Calculate the natural frequency of unsprung mass from FFT/spectrogram

I have a 10ms sampled data of vehicle speeds. I have calculated the FFT of the signal in 10 second windows and have the single sided amplitude spectrum of the time domain data. This data is available for 5 different speed ranges. I a trying to calculate what is the natural frequency of the unsprung mass using the fft data? How do I plot the spectrogram to get this natural frequency?

Here is the code that I have:

function [freq,Response] = FFT_optimum(Inputs)

Fs = 100;%(1/sampling_time)*1000;
window = 10*Fs; % 10 second window
Len = floor(length(Inputs)/window);
limI = window/2;
for i = 1:limI
   freq(i) = (i)*(Fs)/(window);
end 
for k = 1:Len-1     
        InputRange = Inputs(window)- mean(Inputs(window));
        %InputRange = InputRange.*window_function;
        FFT_MAG = fft(InputRange,window);
        FFT_MAG = FFT_MAG/window;%window_length;
        FFT_MAG = FFT_MAG(1:window/2);
        Response(k) = FFT_MAG;
end
end

I would like to calculate the natural frequency of the system after this, by using a spectrogram?

Would someone be able to help me on this?

Thanks!