Temporary Archives >> Towing and Hauling

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jarmstrong
Member
Member # 17179
Reged: 10/26/01
Posts: 758
Loc: massachusetts
why do they call them 5th wheels?
#757947 - 06/04/02 04:32 PM

dumb question to you guys i know but i don't know why
thanks
jason

Popperboy1
Member
Member # 18267
Reged: 12/13/01
Posts: 1511
Loc: Homer Glen, IL
Re: why do they call them 5th wheels? new
#757982 - 06/04/02 05:09 PM

They get their name from how they connect to the truck. The early tractor trucks only had 4 wheels on the ground, not ten like now, so the hitch on the truck is the "fifth" wheel because it looks like a wheel - big and almost round. On our size trucks, the "fifth" wheel is about one foot across. It works by clamping a 2 inch pin in a pair of jaws, with the weight of the front of the trailer setting flat on the hitch. This type of hitch supports up to 1/2 of the trailer weight (CAUTION: do not exceed your rear axle rating), while a frame mounted bumper style hitch can only support about 1/10 of the trailer weight unless an equalizing hitch is used.

Christopher Moehle
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Member # 5312
Reged: 02/10/00
Posts: 23
Loc: San Antonio, TX
Re: why do they call them 5th wheels? new
#758005 - 06/04/02 05:28 PM

Popperboy1, you're right, but it goes back a bit further, 1874: Fifth Wheel , definition 1--unless you're talking about a person, definition 2.



jeffrall
Member
Member # 12754
Reged: 04/08/01
Posts: 391
Loc: Spring, Tx
Re: why do they call them 5th wheels? new
#758290 - 06/04/02 10:08 PM

Popperboy
You are sort of right. As the definition states it is a "fifth wheel" because it acts like a fifth wheel to support the trailer. If that "wheel" where not there the trailer would nose dive. A conventional "four wheeled" trailer can usually support itself if it is unhooked and the jack is stowed.

Just my .02

Jeff

tgdmo
Member
Member # 12631
Reged: 04/04/01
Posts: 129
Loc: Missouri
Re: why do they call them 5th wheels? new
#758316 - 06/04/02 10:40 PM

On early four wheel horse drawn wagons the assembly that allowed the front axle to turn had a round wheel shaped component, the fifth wheel. It was lubricated with the same stuff used on the wheels, lard I think

Tim

Popperboy1
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Member # 18267
Reged: 12/13/01
Posts: 1511
Loc: Homer Glen, IL
Re: why do they call them 5th wheels? new
#758817 - 06/05/02 01:14 PM

jeffrall, I said:
In reply to:

It works by clamping a 2 inch pin in a pair of jaws, with the weight of the front of the trailer setting flat on the hitch. This type of hitch supports up to 1/2 of the trailer weight (CAUTION: do not exceed your rear axle rating)



Why was I only sort of right?

jeffrall
Member
Member # 12754
Reged: 04/08/01
Posts: 391
Loc: Spring, Tx
Re: why do they call them 5th wheels? new
#759295 - 06/05/02 10:29 PM

Popperboy
Two things
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.

Second
As I said in my earlier post if you go to the dictionary link that Christopher Moehle posted and read the fifth wheel definition : a horizontal wheel or segment of a wheel that consists of two parts rotating on each other above the fore axle of a carriage and that forms support to prevent tipping b : a similar coupling between tractor and trailer of a semitrailer. I take that to mean a "fifth wheel" on a trailer to keep it from tipping but I do see how it can also be interperated as part of the tow vehicle.

So I guess we were both right now that I read the definition again. But I still dissagree on the ammount of weight that is supported by the hitch

Jeff

Popperboy1
Member
Member # 18267
Reged: 12/13/01
Posts: 1511
Loc: Homer Glen, IL
Re: why do they call them 5th wheels? new
#759363 - 06/05/02 11:23 PM

jeffrall said:
In reply to:

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.

DezRacer
Member
Member # 21152
Reged: 04/18/02
Posts: 272
Loc: Orange County, California
Re: why do they call them 5th wheels? new
#760460 - 06/06/02 11:11 PM

Regarding the semi, they are 80,000 lbs gvwr. 12,000 allowed on axle 1, 34,000 on axle 2,3, and 34,000 on 4 and 5.

Popperboy1
Member
Member # 18267
Reged: 12/13/01
Posts: 1511
Loc: Homer Glen, IL
Re: why do they call them 5th wheels? new
#760829 - 06/07/02 10:50 AM

Thanks, DezRacer. What's a couple of tens of thousand pounds among friends .

34,000 for the front of the trailer and 34,000 for the back of the trailer. Looks like about half on each end to me, if you don't worry too much about how much weight the rear of the tractor weighs.

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