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Roll-coupling for Safety and Productivity

Roll-coupling is a pivotal connection between two or more vehicles that transmits roll motion from one vehicle to another without restricting yaw (turning corners) and pitch (driving over bumps) motion.  The objective of roll-coupling is to transfer roll stability from one vehicle to another to prevent a roll-over event from occuring.  During the onset of a potential trailer roll-cover, the truck can feel the truck responding to accumulating torsion through a roll-coupling hitch and take remedial action to prevent an accident from occuring.  In more extreme situations, truck and trailer combinations in motion are out of phase with each other; roll-coupling serves to balance roll forces working in one direction on one vehicle with forces working in the opposing direction on the second vehicle.  None of this is possible when using oscillating pintle hitches and couplers.

Mr. Greg Gilks, P.Eng., the former Director of Commercial Vehicle Safety Enforcement in B.C. explains, "The roll-coupling hitch is an innovative device which will improve highway safety by providing roll stability between the truck and trailer.  Conventional pintle hitches do not transfer roll stability, so any instability in the trailer cannot be countered by the stability of the truck.  The roll-coupled hitch not only provides a transfer of roll stability, but it also allows the driver to feel trailer instability and take corrective action before the trailer rolls over."

Fig. 1 - Roll Coupling

Roll-coupling replicates the performance of a universal joint used for transmitting a torsional load through a driveline.  Torsion can be measured as can the effect of roll-coupling on truck and trailer combinations.  The measurement is called "Load Transfer Ratio".

Load Transfer Ratio (LTR)

When they can't stop in time to prevent an accident from occuring, truck drivers must be prepared to evasively maneuver a truck and trailer(s) around an obstacle such as an automobile or a pedestrian.  This predictable event is used to establish a dynamic roll stability benchmark called "Load Transfer Ratio".  The TAC (Transportation Association of Canada) standard is <0.60.  When LTR reaches 0.60, sixty percent of the weight normally riding on one side of the vehicle has transferred over to the other side.  The net result is that eighty percent of the weight is riding on one side of the vehicle and only twenty percent on the other during a "normal" evasive maneuver.  When LTR = 1.0, the vehicle will roll over.  Vehicles become even more unstable during an extreme maneuver and when exposed to a variety of de-stabilizing conditions i.e.: crosswinds, underinflated tires, uneven road surfaces, unbalanced or poorly secured loads, high loads (CG), excessive speed etc.  Experts warn that there is "no room for compromise" on LTR.  The LTR limit identifies the maximum risk that can be imposed on the public for vehicles operating on the public highway system.

Wolf Trailers has been working with FPInnovations (Feric Division) on roll-coupling since 2008; Mr. Séamus Parker, P.Eng., R.P.F. said "Roll-coupling hardware is the only option that will enable the performance criteria to be achieved under current dimensional allowances in western Canada, and that will also facilitate straightforward regulation enforcement."

Mr. John Woodrooffe, P.Eng. (University of Michigan Transportation Research Institute) tells us "Load Transfer Ratio is the most comprehensive and informative vehicle dynamic measure for vehicle configuration safety analysis.  It is a direct measure that relates to differential wheel loads (side to side) and is a direct measure of how close a vehicle is to rolling over.  For this reason, any vehicle with load transfer characteristics that are above the TAC recommended value of 0.60 is considered to be unacceptable.  Other performance measures can have some leeway with respect to published criteria provided the infrastructure and operating conditions are compatible with the more lenient criteria. Since Load Transfer Ratio directly addresses dynamic stability, there is no room for compromise."


Since 1991 the Federal – Provincial – Territorial Memorandum of Understanding on Interprovincial Weights and Dimensions (MoU) has limited the weight of pony trailers, A-train doubles and full trailers and to less than legal axle weight to improve stability. Note that the vehicle configurations (Fig.2) with a LTR > 0.60 are not roll-coupled; the other vehicles are.

Fig. 2 - Unstable Vehicles Identified by The Transportation Association of Canada: 


                                                                                                                                                        Mr. John Pearson, P. Eng., 
                                                                                                                                                        Council of Ministers Secretariat, Task Force
                                                                                                                                                        on Vehicle Weights and Dimensions Policy

Fig. 3 - Unstable Vehicles Illustrated:

Truck/pony trailer, truck/full trailer, A-Train Double and long combination vehicles that would otherwise have a load transfer ratio of 0.60 and higher should be roll-coupled when operating on public highways. 

Fig. 4 - The Effect of Roll-Coupling on LTR

Note that roll-coupling improves stability even after up to 9,000 kg is added to the load.  Roll-coupling also significantly improves the dynamic performance of trucks and trailers with long hitch offsets.

Hitch Offset

Truck hitch offset is measured from the turning center of the driving axle group to the articulation point of the hitch.  Increasing hitch offset (see Fig. 6) decreases vehicle stability.  The TAC standard for hitch offset is 1.8 m however roll-coupling enables vehicles with hitch offsets up to 3.5 m to comply with critical TAC performance standards.

Fig. 5 - Hitch Offset Illustrated

Increasing hitch offset distance increases the lateral movement of the hitch and trailer drawbar relative to steering inputs by the driver regardless of the type of hitch that is used.  Roll-coupling reduces the negative effects that steering inputs have on trailer stability however shorter hitch offsets should be used whenever possible.

Fig. 6 - The Effects of Hitch Offset and Roll-coupling on LTR*

*Simulations were based on a tri-drive truck/ quadaxle full trailer carrying 34 tonnes on the trailer (63.5 MT GVW).  The TAC standard for hitch offset distance is 1.8 meters; Hitch offsets longer than the TAC limit for truck/pony trailers and truck/full trailers are unsupported by engineering work with the exception of roll-coupled vehicles. 

Benefits of Roll-coupling

Roll-coupling is an advancement in transportation technology that enables all truck/trailer combinations in Canada to be productive and comply with critical TAC safety performance standards.  Roll-coupling enables vehicles that are otherwise too unstable to carry full loads to increase productivity up to 20%.  This not only means safer highways for motorists and more profit for industry but through improving efficiency, roll-coupling reduces traffic congestion, fuel consumption and exhaust emissions. 

The net effect of improving transportation efficiency is lower freight costs to move raw materials from the source, through production and on to store shelves.  This reduces the cost of living for Canadians and makes us more competitive in the global marketplace; that creates employment.

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View photos of roll-coupled vehicles and find out more about regional roll-coupling projects.                                                           Updated November, 2013


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