Frame camber - anyone familiar with it? (not axle camber)

[comfun1]

The end of January I took my 27rr in to the dealer for some warranty work. It had a couple minor items and a couple more difficult problems. The two bigger problems were the outside tread on the right rear tire wearing out and the sheet metal buckling at the top on the left side of the trailer. After the service department examined it they make the minor repairs and then take measurements and correspond with Lippert. Lippert is shipping a new axle. After they contact Forest River, Forest River ships a new piece of sheet metal. Apparently this is no easy task since it is one long continuous piece and the trailer is 34'-6" long. They told me it comes crated and is delivered on a semi and takes a number of men to carefully unload it. At any rate, the axle is being shipped and the metal is at the dealership. I talked to the service department yesterday and they are waiting for Forest River to send someone from the factory to look at the trailer because they think there is something wrong with the camber of the trailer frame. I understand how axle camber is changed but I am having difficulty understanding how frame camber can be modified. Has anyone heard of this before? I am starting to get a little worried, it is beginning to sound compicated.

[geotex1]

I am familiar with it, but your issue is not due to design camber but due to frame deflection. Can it be corrected - possibly. Not the place on a treatise about engineering, but as long as Lippert is confident the frame has sufficient strength and there is no current structural distress beyond the obvious aesthetics, the deformation can be corrected and the frame stiffened and reinforced. Removing the deflection is the easy part - just requires a level working platform, jacks, and careful measurements. Keeping the deformation from returning will require stiffening and reinforcement. That may be as simple as installing vertical stiffeners at regular intervals or may require adding some thickness to the web of the frame rails to change bending behavior. Let them look at it and tell you what they're going to do. Further, understand what they plan to do because the added weight of reinforcement and stiffeners could eat into CCC. Regardless of what their approach is, I'd have a much longer written warranty on the frame done as part of the deal.

[comfun1]

What is "CCC"?

[youroo]

Since your unit is a (Toy Hauler) they will be looking at (Rear Overload Situation)! Just be ready! Youroo!!

[Mr. Dan]

Originally Posted by comfun1

What is "CCC"?

Cargo Carrying Capacity...
from Glossary of Terms
http://www.forestriverforums.com/for...rms-11651.html

[geotex1]

Cargo Carrying Capacity. (EDIT: Mr. Dan is a 30-sec faster typist than I)

Good call youroo - I didn't recognize the model number and didn't look it up either. However, you are correct and overloading the garage can definitely cause excess deformation of the twin rails! I see the CCC of the model is only about 2,000# so after ~1,000# is eaten up by usual move in and ready to camp items doesn't leave a lot left to park in the garage.

[comfun1]

Interesting, before this trailer I had a 21rr and the sheet metal buckled on the top of it also. It did it on both sides but not to the point of creasing the metal. When I took it in for warranty they said the metal had not been stapled at the factory.

[geotex1]

Before OP you think the worst, do understand that frames are often designed and built to have quite a bit of flexibility. Especially longer ones. The problem can then be what is built on top of the frame may not have the same flexibility. It's what we engineers call strain incompatibility. So, maybe your frame flex is perfectly within design tolerance, but the aluminum siding cannot take it. The top of the wall is where the greatest tension forms as the frame bends over the axles and a single length of siding gives absolutely no expansion.

[comfun1]

I understand the frame is going to flex but it appears to me that when hitting dips or going over railroad tracks or other such imperfections in the road, the axles will be forced upward and the portion of the frame directly above the axles will be deflected upwards. To me this would seem to result in a stretching force on the uppermost sheet metal. When you look at the picture the opposite seems to be happening, the metal has buckled as though it is too long. Also I shot this picture in the summer when it was hot. When I took the camper to the dealer the buckles were gone, only sign of damage left was the creased metal beside the slide. It seems to me the metal needs to be free to expand and contract with temperature changes much like vinyl siding. I'm not certain why the top section is the only strip of metal affected. Perhaps the top section is under the most stress. I appreciate everyone's responses and look forward to them with interest. I'm also wondering if anyone else has had this problem?

[09grizzly1]

The top piece is the only one that is the full length of the trailer on that side. We had a trailer that had vertical strips that allowed for expansion, contraction and general movement, so the top piece was 3 pieces(vertical strips were above the ends of the slide).

