1. A method for evaluating the instantaneous frequency of a mechanical excitation exerted by an uneven surface of a road on a wheel of a motor vehicle which travels on the road and includes a chassis connected to the wheel by suspension devices the method including: acquiring and elaborating at least a first measurement time series that consists of successive measurement samples representing successive values of a first observable parameter related to instantaneous height of one of the chassis and the wheel; filtering, which uses at least the first measurement time series, to produce at least a first derived principal time series which consists of successive estimates of the instantaneous height of the chassis, and a second derived principal time series, which consists of successive estimates of the instantaneous height of the wheel, extracting a time lag, using at least the first derived principal series and the second derived principal series, to produce a preparatory time series that consists of successive estimated values of an instantaneous time delay between the instantaneous height of the chassis and the instantaneous height of the wheel, wherein each estimated value of the time lag is obtained by optimization of at least one function for correlation of the first derived principal time series and the second principal derived time series, and estimating a frequency, using the preparatory time series to produce a final time series, consisting of successive estimated values of the instantaneous frequency of the mechanical excitation, wherein each frequency value is obtained, in an observation time window defined by the corresponding estimated value of the instantaneous time delay, as the frequency for which the instantaneous time delay constitutes an instantaneous phase shift.