Frame flex

[Jim55]

I’m the original poster and I appreciate all the replies. It has proven quite interesting.

Jim

[Baldicon]

Dealer said that the broken wall studs is usually a result of dropping the trailer onto the truck or hitting the back of the truck cab in a tight turn. Those of you that have gone through this frame flex problem, did any of you have either occur before you noticed the flex?

[Leftie Canuk]

Originally Posted by jetskier

To further elaborate on my previous post...the reason the studs are breaking there is the re-entrant vertical force on the aluminum wall stud caused by the discontinuity of the wall panel at this location. Even though this is not a place of maximum moment in the frame, there is upward curvature in the steel frame when the trailer is hitched up. The measured relative deflection at the front of the slide opening from this curvature creates a force that is too much for the aluminum stud framing and it's connection to resist. After many fatigue cycles, the aluminum fractures and busts out the siding panel. Having a continuous horizontal framing member in the wall panel below the slide accomplishes a few things. 1) It provides a boundary member on the bottom of the slide opening to resist moments, shears and deflection, 2) it removes the re-entrant forces at the lower front corner and 3) it provides more length to make the connection of the wall to the frame at the slide. This is likely the reason for the 2020 design change under the slide.

And yes, if you haven't figured it out, I am a structural engineer.

Jetskier: Thank you for your explanation !
I am interested in your thoughts on two aspects of this frame flex issue: (1) Where is the greatest moment on the frame when hitched up, in your opinion, and (2) in your opinion, do air ride pin boxes /fifth wheel hitches help to reduce moment, or do they change the location of the greatest moment ? I think a lot of forum members will be interested in your thoughts.

[jetskier]

Originally Posted by Leftie Canuk

Jetskier: Thank you for your explanation !
I am interested in your thoughts on two aspects of this frame flex issue: (1) Where is the greatest moment on the frame when hitched up, in your opinion, and (2) in your opinion, do air ride pin boxes /fifth wheel hitches help to reduce moment, or do they change the location of the greatest moment ? I think a lot of forum members will be interested in your thoughts.

Believe it or not, the maximum moment should occur in the frame section over the axles. If the frame's uniformly loaded along its entire length, the base of the neck is about 1/3 the moment of the moment over the axles on a 40' frame.



The air ride pin boxes are basically analogous to springs and shocks in the coupler. The air bag acts as the spring and provides stiffness and the shocks are the dampeners to motion of the spring. They absorb and dissipate energy. I can't see there being a huge reduction in forces applied to the frame when the frame is subjected to forces from emergency braking, hard acceleration from a stop or the jarring force from hitting a pothole. In most situations, it softens the forces between the tow vehicle and trailer and thus reduces the stress on the trailer frame. How much, who knows. You'd have to wire up the frame with a ton of transducers and do a lot of data collection and analysis. Sounds like a good Master's thesis project.

[SRD]

Originally Posted by Baldicon

Dealer said that the broken wall studs is usually a result of dropping the trailer onto the truck or hitting the back of the truck cab in a tight turn. Those of you that have gone through this frame flex problem, did any of you have either occur before you noticed the flex?

Hi Baldicon
We have owned our 2012 3100rl 5th wheel since 2014. We have not dropped it on the truck or hit it with our cab while backing up, so I would not put a lot stock in your dealers explanation.

Steve

[jeffba]

Originally Posted by Baldicon

Dealer said that the broken wall studs is usually a result of dropping the trailer onto the truck or hitting the back of the truck cab in a tight turn. Those of you that have gone through this frame flex problem, did any of you have either occur before you noticed the flex?

Dealer is full of BS

[Montana Man]

Originally Posted by Baldicon

Dealer said that the broken wall studs is usually a result of dropping the trailer onto the truck or hitting the back of the truck cab in a tight turn. Those of you that have gone through this frame flex problem, did any of you have either occur before you noticed the flex?

Sounds like to me the dealer is accustomed to finding ways to blame the owner, not the poor design. I wouldn't trust him for anything.

[richfaa]

The dealer did not lie although hitting the back of the truck cab in a tight turn would normally cause a lot more damage. Dropping' the RV on the truck would be the same as hitting a big pot hole.

[GHallof]

So gents. I do not intend to highjack the thread for my own purposes. I read through different postings/topics attempting to expand my knowledge as much as possible.

I have to say reading through this post caused me to have a bit of an OH CRAP moment for sure.

I recently noticed a slight bulge, PS under bed slide, just above basement door opening. See pics below. Based on what I have read. Is this what spells out the result of Frame Flex? How concerned should I be? Hopefully the pics show enough of the deformity.

[Hblick48]

That is not where frame flex usually manifests itself. Normally happens on the front bottom of the bedroom slide.

Looks like moisture has gotten into the luan causing it to swell. Look for places where water can get under the skin, probably at the lower right slide opening.

[Baldicon]

My suspect area is in front of the slide, but I have a different model. But in following this issue on the forum, I have not seen this problem at the rear of the slide.

[Reelcamper92027]

The photos that MikenDebbie posted, the 2nd two, just prove what our problem is. I have never hit my truck with the cap or dropped the trailer on my truck. Not even rough release from the hitch. I noticed a little separation in wall seam just under our bedroom slide last fall when I was going around the trailer and re-sealing a couple silicone locations. Winter prep. This spring didn’t look any different. Got the trailer ready for our first spring Florida of 2023.There and back 2500 miles. Back home I really did see much more of a separation. Doing some research, sound like the us owners are on our own.
Very good information. I am going to reach out the Keystone. We have been loyal customers for the past 4 5th wheels. First time anything this ever happened.

[phillyg]

Originally Posted by Jim55

Thanks for the quick responses. Do you have any idea if there are any statistics that would indicate which models this problem is prone to occur in?

Jim

Only realistic statistics could be provided by mfgrs, IMHO. Everything you see here is anecdotal.

[wadek01]

Originally Posted by bshgto

that`s funny mine has it, I fixed it cause camping world just kept looking at it with a question mark look on their face and finally said you can`t fix a vacuum bonded wall that strengthens the frame.
Many many new campers on facebook that have had or have it along with varing brands with the lippert frames. Many newer campers on RV Horror stories complain about this with some get fixed, some get fixed then breaks again, some never get fixed. Not until 2020 did they change the wall design on the nose that they think will fix this issue, but the jury is still out on this one as it has not proven themselves to do a correct redesign that is guarantied to work. I wouldn`t let this issue stop your purchase as said it is the luck of the draw or the roads you travel which I think is the real problem. Mine didn't start till I hit a pot hole that rattled my teeth. I would look for the redesign nose on the unit your interested in or just role the dice. Good luck, Lifes short, keep moving forward.

Can you describe how you fixed it? I’m considering going at it from the inside.