Roll-Coupling Hitches

Oct 2, 2013 - 12 years ago

The Transportation Association of Canada (TAC – formerly RTAC) developed critical performance measurements for commercial vehicles and limits to provide guidance for crafting regulations and evaluating permit requests from industry. The dynamic performance measure used to evaluate roll stability is “Load Transfer Ratio” (LTR). LTR is a physical measurement of weight transferring from one side of a vehicle to the other while negotiating a corner and when performing a predictable avoidance maneuver around a vehicle or pedestrian etc. when a truck driver doesn’t have time to stop the truck to avoid a collision. The LTR limit is <0.60. A vehicle experiences LTR = 0.60 or when 60% of the weight that normally is carried on the wheels on one side of the vehicle has transferred to the wheels on the other side resulting in 80% of the weight being carried on the wheels on one side of the vehicle. As variables change, LTR increases until 100% of the weight (LTR = 1.0) has transferred to one side of the vehicle and the vehicle rolls over.

Traditional Hitches

There are two common groups of hitches used in Canada and around the world. Fifth wheel hitches provide roll-coupling (after lash in the hitch has been taken up) when one vehicle in a combination is rolling relative to another. As illustrated by TAC below, the lack of roll-coupling between vehicles (top three combinations) using pintle hitches is cited as the cause of roll instability and the reason behind reducing the amount of weight these vehicles can haul in Canada.

Stability and Handling - Compariso of Vehicles in the Current Truck Fleet

The stability and handling characteristics of heavy truck configurations depend upon the actual dimensions of the vehicle, the axle weight and the type of payload. However, for the purposes of comparison, typical performance levels for seven basic configurations of vehicles commonly used within Canada are depicted in the charts shown.

Roll-Coupling Hardware

Roll-coupling hardware is a low-lash coupling that provides the same function as a universal joint – it allows motion around the yaw axis (turning corners) and pitch axis (driving over bumps) and prevents twisting unless the hitch is equipped with optional selective roll-coupling (on highway) or roll-relief for use off-highway.