arched or cambered trailer frames 3

[Tmoose]

We were having a discussion today about the purpose of cambered main rails on trailers that haul around heavy equipment.
One fellow felt that the beam was stiffer as a result of the camber.
I thought it would have little or no effect on stiffnes unless a tension member went from end to end. So the arch is to end up with a flat floor or good ground clearance.

 

[SomeYahoo]

Sorry JAE, but I have to disagree. If a long container was placed on a flat trailer, the container would not push the trailer down such that the container was point loaded on the ends. If there is no contact, there is no load to push it down and there is no end point loading.

On the flip side of that coin, if a flat-bottom container is placed on a cambered-beam trailer and does not have the weight to load it to flat, the container will be point loaded in the middle (one point only). This would be far more unstable no matter how many tie-downs you use and, in addition, shipping containers are designed to be loaded at the corners and not in the middle. You would likely damage the container.

Just my opinion.

 

[rmw]

Regarding contact with the (platform) trailer, normally, in this country, at least, where possible, the load does not actually contact the trailer, but is either placed on wooden beams, or special load carrying devices (such as are used with the aforementioned steel coil.)

It is not uncommon to see long pipe or tubing loads that are supported strategically at the point above or near the fifty wheel, and above or near the centerline between the suspension attachment points. This some time makes the camber of the trailer rails obvious, because the straighness of the long load highlights the "arch" of the cambered rails.

For the purpose of this conversation, the word "arch" applys to the fact that a trailer frame is cambered, not ARCH in the sense of what structural engineers think of. There is no capstone in the middle of a trailer.

Good discussion. Good answers. Thanks to the structural participants for answering the call.

rmw

 

[Ron]

Loads on trailers rarely are configured such that they actually span anything. They are most often a series of point loads, even if they are long items such as pipes and the like. Further, they contribute nothing to the stiffness or structural resistance of the load/trailer deck interface. As such, the reaction points are only the 5th wheel and the axle(s), leaving free deflection in the middle. The use of a cambered main frame allows smaller, lighter members to be used thus decreasing costs and weight. As JAE noted, the deflection characteristics don't change, its just where they start that changes. That's important for the points mentioned about aesthetics and the need to reduce the "notice" factor.

 

[JAE]

Gensetguy - I would respectfully disagree with you.

ANY beam loaded with twin point loads, say at the third points or quarter points would ALWAYS deflect downward in a reverse arc - this is simple statics and mechanics of a beam.

Two point loads DO make a downward arch, which WOULD produce a gap below a rigid element on the bed. I can get the actual deflection formula if you'd like to see it.(don't have it handy right now)

But the element would have to be rigid enough to span across the distance. This would produce a condition where the supported load would have to either span the distance between the ends (if it was rigid enough) or sag along with the trailer until the "span" of the loaded element was reduced such that it would still span over a smaller gap.

I believe you are correct about the reverse condition, where the camber is not quite overcome by the load and the element is supported in the center, or probably end up with one end in the air.

But the subsequent comments are also valid, I think, in the multiple loaded pieces are the norm.

Perhaps someone else could comment on another thought I had - that the camber keeps the hitch mechanism in a proper angle to maintain the connection? Any validity in that?

 

[flamby]

My doubts..

I am not fully sure that it is an Industry-wide accepted practice to camber the truck/wagon/trailer beams. There is not any clear advantage and at the same time cambering is a costly exercise. It is also difficult to believe that the deflection is actually perceptible to an observer (cop)just by looking at the loaded trucks.

Tmoose can verify whether it is indeed so that these trailer beams are invariably cambered.

 

[Ron]

JAE....the "hitch" mechanism of a 5th wheel hitch can rotate over 15 degrees, so it should be able to accommodate most any level of deflection.

flame...the deflection, particularly in aluminum trailers, can be quite pronounced under load when the smaller sections are used. Comparing older trailers to newer ones shows the significant size reduction in the main frames. Further, older trailers were 34 to 40 feet in length while newer ones are 40-45 feet long, have slide-adjustable axle trucks, and must often endure the fatigue of 500,000 road miles or more.

Container trailers are usually much shorter and stiffer than the trailers discussed. They are designed for the containers and are built for short distance logistics, not long-haul, over the road considerations. I doubt you would see much if any deflection in a "container" trailer.

 

[SomeYahoo]

JAE - After thinking about it last night, I now have to agree with you. I was thinking about a conatiner that spanned the entire length of the trailer (fifth wheel to axles) and that is very unlikely. Containers of this type have specialized trailers.

Speaking with an engineer who has experience in this design (dealing with the regulations and such), at least in the case of a low-boy trailer, the reason is ground clearance. There is a minimum required clearance (from the DOT) and if you are under it, you are ticketed. Another vote for ground clearance.

 

[rmw]

There are other issues that involve ground clearance other than dimensions dictated by regulation and inspectors "eyeballs" and that is rail road track clearance. In the USA, at least, a commercial driver license (CDL) holder can get a major 'ding' on his driving record (along with the monetary fine) if he/she high centers on a Rail Road crossing. I can attest to this as a CDL holder, and I have to be aware of that proviso should I choose to operate a commercial motor vehicle, which I do from time to time.

