Originally Posted by KTM Guy
While I definitely don't want to get into a argument, unless I missed something in high school physics, all three Sidewinder products increase the lever (or moment arm) acting on the 5er frame. All three models extend the distance between the pin-box attachment to the trailer frame and the hitch that carries the weight. That creates a longer lever. You are absolutely right that they move the pivot point rearward. But that doesn't mitigate their greater stress on the frame.
Also there is a disadvantage to permanently moving the pivot point rearward. The beauty of a 5th wheel hitch is that placing the pivot point above the rear axle eliminates sway and porpoising caused by a pivot point at the rear bumper. The farther rearward from the axle that one moves the pivot point, the more susceptible the rig becomes to sway and porpoising. If one moved the 5er hitch pivot point all the way back to directly above the bumper, one would have exactly the same dynamics as a bumper-pull setup (except for the effects of the higher attachment point, which I'm not smart enough to predict).
Before I installed a sidewinder I would check with somebody in the engineering office of my trailer manufacturer.
I agree with the concern about sway when the pivot point is moved rearward, but that is something that no one reports experiencing. A close friend has a Sidewinder on his fiver (installed at the factory), he has a long bed truck so if he experienced any sway he would remove it, but instead absolutely loves the unit. 20 inches behind the axle is apparently not nearly as bad as +48 inches for a TT. Porpoising shouldn't be an issue since the pin weight is still carried at the axle. Only thing I can think of is that seperating the pivot point and the weight point results in better towing characteristics than a TT setup.
I disagree that the Sidewinder increases the lever arm, at least in my case it wouldn't. My Lippert 1621 extended pin box has the same length, king pin to mounting bracket as the Sidewinder replacement. I don't know about other setups, there are six different Sidewinder pin boxes, but lengths are not published and I don't know the lengths of other factory approved extended pin boxes so that info doesn't help me anyway. But it does not make sense for Reese to recommend a Sidewinder with a longer pin box than a factory approved pin box since even the factory approved length is no longer necessary.
I do agree that the owner should never use a longer pin box of any type than what the factory approves as the maximum length. That includes a gooseneck extension (adaptor) as well since it increases the lever arm, yet many owners use them without concern.