The three main requirements of a vehicle-trailer connection system are: en routeudstability, over- or under-steering restraint, minimum off-tracking along curved path. Linkingudthe two units by four-bar trapeziums, wider stability margins may be attained in comparisonudwith the conventional pintle-hitch for both instability types, divergent or oscillating. Theudstability maps are traced applying the Hurwitz method or the direct analysis of theudcharacteristic equation at the instability threshold. Several types of four-bar linkages may beudquickly tested, with the drawbars converging towards the trailer or the towing unit. The latterudconfiguration appears preferable in terms of self-stability and may yield high critical speeds byudoptimising the geometrical and physical properties. Nevertheless, the system stability may beudimproved in general by additional vibration dampers in parallel with the connection linkage.udMoreover, the four-bar connection may produce significant corrections of the under-steering orudover-steering behaviour of the vehicle-train after a steering command from the driver. The offtrackingudalong the curved paths may be also optimized or kept inside prefixed margins ofudacceptableness. Activating electronic stability systems if necessary, fair results are obtainableudfor both the steering conduct and the off-tracking.
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