why do they call it a 5th wheel

A bit more detailed. A fifth wheel is not a wheel at all. It is a greased hinged bearing plate that connects power and load. 1) A horizontal ring or ring segment, consisting of two bands or plates that slide on each other, placed above the front axle of a carriage and designed to support the forepart of the body while allowing it to turn freely in the horizontal plane. 2) A similar device used as a coupling to connect a semi-trailer to a tractor. 5) a superfluous, unneeded, or unwanted person or thing. Although a relatively simple mechanical device, the fifth wheel occupies a pivotal position in the overall mechanical assembly of which it is a part, serving multiple functions simultaneously.

In the tractor and semi-trailer assembly, the fifth wheel is the connecting device on the tractor unit which grasps and locks onto the kingpin of the trailer unit. The fifth wheel permits horizontal motion: the kingpin swivels freely as the weight of the trailer rests on the flat bearing plate, allowing the truck and trailer to flex and negotiate turns. The fifth wheel also pivots in the vertical plane, permitting independent flexing of the two units over uneven or hilly road surfaces.

As a mechanical device, the fifth wheel is, therefore, both literally and figuratively pivotal. It rotates, permits rotation, connects, and bears weight. The near complete reversal of meaning as the mechanical device is translated into the social sphere is puzzling. We suggest that a reframing of the social expression "fifth wheel" --an unwanted or superfluous person-- is clearly in order. This site will, among other things, devote itself to this undertaking
First I have never seen a fifth wheel trailer that had half of its weight being supported by the tow vehicle, more like 18-25%MAX. I am talking R. V's now, not semi trailers, but I don't even think that they have 50% of their weight on the pin fully loaded.

I was refering to all 5th wheel setups, that's why I said "up to 50%". Most travel trailers are as you said in the under 25% range, with some down to 15% or so. As for as semi trucks, take a good look at how tractor trailers are set up.

Yes, they cary about 50% of the trailer weight on the truck's rear axles (you have to subtract the weight of the tractor's rear end from the trailer weight to get the exact amount of trailer weight it can cary) and the other half (about) of the trailer is on it's rear wheels. These dual wheel axles cary about 15,000 pounds each, with about 7,500 pounds on the single wheel front axle, depending on state highway weight limits per axle. Some trucks can cary twice that weight per axle, and you can add extra axles for super heavy weight.

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