We describe an on-line observer that allows to monitor the soil-tire contact longitudinal stiffness of off-road vehicles equipped with low cost sensors. The knowledge of such a value is a milestone in the development of active security devices, or at least a mean to inform the driver about the longitudinal stability of his vehicle. Indeed, these vehicles are frequently subject to ground changes and high slopes, causing fatal loss of control – slipping, rollover, etc. – every year. The observer 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.