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202 Multibody Systems Approach to Vehicle Dynamics
ride modes much above 2.5 Hz, even quite aggressively suspended ones.
There is thus a ‘window’ in which we position the primary ride behaviour
of our vehicles to make them compatible with the operators.
For autonomous vehicles that never carry humans, these restrictions are
relaxed and the suspension primary ride frequency can be chosen on the
basis of some other functional aspect.
4.10 Case study 5 – Suspension vector analysis
comparison with MBS
4.10.1 Problem definition
The following study is intended to demonstrate the application of the vector
theory outlined in Chapter 2 to a range of suspension analyses. Before
the advent of computer programs to analyse the motion of suspension linkages
vehicle designers resorted to graphical methods or simplified calculations
often using two-dimensional representations to study the suspension
in a fore–aft or transverse plane. These methods are still taught and
included in many texts addressing vehicle dynamics. They can develop an
understanding of suspension design, and the effect on total vehicle performance,
that can be lost using the automated methods in modern computer
aided analysis.
The aim of this textbook is to bridge the gap between traditional vehicle
dynamics theories and the multibody systems approach. The following calculations
are typical of the processes carried out using MBS software.
While the methods used here do not represent exactly the internal machinations
of, for example, the MSC.ADAMS software they do give an indication
of the computational process involved. The example chosen here is
based on a typical double wishbone suspension system. The answers
obtained are compared with those running an MSC.ADAMS model using
the same data. The calculations will include a series of analyses including:
● Geometry analysis
● Velocity analysis
● Acceleration analysis
● Static force analysis
● Dynamic force analysis
The geometric data required to define this problem is defined in Figure
4.67. Note that in this example the x-axis is orientated towards the front of
the vehicle rather than pointing to the rear as used generally throughout this
text. This is therefore the front left suspension system on the vehicle.
4.10.2 Velocity analysis
The starting point for this analysis is to establish a boundary condition, or
road input, at the tyre contact patch P as the vehicle negotiates the road
hump shown in Figure 4.68 with a forward speed of 15 m/s. The analysis is