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Bucking and Chucking with Demco Hijacker on 2020 GMC Sierra 3500 With Factory Puck System


I have a 2020 gmc 3500 that I installed a demco 21k hitchhiker slide hitch in. New truck and new trailer and my wife refuses to ride in it because of bucking and chucking. It looks to me like the hitch pin is too far to the rear behind center of axel and possible causing the problem. Is there a way to adust the hitch forward with the gm puck system. Or any other help would be appreciated.


Expert Reply:

I reached out to my contact at Demco to see if they have ran into this problem as well, as of right now they have not had trouble with the new GM system. When GM redesigned the full size trucks, they also redesigned the factory puck systems which did move it back towards the tailgate slightly. I am not aware of any way to move the pucks forward or adjust the hitch closer to the cab. However moving it back slightly like GM did helps with clearing the cab in tight turns if you have a short bed truck, but as you and your wife experienced it can also cause bucking - the king pin moving up and down inside the head causing a rough uncomfortable ride. Chucking is slightly different - where the king pin is moving inside the fifth wheel head back and forth, again creating a very uncomfortable ride.

Both of these can happen when towing a 5th wheel, but the Demco Hijacker has a wrap around jaw that provides a secure connection to the kingpin, and since it has tighter tolerances around the king pin it is designed to reduce chucking. Also the autoslide feature of the Hijacker requires the king pin to be stabilized to the fifth wheel head. Normally the kingpin rotates inside the head, but the autoslide keeps the pin stationary and the entire head turns. This is accomplished with the capture plates on the sides of the head holding the pin box against the head.

If the capture plates are not adjusted properly it can lead to a rough ride because of play in the head and kingpin, or the auto slide feature not working properly. Also, pin boxes and the capture plates or locking plates used are specific, and which model you have may require separate capture plates, or locking plates. If you provide me with the part number of your current pin box I can make a recommendation and see if the capture plates you have are the correct ones.

Another thing to check is the adjustment of the Autoslide gear bolt. This should only be adjusted if there is too much for and aft movement in the slide assembly when stopping and starting in the straight towing position. Some movement when in a turn is normal, but there should be a minimal amount when the truck and trailer are in line.

One last thing to check is if your trailer is riding level when you're towing it. This can greatly effect the quality of the ride.

If all of those things are in adjustment and you are still experiencing a rough ride a cushioned pin box will cut down on the jarring ride. They use an air bag inside to absorb the road shock so you won't feel it. I linked our selection of cushioned pin boxes, as well as instructions for the Hijacker.

I hope you and your wife can enjoy your new truck and camper!

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