4569846498

Modelling and analysis of suspension systems 171
Variables such as BZ-AZ are defined using system variables which measure
components of displacements between markers, such as DZ(1414,1411).
The REQUEST statement REQ/1 demonstrates how to access the information
calculated by the VARIABLE statements.
The alternative method of writing a FORTRAN subroutine is demonstrated
in Table 4.7 by the listing of a user-written REQSUB developed specifically
for a double wishbone suspension. The subroutine would be called
from the main data set as follows:
REQUEST/id,FUNCTIONUSER(1,par1,par2,par3,par4,par5,par6,
par7,par8,par9)
where the parameters par1, par2, …, par9 are the various items of data outlined
in the subroutine.
4.5.8 Calculation of wheel rate
The wheel rate for a suspension system can be thought of as the stiffness of
an ‘equivalent’ spring acting between the wheel centre and the vehicle
body as shown in Figure 4.31. This is the definition most useful for developing
basic full vehicle MBS models where the wheel will be modelled as
rigid with a separate tyre model. This differs slightly from other definitions
sometimes used for wheel (or suspension rate) where the force displacement
curve is measured at the centre of the tyre contact patch. In a quarter
vehicle MBS model this would simply involve moving the point of jack
contact with the wheel from the wheel centre to the tyre contact patch.
The wheel rate should also not be confused with the term ride rate. This is
associated with the force displacement relationship between the vehicle
body, or sprung mass, and the ground. To derive this with a quarter vehicle
model it would be necessary to model an additional spring, representing
Fw
kw
VEHICLE BODY
w
kw
Fs
ks
Equivalent spring
acting at the wheel
centre
w
Fw
s
ls
A
lw
Fig. 4.31
Equivalent spring acting at the wheel centre