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Was this the OEM pin box or an aftermarket addition? Does it move freely when you do it by hand without the truck hooked up?
The USUAL reason for this not being exactly straight with the pin box as you describe is from either the pin box being slightly off center with regard to the axles and tracking OR the weight isn't distributed correctly on the trailer and it allows the unit to be pulled somewhat to the side. I really don't personally care for this sort of pin box for this reason, but I do see that sometimes the extra ability to get around in tight places can be a benefit. It shouldn't really hurt anything so long as the tires aren't wearing unevenly AND it isn't allowing the trailer to track too far to one side of the truck as that can make it handle somewhat unevenly and on wet pavement, that might be an issue. Hope that proves helpful here. Please get back to me if you need more as all follow ups are included in our fee.
You would need to weigh each side separately on a scale............most trailers are NOT the same from side to side. That can make them pull unevenly. You might contact the maker of the pin box and see if they have anything else to offer, but I see this question from time to time and I have never heard of any other reason IF the pin box isn't the problem being bound up in one direction.
The only want you will solve this issue it to remove that and go to a non-pivoting pin box. You asked for a cause and I have you several. Most you either can't correct or checked out. So, you can live with this or buy a new pin box. End of story.......and no offense taken. We don't give answers based on whether or not it is popular or not.......just based on facts and years of experience. I never sell those and this is one reason why. Hope that clarifies this some for you.
Rare for that............and we addressed the pin box being off center. Let me know if you resolve it using any of these other than replacement. Always open to something I haven't seen or heard of before.