Now, understanding the industry is part of the answer, too. In the truck transportation industry in most developed countries of the world, weight, that is to say 'payload' capacity is EVERYTHING!!!! Therefore any design consideration (arched or cambered beams) that would produce a smaller, and hence, lighter frame member is a feature that would set you apart from the competition.

And, Ron, not to be picky, but the truck/trailers in our country can be expected to go several millions of miles before they find the scrap heap. Your point is excellent otherwise in emphasing that they have to be built tough.

Flame, I have carefully used the word 'platform' trailers in these posts, because, I, like you, am unaware of any cambering of the frames on other than flat beds, low boys, drop decks, etc. And, not every style of platform trailer out there has a cambered frame. Pulp wood hauling trailers, for example, have to be built so stout in order to survive the rigors of going into the woods to be loaded, that some of the lightweight designs where you find the cambered beams would never survive the twisting and torsional loads put on the frame members.

The cambered beams are especially prevelent in trailers that are designed to carry point loads. This would include the 50,000 steel coil I keep mentioning, or a low boy that may have a huge transformer sitting in the middle of a long span.

Those parts of the discussion I can vouch for, the conversation regarding the engineering behind reason for the cambering is what I am benefiting from here.

rmw

 

[TheTick]

Ground clearance doesn't really matter for a regular flatbed trailer, though, and these are still cambered.

I'll concede that I may not have the correct reasoning behind arching trailers. All I remember is that we spent about half of a mechanics of materials class discussing it. There was more than aesthetics at stake.

 

[patprimmer]

I know from personal experience that drivers judge the weight on their truck by both spring and trailer bed deflection.

It is a lot easier to determine by simple visual inspection that the trailer is still arched or flat. It is a lot harder to determine it is sagged to a predetermined level.

Many trucks are loaded where no scales are available, so the drivers ability to estimate the load is really an important issue, at least in rural Australia

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

"Tmoose can verify whether it is indeed so that these trailer beams are invariably cambered."

All I know about trailers is what I see drivin' down the road. I would have said I've seen plenty of the real long ones that are cambered, whether they are up high, or close to ground to haul a 'dozer. The beam/frame I was talking about would be handling the load all alone, without a cable or rod tension strut paced several inches off the bottom of the rail, like an old trolley.

Actually the cambered trailer was an example thrown into the tail end of a conversation. I had "invited" myself into it when I thought I heard My boss and the new project engineer discussing cambering as a means of stiffening a part of a simple part.

I had thought a bit about almost the same topic almost 20 years earlier. We were thinking of using bowed rafters for the house we were planning to build. I was talking to an outfit that offers them as part of kits. The proprietor or maybe his wife tossed out a catchy statement something like "curved parts are stonger. Always were. Always will be." As it looked (and looks) to me like there is little or no arch action in a typical framed roof whether the rafters are bowed or straight, I researched the subject a little more. I believe According to the "Timber Construction manual" laminated wooden rafters or beams are actually a little weaker in bending than straight laminated beams. The reason is because each of the laminating strips were straight before being bent cold and glued together. For the rest of their life they are trying to straighten out, but are prevented from doing so by the convex/concave glue joint. So the extreme fibers on the concave face are in tension when the rafter/beam is just sitting there, and adding real load to the convex side adds to their non-zero starting tension.

 

[pba]

There is a lot of talk in the above about cambering being expensive. I know nothing about vehicle trailers but find this hard to believe. Even if a significant cost could be associated with cambering it is only one process among the many required to form a trailer.

I'd guess that cambering offers SOME material savings, SOME benefit to the 'laymen' who have to load the trailers, and SOME improvements in the way ground clearance is achieved (if not the actual ground clearance required.

Another thought, a cambered surface will shed water...

 

[SomeYahoo]

The cambered beams are actually three plates. One with a double arch (think of the waste in drop), a top plate, and a bottom plate. The center web is cut and the top and bottom flange are heated or in some other way bent around the arced web. Then (making sure it is square to the world) the three pieces are welded up (most of the time they are stitched).

This seems a lot more expensive than an extruded I-Beam to me.

 

[zeusfaber]

"This seems a lot more expensive than an extruded I-Beam to me."

True, but for lots of trailer designs, a rolled section isn't ideal anyway - fabricating a beam allows the shape and dimensions to be tailored at will (and varied along the length of the beam).

Imagine the junction between the gooseneck and main deck on a stepframe trailer - this is a real stress point. With fabricated rails, you can make it so that the web is one continuous piece of metal all round the transition. With rolled section, you're looking at having to put welds in all the wrong places.

Similarly, with fabricated raves, you can build in details like wheel arches without compromising the structure too badly.

A.

 

[zeusfaber]

Rather belately, I had a bit of a trickle of brains to the head.

So far, we've talked a lot about big individual loads.

Driving home the other evening, I passed a trailer carrying a large number of tall boxes, each pushed flush against the next. It struck me that any sag in the trailer would make these loads touch and fret at the top, while separating them at the bottom making them hard to secure.

