4569846498

148 Multibody Systems Approach to Vehicle Dynamics
events. It is strongly suggested that the simplest interpretations of the
vehicle kinematic measures be used and comparisons drawn between these
simple measures. For comprehending the effects on a full vehicle, a full
vehicle model is recommended.
4.1.6 Component loading environment
Multibody systems models of the quarter vehicle type are often used to distribute
design loads through the different suspension members with a view
to sizing them intelligently in the first instance. A distinction needs to be
drawn between design loads and service loads when discussing multibody
systems analyses for this purpose. Design loads are calculated by postulating
notional extremes for possible loads that may be induced in reality. An
example of this might be the case where a car has been parked between two
kerbs, between which it is a snug fit, and the driver then applies maximum
effort to the handwheel. The exact philosophy for the selection and implementation
of design loads is typically historical; they have been empirically
defined and honed over many years of development experience. In practice
they often correspond to events described as ‘extreme service loads’ or
‘abuse loads’. They are events that the vehicle must survive but may need
some attention immediately following it; in the steering example given, the
steering alignment may be distorted by events but the vehicle would probably
be required to remain capable of being driven. In industry many companies
have standard cases such as the 3g bump case where the static wheel
load is factored up to represent dynamic abuse situations such as striking a
road hump at speed. The specification of abuse loads can be traced back to
a publication in the Automobile Engineer by Garrett (1953) where a range
of recommended wheel loads were proposed for the design of vehicles of
that period. Design loads are frequently viewed as ‘one-off’ events in the
life of the vehicle.
In contrast to design loads, there is another category of loads to which the
vehicle is exposed. These are the service loads. This is the loading environment
to which the vehicle is subject during its durability sign-off testing.
Durability sign-off criteria vary widely but they typically consist of a specified
number of repetitions of different events at prescribed speeds and
loading conditions. They induce a large number of repetitions of events
that are probably more commonplace in the life of a vehicle – driving up
kerbs, for example. In contrast to the design events, the vehicle is expected to
emerge largely undamaged from the durability sign-off procedure –
although no expectation of remaining service life is usually associated with
durability sign-off. The relationship between durability sign-off criteria
and actual usage of the vehicle is another question entirely; durability signoff
procedures themselves are often compiled in the light of historical warranty
costs and other such influences.
There is, then, a key difference between design and service events. The
design events are slightly fictitious and are usually analysed using a static
or quasi-static procedure. While of questionable ‘accuracy’ they are of
tremendous value. They may be calculated with the scarcest data about the
vehicle and allow the early intelligent sizing of many different components,
once applied to the wheel of a quarter vehicle model and distributed about