We describe an on-line observer that allows to monitor the soil-tire contact longitudinal stiffness of any off-road vehicle equipped with low cost sensors – relatively to the machine cost. 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 its vehicle. Indeed, they are frequently subject to ground quality changes and high slopes causing deadly roll-overs every years. The observer have been tested on a grape harvester in various conditions and its output compared with an estimation based on expensive but precise sensors.