This post was written by member Durb. I am posting it for him as he had a computer problem.
BarneyFrom Durb:
Following is my attempt to explain why a fifth wheel trailer creates chucking forces. Please pardon the cheesy images.
As the front tires of a tandem axle trailer rolls over a bump the front of the trailer lifts up. As the front tires roll off the bump the rears roll up causing the front of the trailer to lower and the rear to lift.
This causes the trailer to rotate around an axis that is parallel to the axles and is located about here.
For demonstration purposes, if the hitch were the same height as this rotational axis all the forces would be bouncing forces (up and down). If the hitch was located directly above the axis all the trailer’s forces would be chucking forces (fore and aft).
Since the kingpin of a 5th wheel is neither horizontal nor vertical to the axis, the forces exerted by the trailer are a mix of bouncing and chucking forces. The greater the angle the more chucking forces relative to the bouncing forces.
There are two ways to alleviate chucking forces created by the trailer. First is to reduce the trailer’s proclivity to rotate. Second is to reduce the hitch angle and reduce chucking forces relative to bouncing forces.
Why does trailer length matter? - Shorter trailers have a built in higher hitch angle so more chucking. In addition to lower hitch angles longer trailers have more polar mass further from the rotation axis so they rotate less.
Why does increasing pin weight help? - Moving mass forward causes the truck to squat more reducing hitch angle. Moving mass forward shifts weight from the axles raising the rotational axis and reducing the hitch angle. Moving mass forward also creates polar mass and reduces the trailer’s tendency to rotate.
Does trailer design matter? - Yes, other than length, trailers with centralized mass such as center kitchens and tanks over the axles rotate more easily. Trailers with weight on the ends (polar mass) are less prone to rotating.
Does axle design affect chucking? - Yes, good equalization and trailer shocks will reduce the tendency of the trailer to rotate. Wider spaced axles will raise the rotation axis reducing hitch angle and also reduce the tendency to rotate. Trailers with stiff suspensions relative to trailer weight will rotate more.
Why does towing nose high increase chucking? - No worse way to increase the hitch angle and increase chucking relative to bouncing.
Does filling the fresh water tank help? – Yes, if the tank is forward of the front axle. This increases polar mass making it more difficult for the trailer to rotate. The extra weight squats the truck reducing the hitch angle.
Do air bags on the truck help? - No, not for chucking. Raising the back end of the truck and kingpin increases the hitch angle and causes the chucking to become worse relative to bouncing.
Do shocks on the truck alleviate chucking? - No. The truck’s suspension is designed to quell vertical motions, chucking is fore and aft. Chucking forces will be felt in the cab independent of the truck’s suspension.
Does the hitch cause chucking? - No. The chucking forces are created by the trailer. Most hitches just transmit those forces very effectively to the truck. Some hitches with loose kingpin connections make noise due to chucking forces. A hitch with a tighter hook up will eliminate the noise but the trailer generated chucking forces will still be there.
Why don’t travel trailers chuck? - Their hitch ball is level with the rotational axis therefore the hitch angle is zero and all trailer generated forces are bouncing forces.
Durb
