625 ILCS 5/1-120.5
A coupling device connecting 2 or more vehicles operating in combination. The lower half of a fifth wheel assembly mounted on a truck tractor or converter dolly must be secured to the frame of that vehicle with properly designed brackets, mounting plates, or angles and properly tightened bolts of adequate size and grade or devices that provide equivalent security. The installation shall not cause cracking, warping, or deformation of the frame. The installation shall include a device for positively preventing the lower half of the fifth wheel assembly from shifting on the frame to which it is attached.
The upper half of a fifth wheel assembly must be fastened to the motor vehicle with at least the same security required for the installation of the lower half on a truck tractor or converter dolly.
Every fifth wheel assembly shall have a locking mechanism. The locking mechanism and any adapter used in conjunction with it must prevent separation of the upper and lower halves of the fifth wheel assembly unless a positive manual release is activated. The release may be located so that the driver can operate it from the cab. If a motor vehicle has a fifth wheel assembly designed and constructed to be readily separable, the fifth wheel assembly locking devices shall apply automatically on coupling.
The lower half of a fifth wheel assembly shall be located so that, regardless of the condition of loading, the relationship between the kingpin and the rear axle or axles of the towing motor vehicle will properly distribute the gross weight of both the towed and towing vehicles on the axles of those vehicles, will not unduly interfere with the steering, braking, and other maneuvering of the towing vehicle, and will not otherwise contribute to unsafe operation of the vehicles comprising the combination. The upper half of a fifth wheel assembly shall be located so that the weight of the vehicles is properly distributed on their axles and the combination of vehicles will operate safely during normal operation.
625 ILCS 5/1-120.5