Apparatus for estimating a grip factor (G) of at least one wheel of a road vehicle ; the apparatus comprising: - at least one steering factor detecting device configured to cyclically detect at least one of the steering parameters comprising an actual rack (RF) force (R) applied to a steering system of the road vehicle , which extends from a steering wheel to a suspension of the road vehicle ; - detection elements configured to cyclically detect actual vehicle state variables ; - a control unit , configured to cyclically process, depending on at least the actual state variables of the vehicle , an actual slip angle (α) of said wheel of the road vehicle ; - a storage unit , within which, in a consultation table (CT), a plurality of grip curves (GC) correlating a plurality of values of the steering parameter comprising the rack force (RF) to a plurality of values of the slip angle (α) are recorded; wherein on a same curve, the value of the grip factor (G) remains constant; wherein the control unit is configured to cyclically estimate at least one raw value (RG) of the grip factor (G) of the at least one wheel based on the position of a current condition (AC) within the consultation table (CT), as a function of the actual slip angle (α) and the actual rack force (RF); characterized in that the control unit is configured to process a finite value (FV) of the grip factor (G) by filtering the raw value (RG) of the grip factor (G) according to at least one reliability index (Q); in particular, the control unit is configured to perform a weighted average of the last n raw values (RG) wherein the weight is given by the reliability index (Q). - at least one steering factor detecting device configured to cyclically detect at least one of the steering parameters comprising an actual rack (RF) force (R) applied to a steering system of the road vehicle , which extends from a steering wheel to a suspension of the road vehicle ; - detection elements configured to cyclically detect actual vehicle state variables ; - a control unit , configured to cyclically process, depending on at least the actual state variables of the vehicle , an actual slip angle (α) of said wheel of the road vehicle ; - a storage unit , within which, in a consultation table (CT), a plurality of grip curves (GC) correlating a plurality of values of the steering parameter comprising the rack force (RF) to a plurality of values of the slip angle (α) are recorded; wherein on a same curve, the value of the grip factor (G) remains constant; wherein the control unit is configured to cyclically estimate at least one raw value (RG) of the grip factor (G) of the at least one wheel based on the position of a current condition (AC) within the consultation table (CT), as a function of the actual slip angle (α) and the actual rack force (RF); characterized in that the control unit is configured to process a finite value (FV) of the grip factor (G) by filtering the raw value (RG) of the grip factor (G) according to at least one reliability index (Q); in particular, the control unit is configured to perform a weighted average of the last n raw values (RG) wherein the weight is given by the reliability index (Q). <EOS>