'Model may not give enough information' error when using non-powered (free rotating) 'Tire' blocks

I am trying to model a 2 wheel drive vehicle using Simscape Driveline. My model model consisting of four 'Tire (Friction Parametrized)' blocks connected to a 'Vehicle Body' block, as shown below:

The rear tires are powered but the front tires are non-powered and rotate freely. All the four tires are connected to the 'H' port of the 'Vehicle Body' body. Everything simulates fine when the 'Rolling Resistance' is set to off on the front tires. However, when 'Rolling Resistance' is enabled on the front tires, the model throws an error with the following error excerpt:

Error: An error occurred while running the simulation and the simulation was terminated
    Caused by:
        ['TireTest/Solver Configuration']: Initial conditions solve failed to converge.
            Nonlinear solver: Linear Algebra error. Failed to solve using iteration matrix.
                The model may not give enough information to make it possible to solve for values of some of its variables. Specific advice is given below.

all components and nodal across variables involved
Cannot solve for one or more variables:
'Tire_Left_Rear.fundamental_clutch.clutch.t' (Torque)
'Tire_Right_Rear.fundamental_clutch.clutch.t' (Torque)
'Tire_Left_Rear.wheel_axle.f' (Force)
'Tire_Right_Rear.wheel_axle.f' (Force)

Equations (including switched-linear equations) of one or more components may be dependent or inconsistent. This can cause problems in transient initialization.

Why does this error show up? Are there any workarounds?

Answer 1

[MathWorks Support Team]

This issue arises because the system is underdefined from the perspective of the solver. This scenario occurs because all four tires are directly coupled to each other through the hub port ‘H’. With the front tires set as free to rotate and frictionless, this coupling is effectively nullified. However, upon introducing rolling resistance in the front tires, new variables emerge for the solver, but new constraints do not, resulting in too many dependent initialization states. Due to this underdefined state, simulating the system shows an error for the rear tire blocks even though the settings are changed for the front tire blocks.

Some possible solutions for this issue are as follows:

1. Connect a ‘Mass’ block of any value at the junction between vehicle body’s ‘H’ port, and the ‘H’ ports of the four tires.
2. Add a ‘Translational Spring’ block with very high stiffness with one end connected to the vehicle body’s ‘H’ port, and the other end connected to the ‘H’ ports of the four tires.
3. Add a spring and damper combination to the ‘H’ port of the rear tires, before connecting them to the ‘H’ ports of the vehicle body and the front tires.
4. Do not specify tire inertia on the rear tires.

All of the above solution add additional states to the model and help the solver initialize.