[comfun1]

I contacted my dealer yesterday to see how things were coming along and this is what I found out. The two minor problems, refrigerator door and rear screen door, were fixed. The new axle has been installed and they are in the process of installing the replacement metal. The service department said that because the metal was very thin and over 30' long that it took 8-12 men to install it. He said one slip and it would end up in the trash. When I asked about the frame camber and the factory rep he said he didn't know anything about that but thought the trailer would be done by the end of the week. I know the service department is big, (supposed to be the largest in Oklahoma) perhaps this explains the breakdown in communication. Hoping all is well.

[OldCoot]

Can someone explain what frame camber is?

[comfun1]

I have tried to find this out myself, maybe others can help. The best explanation I can understand from reading about it is as follows. If you see a large empty flat bed semi trailer, they often are arched upwards. This is positive camber. When the appropriate load is applied, ( dozer, excavator etc.) the "arch" straightens out.

The problem I have understanding this is how this is different than an arched leaf spring with a load.

[OldCoot]

Originally Posted by comfun1

I have tried to find this out myself, maybe others can help. The best explanation I can understand from reading about it is as follows. If you see a large empty flat bed semi trailer, they often are arched upwards. This is positive camber. When the appropriate load is applied, ( dozer, excavator etc.) the "arch" straightens out.

The problem I have understanding this is how this is different than an arched leaf spring with a load.

I had never heard it called camber before. Learn something every day.

[DDC]

Never seen or heard about camber on an RV before, large trailers that carry thousands of pounds are laid out differently, i.e., the wheels are at the back and the trailer and the bed flattens with the load, not so on a fifth wheel.

[geotex1]

I missed OC's question earlier... OK cambering a beam is giving it a crown (positive camber) by basically prestressing the beam so that deflection under load basically flattens out the beam versus inducing a sag (negative camber) in it. Camber actually does nothing at all for structural capacity. It's basically an aesthetic consideration that provides engineering efficiency because you can use a smaller cambered member versus using a heavy straight one to achieve the same deflection limit.

Now as a structural engineer, you are taught you don't camber short beams (less than ~25 foot), shallow sections (web less than ~12" depth), beams used in moment frames, and beams that are cantilevered. Mechanical engineers, where crossing the domain of structurals (yeah, I know), challenge these rules somewhat because they depart from AISC standards and are working with fabricated beams (like trailer frames).

As the OP raises concern, and why structurals avoid camber in cantilevers is because of a spring-like effect. It's not at all the action of a leaf spring where spring steel behavior is different (and can take the cyclic deflection), but is the eccentricity once loaded that causes a greater reaction at the rigid support that over-stresses - liken to why the really, really large fellow shouldn't use the diving board at the pool.

With a trailer frame, when hitched, each frame rail can be treated a simply support beam with the pin corresponding to the coupled hitch and the roller corresponding to the axles. The length of span from axle to bumper gets interesting in the case of a toy hauler where the greatest load could actually occur versus in the cabin (and the span between coupler and axle where typically cambered). So, in the case of a toy hauler garage frame, things get interesting if camber is induced, CCC is not stellar and the owner goes for using all of it (and often more) to stuff the garage.

Lippert does camber several of the longer camper frames. They have to because of the shallow beams they are using for frame rails to conserve weight and clearance. However, I wouldn't expect it for a toy hauler frame.

[comfun1]

I got a call from my dealer today, my camper is done. I was mistaken about Forest River coming to look at it, it was Lippert that came to check the frame camber. Apparently, Lippert furnishes the axles and builds the frames. All I was able to find out about it was that they did some welding on the frame. He did tell me that frame camber problems don't happen very often. Perhaps when I go to pick it up I will be able to find out more. I believe this is the first Grey Wolf Toyhauler with a slide. Perhaps this caused it to be weaker on the slide side. At any rate I am good to go camping again!

[OldCoot]

Originally Posted by geotex1

I missed OC's question earlier... OK cambering a beam is giving it a crown (positive camber) by basically prestressing the beam so that deflection under load basically flattens out the beam versus inducing a sag (negative camber) in it...

Always heard it called "pre-stressing", not camber.