Flatbed Trailer bow

[bentwings]

Ok I'm not truck driver except my DCTD dually. But here is my question..
Why are large flat bed trailers bowed up inthe middle.???
I worked in a trailer fab shop for a while and there was about a 4 inch bow put in flat beds. I asked the question then and got more BS answers than about any other question asked.

After this stint, I waded thru Mech Eng school for 5 years and now 25 years later the question is still unanswered. I also did a bunch of structural analysis and it all works out the same......It simply makes the trailer flat when loaded. I guess this is better looking than bowed down inthe middle. It also maybe provides a guide to how heavy the load is with out weighing.....ie flat equals maximum load. If load is concentrated in the middle. Now if you look at Rail road tankers you will see the older ones are pot bellied. I think this is from fatigue of the rail system. Rail stuff really takes a beating. I see new ones straight as ruler but the same type 20 years later is potbellied.

Any takers??

 

[scrappy]

Ok I'm not truck driver except my DCTD dually. But here is my question..
Why are large flat bed trailers bowed up inthe middle.???
I worked in a trailer fab shop for a while and there was about a 4 inch bow put in flat beds. I asked the question then and got more BS answers than about any other question asked.

After this stint, I waded thru Mech Eng school for 5 years and now 25 years later the question is still unanswered. I also did a bunch of structural analysis and it all works out the same......It simply makes the trailer flat when loaded. I guess this is better looking than bowed down inthe middle. It also maybe provides a guide to how heavy the load is with out weighing.....ie flat equals maximum load. If load is concentrated in the middle. Now if you look at Rail road tankers you will see the older ones are pot bellied. I think this is from fatigue of the rail system. Rail stuff really takes a beating. I see new ones straight as ruler but the same type 20 years later is potbellied.

Any takers??

My guess is so that the trailer deck is flat when loaded beyond that I have no idea.

 

[bentwings]

I agree That's about the most logical reason. How about if the bow is gone so is the trailer....she's done over stressed past the yield point. Get rid of it for non road use

 

[EMD Diesel Power]

Its the same reason that highway/rail bridges are bowed slightly in the center.... Most appear straight... but if you look real close, they're raised in the center.

Has to do with loading dynamics and stuff. I havent really quite figured it all out either, but basically I've figured that an upward curve beam that is loaded upon.... will go into compression (squeezed together).... as it straightens out to a level stance.

Whereas a level or swaybacked beam will go into tension (pulling apart) as its loaded.

Basically a swaybacked or level when empty flatbed is at or very near the end of its useful life.... load capacity has probably been reduced at that point and will fail if overloaded beyond its original rated capacity. (the safety factor is gone)

 

[PackRat]

Simple physics.

Take a yard stick, and lay it across 2 books(desks, chairs, whatever).

Push in the middle. Flexes easily.

Drill a hole near each end of the yardstick, and tie a string to one end. Flex the yardstick, and tie it, so it has about 2 inches of bow in the middle, when relaxed against the string.

Place the yardstick across the same books(desks, chairs, etc), with the bow up.

Press down in the middle. Much stronger.

Picture the yardstick as the trailer deck, and the string, as the frame beneath.

Try and figure out how to create the same effect, with the yardstick flat, or bowed down. It is much more complicated, and therefore, less efficent, less cost effective, and requires more material(read weight), for similar strength.

Hope this clears things up, and makes sense.

 

[Diezel_Cowboy]

Ok I'm not truck driver except my DCTD dually. But here is my question..
Why are large flat bed trailers bowed up inthe middle.???
I worked in a trailer fab shop for a while and there was about a 4 inch bow put in flat beds. I asked the question then and got more BS answers than about any other question asked.

After this stint, I waded thru Mech Eng school for 5 years and now 25 years later the question is still unanswered. I also did a bunch of structural analysis and it all works out the same......It simply makes the trailer flat when loaded. I guess this is better looking than bowed down inthe middle. It also maybe provides a guide to how heavy the load is with out weighing.....ie flat equals maximum load. If load is concentrated in the middle. Now if you look at Rail road tankers you will see the older ones are pot bellied. I think this is from fatigue of the rail system. Rail stuff really takes a beating. I see new ones straight as ruler but the same type 20 years later is potbellied.

Any takers??

I find it very hard to believe that you went through 5 years of Mechanical Engineering and don't know the answer to this question!
Maybe you should go back to college to learn how to be a better TROLL!

 

[TPCDrafting]

Packrat is right. It's called beam camber. It is actually used in structural steel design in many buildings as well. Many of these cambered trailers you will also see that the trailer axles are spread apart farther than normal as well. This spread doesn't really give the trailer any more capacity, but the DOT will allow heavier loads on this style for bridge and road rating reasons. I also would say that with the spread axles and camber that an unloaded trailer would experience less wear on the front trailer axle and able to make sharper turns w/o sidewall and tread wear on the tires.

 

[bentwings]

I understand the structrual beam, the lower member is put in tension and is stong enough to prevent fatigue , yield to limit, and rupture. No problem there. Good example with the yard stick. The string represents the tension member and related to trailers it is the lower member of the usual pair of lengthwise beams. As long as this member especially is not over stressed or forced to yield the trailer will still have an upward bow if designed with that feature. (just to eliminate the totally flat ones). Now if the beam is deep enough there will be quite a large value of stiffness as a result of this member shape by itself (the web or the vertical member). Then by adding the lower tensioning member it becomes even greater. The upper member is in compression which has quite a high rupture strength also. More important with wood structures such as bridges. Leave that for now.

