Fifth wheel frame test
With all of the posts about the frame around the fifth wheel pin boxes failing, I decided to do a test on ours.
But first, some background. What I have seen from a few posters seems to indicate that the very front crossmember under the front of the fifth wheel is flexing upward, which in turn puts a torque action on the rear crossmember, the one that the rear of the pin box assembly is welded to. That flexing is causing the metal in the crossmember to fatigue, and crack, sometimes with disasterous results.
One poster posted pictures of how the very front crossmember failed completely, and the pin box pushed everything upwards, the bedroom floor, the bed, and actually tore the interior finish panels right off the walls.
So my test is concentrating on that front crossmember, and how much it flexes. I ran a string under the front cap from side to side, and pulled it tight, taping it at both ends. It made an excellent straight-edge to take measurements from.
I then took measurements of the gap between the string and the bottom of the front cap, at each side of the pin box. I measured with the fifth wheel lowered onto the truck, and with the camper raised and the weight removed from the truck. The difference in the measurements was 1/8th of an inch, which doesn't seem too bad. Obviously, if the frame was built like a tank, there should be no difference, but we all know how these things are built!
As the bottom of the front cap has a natural curve to it, there is a permanent gap of about 1/2 inch between the string and the bottom of the cap.
The purpose of the test was to establish a base line. As our camper is quite new, and it hasn't seen many miles yet, I am assuming the frame is still good, so a 1/8th inch flex is my base line measurement. And the permanent gap of 1/2 inch is another base line.
I can now check it regularly, and if either measurement starts to increase, I know to be alert. If the 1/2 inch gap starts to increase, I know that the front crossmember may be bending. If the 1/8th inch differenece between loaded and unloaded starts to increase, I may have a cracked front crossmember, which would allow more flex.
It is inconceivable that we should have to be doing such things, as our campers should have been built properly from the beginning, but we know that is not the case. It might be a slight inconvenience to have to do such checks, but it makes me feel a lot better.