Maybe not the primary reason for the camber, but certainly one to make you glad it's there.

A.

 

[TM9design]

Trailer frame main rails are cambered by taking a rosebud on the oxy/acy torch and heating the the rails in about the middle, this brings the ends up when it cools. Then when the rest of the camper is built, or the car/equip. whatever is loaded on the frame,it goes to a fairly straight shape.
It's not done by mech. bending as far as I know. They just typically use the principle of heating a member on one side only to red hot and this makes it settle into curve.
Unless it is a really light trailer, carrying a load on it w/out camber would probably make the ends to sag downward visibly.
I have seen this done to re-straighten(re-camber) a large mobile home frame by welding beads along frame rails with a stick welder, you see a lot of this in Elkhart Indiana-believe me!

 

[Lookin4a6FigSalary]

Hi Guys,

I'm a newbie here, so I hope I don't step on too many technical toes in this message.

I would like to respectfully disagree with all of the given reasons why trailer frames are cambered. I think the real answer is something else.

Everything wears out. Tires wear, brake linings wear, brake drums wear out, light bulbs burn out, paint wears out, and believe it or not...trailer frames wear out! Trailer manufacturers realize that over extended time/miles/bumps/overloading the frame rails will tend to relax. So to make their frames last longer, they are built with a positive camber. The more camber, the longer the service life of the trailer.

Having some engineering background, having worked for a small volume trailer manufacturer and having been around the trucking industry, all cause me to believe in my above stated opinion regarding trailer camber.

In the trucking industry, a flatbed trailer that has lost its camber is called a "sway-back". I think the term is a carry over from the old west describing a horse with a bad back. Sway-back trailers are not very valuable/useful/desirable in the trucking industry, ie the service life is out of them.

Many times trailer frames are made out of T1 steel which typically comes in plate form. T1 is much stronger than A36 steel which I beams are commonly formed from. So to make a strong trailer frame, 3 pieces of T1 steel are used, the top flange, the web, and the bottom flange. A good trailer manufacurer welds all 3 together with a certain amount of camber. Trailers made from A36 I beams are not strong and are heavy. Heavy equals less payload. However on some applications like the skeleton frame under container boxes the frames do not carry much weight and therefore can be made out of the cheapest steel available (usually A36). Container boxes are self-supporting and focus their weight to the hard points in the corners which are typically immediately above the fifth wheel and rear axles of the trailer hence the trailer frame bares no weight.

As you notice flatbed trailers on the road, look to see how they are loaded. Let me give you an example using bricks. I good driver will have 2 piles of bricks on his trailer. The first bunch will be as far foward as possible (above the fifth wheel) and the second bunch as far back as possible (above the trailer axles). This method of loading puts the least amount of load on the middle of the trailer which stresses the trailer frame the least which causes the trailer to last longer. A poor driver puts all the bricks in one bunch and in the middle, stressing the frame the most and "wears out" the frame slightly faster than the good driver.

Lastly, being somewhat associated with the trucking industry in the past, you guys have seriously overestimated law enforcements ability to detect overweighted commerical vehicles. Weighmasters and police are really inexperienced regarding this topic. With cambered trailers and air ride suspensions, it is almost impossible to detect an overloaded vehicle. The only thing to look for is the amount of tires are squatting...and this is difficult thing to do visually. And with cb radios, cell phones, and truckstop gossip, many overloaded trucks bypass scales and/or pass by scales when they are closed. The overloading issue is furthermore confused because different states have differing weight laws. The whole thing is a colossial governmental reglatory mess, ie tax dollars at work.

Anyway, these are my thoughts, I could be wrong, (heck...most of the time I am wrong) but I think the bending/cycle fatigue/service life of the frame rails is the primary purpose for the camber.

Wayne
bulldozermaki@gmail.com

 

[GregLocock]

The camber of the frames will have no (significant) effect on their fatigue life.

Cheers

Greg Locock

 

[zeusfaber]

Wayne,

I'm glad to see it's not just here that regulatory enforcement is a mess.

For Heavy (Special Types) haulage in the UK, you have to give a couple of working days' notice of your load and route.

Once you've got clearance on that, you're not allowed to deviate from that route - and the legend says that at least one haulier has got away with telling the traffic policeman that this prevents him from diverting to the nearest weighbridge.

A.

 

[venom37083]

Hey

Good topic! I work at a company were we build mobile home frames, and it is a similiar process, with the same goals in mind as a tractor trailer frame. Adding camber to a frame does stiffen the metal if it is burned in with a welder in a arched position. And having the camber in the frame helps distibute the weight across the frame( how much would be hard to say). Having camber in a frame allows that beam to hold heavier loads than it would if it was staight. By cambering a frame you can get a stronger frame for less money. As far as the placement of the load on a frame, consider the wear and tear on it and the truck. Some loads even though they may be smaller can still have quite a bit of weight(bricks for ex.), could ride different from one frame to another. The camber of the frame should be a issue for clearance except for maybe on a lowboy. Just some thoughts for you to consider. I'm a newbie to this site,but I have enjoyed all of your committs.