[DonRotolo]

I think I understand the question. Measure the angle between the axis of the two rear wheels and the axis of the two front wheels. Straight ahead the angle is zero, and if the front wheels are turned completely to one side the angle is 90 degrees.

Using that angle you can calculate the turning radius, but you need to know the distance between the front and rear wheels.

If that seems difficult, set the wheels at known angles (like 10, 20, 30 degrees) and measure the turning radius directly (draw a line on the floor and then measure the curvature)

Don