That's the only thing that is easy to measure. Load the tire with different weights and measure the deflection. Just two - three points can give you an estimate of linear stiffness.
there is no single 'radial stiffness' of a tire. Not of the real one, and not of a good simulation model. It will strongly depend on radial deflection, camber angle, inflation pressure, actual tread depth, temperature, rolling speed, slip state, and more. For FTire, there are tons of tools available within Adams/Car -> component analysis -> cosin tools to analyze all this. If a magic-formula-type tire model tries to come up with one value of radial stiffness, you shouldn't trust it ;-)