To some extent there is flex built in otherwise the trailer may fracture if the rear wheels go over something which causes only one corner to be supported. (extreme twist) I would guess that tying things down is a bit of a trick to maintain some flexibility yet secure the load. Someone said there are specific number of binders and placements depending on the load.

The part about axel spacing and the DOT is interesting. I suppose you can liken it to troops spreading out and not walking in step on bridges. Makes sense. I learned something today. As they say..behind every regulation is an incident. Here's one.. how about the cement trucks that look like caterpilars?? I know this a load carring thing but I would guess that the DOT guy would have a coronary if one showed up at a regulated bridge as you suggest.

The axel placement relating to turns...I have seen some with the front wheels barely touching the ground so when they turn sharp they skid more or simply pick up due to the small twist in the trailer. less weight better mileage, Good I guess. I saw one the other day that had a hydraulic lift on the front axels. I always thought the placement was for handling on the long stretches of road. Aren't they called "west coast" trailers?? As for turning sharper..does not the trailer become more out of track as the wheel base is extended (only the far rear wheels on the ground). The mid position axels on the hotrod trailers make them pretty manueravable in tight places considering their length.

good discussion. Thanks

 

[Diezel_Cowboy]

Bentwings,
Although I find it hard to believe that you are a mechanical engineer and cant figure this out, I will give a few more reasons that the trailer has a bow in it.

First of all the main reason is that the bow adds strength, the weakest point in a suspended constant thickness beam is in its center, the slight bow in the center of a trailer adds strength to the center and compensates for the increased moment in the center.

The other main reason is that flat trailers often haul the entire load, a steel coil for example, in the very middle of the trailer. They do so to distribute the weight of the load equally between the trailer and tractor axles. When hauling anything on a trailer the placement of the load on the trailer is very important to distribute the load to all axles for stability and handling purposes.
Therefore the center of the trailer has to be strong and light so it has a bow in it to add the necessary strength.

 

[TPCDrafting]

Thanks for the insight Copernicus

 

[apextrans]

This principal applies not only to flatbeds, but is the theory behind modern (current edition) car haulers as well. The bottom rail of a car carrier trailer is the main support for the entire trailer system. The trailers are built with an intentional bow inward in the bottom rail. It's done so that as the trailer is loaded down those rails flex out to carry the weight. The main reason for this practice, which I guess applies to flatbeds also, is weight savings. Car Carriers are one of the heaviest unloaded trucks one the road (my empty weight is 45,200lbs.) They build these trucks lighter every year by using thiner wall tubing. The only way to retain the strengh is to bow the main support rails. If you ever get a chance, look at a car carrier up close when it's empty. The bottom rail should look bent inward.

My trucks are very easy on tires loaded or empty, so I don't have the tracking issues a flatbed might have, but I can't turn any sharper then 90*, tractor in relation to the trailer due to the fifth wheel placement. Most of the tire damage I've seen in truckstops is from jacking VERY hard to make a tight turn.

Just my .02

 

[Flagship]

If you start with a flat trailer and load it and it sags in the middle, you'll spend more time being weighed by portable scales, operated by people who should but don't know better, than you will driving. Have the trailer cambered, and you'll look legal and won't be bothered.

All that bowing was started in Eastern Ohio by Ravens and East for the steel haulers back in the late 70's.

I was there.

 

[Hoss6.9]

Trailer Arch...

Hey Guys!

When a trailer no longer has arch left in it and it begins to belly down... It is NOT the end of its useful life. That's part of what trailer shops are for. For a nominal fee they can "re-arch" a trailer and put it back in shape again.... There are some trailers that won't take re-arching as well as others either. Probably due to the age, material it's made of, the way the trailer has been used throughout it's life, etc.... I have a 1978 Ravens all aluminum flat that I had re-arched a number of years back for about $1,000.00 that is still in use today and looks like it has as much or more arch than a new one.... Granted that I don't go put a 50,000#+ donut (coil) in the middle and have it concentrated in four feet everyday but, it handles whatever I throw at it alright..... And yes - if your wagon is leveled and bellying down going down the road - then you are Motor Carrier bait..... And I'm sure that the portables will have your name on them soon....

And as far as the wide-spread suspension goes. They are not just California trailers. Not all states recognize a wide spread for more weight. They do distribute the load out over a larger apce and they also ride tremendously better when it is an air ride suspension. Most trailers with an A/R spread also have individual dump valves on each axle for making turning and the likes easier. For example - When making a normal turn on the highway... You blow off the front axle and it will make the trailer turn easier. Less drag on the lead axle and easier on the tires and when you are backing in somewhere - Dock, Drive, etc... Then your would blow off the back axle and the trailer will pivot on the front axle and back like a MUCH shorter trailer.... I love my wide spreads... Indiana does not recognize a wide spread but, Michigan does. So, since i am so close to the Michigan line - It makes sense for me to have them here.

Hoss....