Chassis Model

The chassis connects the front and rear suspension linkages. The models utilised

here employ a rigid chassis, meaning there are no degrees of freedom between the

front and rear suspensions, the two are simply offset in the x-axis by the wheelbase.

This approach is the simplest and most feasible given the data at hand. Chassis

models including varying degrees of freedom can be developed and included if need

be, ranging from simple torsional models which just incorporate a torsional stiffness, to full FE models of physically modelled chassis’.

The chassis model houses the detailed mass and inertia data for the vehicle. As

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to undergo changes to its architecture and setup in order to create the model of the

GTV. To enable this, all components of the real vehicle that are to be altered are

modelled separately in the standard vehicle model. Components such as the engine

and gearbox all the way down to the rear seats and carpet are included in the model

as separate multi-body masses, enabling their individual mass and inertia effects to

be captured. In the adaption of the SV model into the GTV model, these components

are simply removed, and similarly added for components such as the EM and battery

pack. This allows the mass and inertia properties to be accurately captured by the

models without the need for testing of the real vehicle to obtain such data. The data

required for this section of modelling was again obtained from the OEM, the data for

the parts of interest were extracted from a detailed CAD model, these parts were then

removed from the CAD model and the parts that were not of interest left in and

lumped together with the mass and inertia data given for the vehicle body.

Also incorporated with the chassis model is the body model, this houses specific

data regarding the body in white, such as it mass, inertia and aerodynamic properties.

The complete chassis model including suspension and tyre models is shown in

Figure 3-15.

The chassis model in this state is usable, it can be given an initial velocity, and a

number of steering inputs which enables its lateral response to be studied, it can also

be combined with road models in order to investigate its ride characteristics.

However due to rolling resistance, longitudinal components of lateral tyre forces and

aerodynamic drag forces, the longitudinal velocity of the chassis reduces greatly

during the duration of test manoeuvres. This change in vehicle speed would affect

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chassis model can be combined with simple engine and drive-train models, which

then in turn can be interfaced with a simple driver model/speed controller.

Figure 3-15 Chassis model components with detailed mass and inertia model

Vehicle origin/ axis system Tyre models Instance of front suspension model Instance of rear suspension model Mass model External connection to drive-train and/or brake models (torque, angular speed and position) Mechanical connection to drive-

train models (force)

Vehicle body

3d to multibody adaptor

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To ensure this section of the model is as accurate as possible when compared to

the real vehicle, model based inertia tests were conducted. This consisted of using a

model of an inertia rig, to which the vehicle could be attached and its roll, pitch and

yaw inertia obtained. The rig consisted of a circular ground plane mounted on a

revolute joint which allowed rotation about the z-axis. The vehicle was mounted to

this ground plane so that its Cog was aligned with the rotation axis. A small torque

was applied to the top of the revolute joint and the corresponding angular

acceleration of the vehicle was measured. From the input torque and the resulting

acceleration, the inertia of the vehicle can be calculated. The vehicle was mounted

on its three different axes to obtain the roll, pitch and yaw moments of inertia. The

test setup and inertia rig are shown in Figure 3-16.

Figure 3-16 Inertia rig model (right) and test configuration (left)

Ground Revolute joint Torque source and application Rigid connection to chassis

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The results from these model based tests are summarised in Table 3-3. The slight difference between the Dymola model presented here and the OEM’s model will

correspond to the level of detail used, the OEM’s model lumped all mass and inertia

properties within the body, where the Dymola model presented here, as previously

discussed, uses a more component orientated method to include the individual

properties of components that are of importance to this study.

Metric Unit Dymola Model OEM Model

Vehicle Roll Inertia _{Kg.m2 859.4} 815.5

Vehicle Pitch Inertia _{Kg.m2 3471.7} 3365.0

Vehicle Yaw Inertia _{Kg.m2 3776.5} 3720.0

Table 3-3 Vehicle inertia properties comparison