Most sources recommend that fifth wheel pin weight should be in the range of 15% to 25% of the trailer’s weight. Larger hitch weights make the truck itself heavier and less agile in abrupt manoeuvres. With a more forward centre of gravity (CG), the trailer itself is more stable with a higher hitch weight. Lighter hitch weights mean the trailer CG is located closer to the trailer axles, so the trailer is less stable, while with a lower weight the truck is more agile.
The combination of a more agile, lighter truck, and a less stable trailer, creates a situation where the truck can “overcontrol” the trailer, causing it to skid and swing widely and in the most extreme case, jackknife (in other words, to oversteer)
Conversely, a stable trailer and heavy truck can be difficult to manoeuvre, tending to “plow” into corners (in other words, to excessively understeer).
In the simulations, a double lane change is completed at 104 km/h (65 mph). This simulates an accident-avoidance manoeuvre where the driver temporarily enters the adjacent lane to avoid an obstacle and then returns to the original lane. The test is done at a constant speed.
Within the typical range of 15% to 25%, the vehicle and trailer handle in a roughly similar fashion (Video #1). The truck is slightly easier to control through the manoeuvre with a lighter pin weight, but the trailer swings slightly more.
Video #1: Double lane change at 65 mph. Pin weight is
15% of the trailer weight.
At more extreme pin weights, the effect becomes more exaggerated. At a pin weight of only 3%, the trailer jackknifes and the driver loses control (Video #2). At a pin weight of 50%, the trailer tows steadily behind the trailer, but the truck misses the target path (marked by the pylons) by a significant margin (Video #3).
Video #2: Double lane change at 65 mph. Pin weight is
3% of the trailer weight.
Video #3: Double lane change at 65 mph. Pin weight is
50% of the trailer weight.
It is worth noting the effect of a reduction in speed on these results. When the simulation is run at 88 km/h (55 mph), the trailer remains controllable even in an extreme case when the trailer CG is moved so the pin weight is actually negative (-5%.)
(The simulation does not model component failure. The weight placed the rear axle of the model pickup truck in the situation with a 50% pin weight would be well in excess of the rear axle rating of a similar truck in the real world, creating a significant risk of failure of tires, wheels, or axle or suspension components.)
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