In this paper an on-line observer that allows to monitor the soil-tire contact longitudinal stiffness is proposed. It is focused on off-road vehicles equipped with low cost sensors. The knowledge of such a value is viewed as a milestone in the development of active security devices, in order to warn the driver about the longitudinal stability and/or correct his actions. Indeed, in the off-road applications, vehicles are frequently subject to ground changes and high slopes, causing fatal loss of control - slipping, rollover, etc... The proposed approach has been tested on a grape harvester in various conditions and its output evaluated with respect to an estimation based on expensive but precise sensors.w