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
||Axle number (from front) for which data is required.
||Size of tyres fitted on the axle. Select from drop down
||Single, Single Large, Single Mega or Dual. Select from drop down list.
||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.
||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.
|| 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 carried by the axle group in kgs.
||Unladen vehicle mass carried by the axle group in kgs.
||Payload mass carried by the axle group in kgs.
- Only gross mass and tare mass are entered. The payload mass is calculated automatically from these two values.
- 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.
| 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
- 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.
- 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.
||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
||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.
||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
|Roll Center Height:
||Measured from axle centre in metres. Positive is above the
axle and negative below.
- 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.
- 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
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
|Generic air - low roll stiffness type
|Generic air - high roll stiffness type
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.
|Units used by supplier
|Units required by SRT Calculator
|To convert multiply by
|Roll centre, track width
Other common variations use metric alternatives such kN
instead of N or mm instead of m. These conversions are straightforward.