Flexing 5th wheel pinbox

I have 2010 Cyclone 3950. The pin box seems to flex or twist forward and up an unusual amount when we are towing the unit. I am wondering if anyone else is having this problem. I am starting to think the frame may be too weak or is cracking and allowing the excessive movement.

18 inch pinbox flexes up about 1.5 inches.
 

caissiel

Senior Member
I was told that the flex of around 3/8" to 5/8" is nornal. Above that I would start to worry.
My previous SOB trailer had a lot of flexing and I discovered that at the factory they had burned a hole at the wrong place and it caused the front beam to fail and all the welds failed. They had a repair recall on the models larger then mine but mine was excluded so I had to do the repair myself.
 
Do you mean 3/8 to 5/8 vertically between the pinbox and the front cap? That is about what mine is doing. Tomorrow I will look into a shorter pinbox to give it less leverage.
 

kkamshop

Well-known member
I would have it checked for broken welds. We had a similar problem and we could see movement of the corner brace in the front closet when lowering the pin onto the hitch.
 

caissiel

Senior Member
Most trailers will be designed to have 1 in for every 120 inches to 1 " for every 240 inches of flex that is actualy normal on building structural steel application. Steel has to flex because if it does not it will be way to heavy.

The 5th wheel extension is about 60 inches so having a 1/2" flex would be at its worst. But the pin box should not tilt upward as its welded solid on the frame member. I have been watching mine closely and so far it looks great.

I have seen SOB trailers with cracks under the bedroom slide and they were ok with that, that's why there is a seam on HL side under the bedroom slide.
 
That amount of flex is dependent on the tensile strength or quality of the steel used by Lippert. I am quite sure Lippert and Heartland will not spend any more than absolutely necessary for the steel used in these frames. It's like the axles on these units. We had an alignment done because of severe tire wear and the shop could not believe how soft the steel was in the axle tubes. Next time I will look for quality over amenity in an RV. It won't be a Heartland!

With the weight on the pin of our Toyhauler I would think there would be quite a bit of flex with the cheap steel used in construction. Especially with an 18" long pinbox. Too much flex for me so I am ordering a, short as I can get, pinbox.
 
Top