Effect of Pin Weight on Truck and Trailer Stability

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.)

Weight Transfer Under Hard Braking

What happens when you apply the brakes aggressively while towing a fifth wheel trailer? Does the front of the trailer push down on the hitch and the rear axle of the truck? To find out, I ran a simulation in Carsim 7, using a full-size pickup model, and a model fifth wheel trailer. The loaded truck in the simulation weighs approximately 5600 lbs, and the trailer about 6500 lbs. Yellow tire force arrows show how much weight is on the tire during the braking event (the vertical arrows), and how much braking force the tires are providing (the arrows pointing to the rear).

The results of the simulation show that the truck is able to stop much more quickly than the trailer, even with properly adjusted trailer brakes, so the trailer actually pushes forward on the hitch. Combined with the forward weight transfer of the truck itself, this means weight on the rear wheels decreases significantly under hard braking; enough that the truck’s ABS is activated for the rear wheels.


Video: Weight transfer during hard braking. Yellow arrows
show weight on each tire during hard braking. Brakes applied at 0:17.

There is some weight transfer from the trailer axles to the king pin under braking. To model this, I simulated how the trailer would behave without any truck braking. This would be the situation if the truck brakes failed for some reason, and the driver used the trailer brakes alone to stop the vehicle.


In this video, only the trailer brakes are being used.
Brakes applied at 0:13.

The results of this simulation show that there is some forward weight transfer of the trailer under braking. The force on the rear axle of the truck does increase when only the trailer brakes are used.

Towing Dynamics

After purchasing a fifth wheel trailer in 2008 and enjoying a season of camping with my family, my training in engineering led me to become curious about the forces at work when towing a recreational trailer. I checked the Internet, and talked to other campers about their understanding of how towing works. Most discussions revolve around engine power, published towing capacity, loads on and weights of tow vehicles and trailers, and the tow vehicle wheelbase. Conceptually towing is very simple – the trailer follows the tow vehicle. This is true most of the time, and everything is usually fine as long as it is true. When reviewing accidents, however, we can see that the trailer doesn’t always behave so well - sometimes with serious consequences.

The point of this blog is to try and explain the interactions between tow vehicles, trailers, and the road. I am not an automotive engineer, but would be happy to hear from any who might be able to explain the world of towing for the rest of us.