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Modelling and analysis of suspension systems 149
a conceptual suspension layout. The resulting loading environment for each
component may be used by designers to sketch a meaningful first-sight
design. This can avoid packaging difficulties induced by ill-informed
sketching of components and the subsequent ‘stealing’ of space needed for
section modulus in key components.
Service loads are altogether more detailed. They are typically well-defined
events where road profiles and speeds are well known. Once a reasonably
representative geometry and suspension setting data set is available, the
description of events may be combined with some form of enveloping tyre
model in order to predict load–time histories for each component in the
suspension. This in turn may be passed to finite element-based fatigue calculation
methods in order to evaluate the number of times a component
may be subject to this event before failure. Hypotheses such as the linear
damage accumulation hypothesis (Miner’s rule) may be used to sum up the
contribution of different events to the life of the component. Interested
readers are referred to the SAE Fatigue Design Handbook (Rice, 1997) for
more information on component fatigue analysis.
With the calculation of service loads, instead of a single set of peak loads as
produced from the design loads there is a vast amount of time-domain data.
Typical data rates for service load events are around 200 points per second
and service load events last of the order of 10 seconds. With a repertoire of
up to 20 events describing a typical durability sign-off, it may be seen that
the calculation of service loads results in an increase in the quantity of
results data of the order of five orders of magnitude. Calculation and subsequent
processing times rise too, though not quite by the same amount since
linear static FE calculation times are a significant proportion of the overall
elapsed time following design load calculations. However, as an overall
process time amplification the expectation should be between two and three
orders of magnitude for the calculation and use of service loads as compared
to design loads. For some simple components, the use of a previously verified
and correlated rig test may prove less onerous to the organization.
4.2 Types of suspension system
There are various suspension systems used on cars and trucks and as
described in Chapter 3 specialized versions of MBS programs such as the
ADAMS/Chassis and ADAMS/Car programs provide templates with preprogrammed
configurations of suspension systems commonly used by
automotive manufacturers. Many established textbooks on vehicle dynamics
and some that focus on suspensions provide a detailed treatment of the
various types of suspension system and their function. The coverage here
will be to briefly mention some of the most common systems and then to
direct our attention to the MBS modelling and simulation environment.
The double wishbone and McPherson strut systems are very common and
will provide the basis for the ensuing discussion.
At the time of writing the following configurations, shown graphically in
Figure 4.12, are those, for example, provided with the ADAMS/Chassis
program to model vehicle front suspension systems. Note that the software
follows its roots and uses the Short–Long Arm naming convention used