Questions regarding Cornering Stiffness for RC Car

I am a bachelor’s level student tasked with modeling the vehicle dynamics of an RC Car that will be made to drive autonomously. I lack prior experience or particularly specific education with vehicle dynamics so I have been studying the field a bit, particularly Vehicle Dynamics and Control by Rajesh Rajamani. By advice from a supervisor I will not be using the Pacejka Magic Formula. I think I have a good enough idea of the problem and how to approach it but I’ve hit a wall quite badly regarding the parameter ‘Cornering stiffness’. As I understand this is a property of the tires, and something that I would find on data sheets on tires. However in my case the tires are considerably more simple, they’re just solid rubber rings around the wheels. I have tried to read up on Cornering Stiffness to see if I can determine it myself somehow but I haven’t really found anything I’ve understood.

So, how should I go about determining the cornering stiffness? Otherwise, considering the shape of these tires are there any assumptions I can make that would simplify the model?