic - steer axle 185000 0.8 130000 15 -0.02
Generic steel 1000000 0.97 520000 30 0.2
Generic air - low roll stiffness type 350000 0.97 280000 300 0.2
Generic air - high roll stiffness type 350000 0.97 780000 300 0.2

If the User Defined suspension options are selected the suspension data need to be obtained from the suspension supplier. These may be in units other than those required for input into the SRT Calculator. The table below gives conversion factors for some of the more common alternatives.

Suspension Parameter
Units used by supplier
Units required by SRT Calculator
To convert multiply by
Spring stiffness lb/in N/m 175
Roll stiffness in-lb/degree Nm/radian 6.47
Roll stiffness Nm/degree Nm/radian 57.3
Lash in mm 25.4
Roll centre, track width in m 0.0254

Other common variations use metric alternatives such kN instead of N or mm instead of m. These conversions are straightforward.

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