why flat bed tow trucks use beds with a little bit of an arch to them – instead of being truly flat.
In: Engineering
Couple concepts needed to understand this. First, metal breaks when the stress gets too high. Loading a truck trailer puts stress on it, you can almost think of “load” and “stress” being nearly the same idea.
Second, stress goes in more than one direction. There is compression (squeezing) and tension (stretching). When you load a truck trailer and it sags, the top surface of it is being compressed and the bottom surface of it is being stretched.
So when the truck trailer is manufactured, it can be made with a stress already in it. I’m really going to simplify here: they put an upward load on the beams of the trailer as it is being made, and when it “solidifies” (i.e. the welds or the casting or whatever), the metal is stuck with that stress in it. The top surface is stretched and the bottom surface is compressed and it is bowed upwards. And this is the opposite of what a load on the trailer will do. So say you add a load to the trailer slowly – the trailer is pushed down and that upward bow is less and less – and the built-in stress in the metal is actually counteracting the stress of the load, so the overall stress is DECREASING. At some point the trailer is flat. You can continue to add weight and the trailer will bow downwards. At some point you reach whatever the safe limit is and you must stop. But the whole point is to allow a trailer to carry more weight than it normally could without the pre-stress.
edit: Thanks for the gold!
It’s built-in so the weight will still keep the arch pointing up, it’s not made to go flat or point down under design loads. If it’s overloaded the bow won’t come back up. It’s one visual cue that the trailer’s not ok.
Source: Father worked at trailer factory.
The built-in arch compensates for the sagging due to heavy loads on the bed. So when the load is applied, the bed of the truck wants to be flat, instead of sagging down. This is more important for large trucks than it is for pickup trucks, say, because of the length of the bed.
Paper makers call big loaded (against another big roll) crowned rolls, such that when the 2 rolls are loaded against each other they are “flat” all the way across.
The weight limit without special permit is 40,000 lbs. Many times it’s concentrated to a very small space in the middle. Were it not for the bend, the rear axle tires would raise due to the sag in the middle and would also lift the rear axle of the tractor causing traction problems.
Ah, a topic I can answer.
This is what I am currently driving- well, I’m in the dock but you get the point.
We have upwards of 100,000 pounds (45,000 kg) of steel/aluminum on our flatbeds. The arch is to provide weight support so the deck doesn’t bow under all that weight.
I think the OP asking about the actual curve in the bed at the back where the car pulls onto the bed. I’ve heard it called a whale tail. It allows sports cars that have a very small angle of attack to get onto the flatbed. If the bed was completely flat all those fancy under front bumper spoilers that are on sports cars would hit before the tires get onto the flatbed.
Pretty much all comments have it partially right. The crown of the trailer actually helps with the longevity of the trailer life as well. Each time a trailer is loaded the crown will become less and less until eventually the trailer is flat with no load. The more Crown the trailers manufactured with means generally the life of the trailer will be extended. This is dependent on what kinds of loads the trailer hauls as well.
it is for structural strength, and weight resistance. it is harder to break a arched structure than a flat one. so it can hold more weight than a flat one. when weight pushes the trailer flat its built in arch pushes up providing resistance. where as a flat trailer would bend downword instead of being flat when loaded. making it easier to break under the stress of travel if not imediatly when loaded. damns are arched for the same resion. what is the strongest structure? a flat surface or a arched one? an arched one. something the romans figured out a long long time ago.
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