Most tow trucks now being manufactured include means of lifting and towing vehicles by engaging the tires, axle, or suspension of the towed vehicle, lifting and securing the towed vehicle to these means before transporting the towed vehicle from the load site. These means are called wheel lifts or underlifts. Wheel lift tow trucks can be used to tow light duty through heavy duty vehicles. Axle and suspension type underlift tow trucks are often used to tow medium duty and heavy duty vehicles, which are more likely to have straight axles and more robust suspensions than light duty vehicles.
When a tow truck lifts a vehicle, part of the load from the towed vehicle is distributed to the rear axle of the tow truck, and load is removed from the front axle of the tow truck because of the load moment from the lifted vehicle, for which the fulcrum is the rear axle of the tow truck. These effects on the load distribution for a tow truck result in potentially dangerous conditions. Adding load to the rear axles of the tow truck can cause the carrying capacity of the axle, tires, springs, brakes, and bearings to be exceeded. In addition, overloading the rear tires reduces sidewall flexibility and, consequently, adversely affects handling and control. Removing load from the front axle of the tow truck can result in reduced steering and braking response because these functions depend on the friction forces available between the tires and road surface, and these friction forces depend on the normal force between the tires and the road surface. It is an accepted criterion in the tow truck industry that reducing the load at the front axle of a towtruck by more than fifty percent is not safe.
This invention provides a means of creating a moment at the contact point between the tow truck and the towed vehicle which counteracts the load imposed on the rear axle of the tow truck and the force that is removed from the front axle of the tow truck when the towed vehicle is lifted. This moment is created by attaching a chain, wire rope, strap, spring, wire rope assembly, turnbuckle or similar construct or combination of such constructs from the rear surface of the wheel lift or axle/suspension lift mechanism to the frame, crossmember, or other rigid part of the towed vehicle at a point to the rear of the tires being lifted by the tow truck. This construct is then tensioned before the vehicle is towed, so that a moment is created in the tow truck boom that will be transmitted to the tow truck frame and body, thereby redistributing the loads on the front and rear axles of the towtruck in a less disadvantageous manner.