6-wheeled wheelchair model in gazebo

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Hi,

I tried to simulate my model of a 6-wheeled wheelchair on gazebo. The idea is to have two bigger wheels on the centre to control independently with motors, and four smaller wheels (2 in the front and 2 in the back) to balance the wheelchair. These four wheels will not be motorised. The figure shows the model.

However, with my model, the wheels do not ideally touch on the ground. What's the best way to align the six wheels, so they all touch the ground? Is there a way to add something like shock absorbers or add some elasticity to the wheels? Or something else that allows the six wheels to touch correctly on the ground?

I'm using ROS Melodic and Gazebo 9.

Thank you!