5th Wheel Hich Position?

A fifth-wheel trailer is one that has a hitch that connects to the towing vehicle at a point above the rear axle of the towing vehicle - not at a point on the rear of the vehicle as with a pull-behind trailer.
For example, this is a 5th wheel:
http://www.theironworker.com/Fifth%20Wheel%20Trailer.jpg
My question concerns position of the point of attachment, let's say, the position of the king pin. For normal pulling of the trailer, where is the best point to have the king pin? Should it be directly over the axle of the towing vehicle or should it be ahead of the axle? If ahead, by how much? In what ways does handling change if the position of the king pin is changed?
This page http://www.etrailer.com/faq/fifthwheelfaq.asp discusses using a slider arrangement to move the king pin for maneuverability. It doesn' t discuss the optimum position for towing.
Instructions that came with a hitch say to mount 0 to 6 inches ahead of the axle, but provides no further information.
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Each truck's manufacturer has their own specifications as to the proper location. Many say to position the center 2" - 4" forward of the rear axle while others specify a position directly over the axle.
You may wish to peruse the installation instructions on the Reese website for hitch rails similar to the ones you are interested in on the make and model truck that you have.
http://www.reeseprod.com/support/support/su_install.shtml#5thwheel will take you directly to their support page.
The reason for the placement of the hitch centerpoint above or very slightly ahead of the rear axle is to allow that portion of the weight of the trailer that rests upon the hitch to be transferred to the rear axle without disturbing the balance of the truck. If the point of weight transfer falls behind the rear axle then the front (steering!) wheels are levered upwards with a corresponding reduction in vehicle control.
Doing a wheelie down the highway while pulling a trailer will attract LOTS of unwelcome attention! <G>

The sliding hitch mount is intended to provide adequate clearance, during tight turns, between the rear of the cab and the front of the trailer when a short (6' or less bed) wheelbase truck is used as a tow vehicle.

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RAM^3 wrote:

I would think that the weight of the engine would be one of the considering factors for hitch location. Such as the Ram hitch is over the axle, but my 94 GMC with a 350 gas engine is 3" forward. Both trucks are almost the same length and wheel base. The Ram clearance is 37" (pin to cab) but the 5th wheel is quite a bit wider. I do not know what the turn angle is, but it will be far less than 90deg. I might consider on installing a slider so I can gain cab clearance of a couple of inches at 90deg, that would be aproximately 11" plus the desired clearance. My rig is 96" wide, pin location is aproximately 1" forward of front cap. I know the Cumins engine is heavy but I dont think I would consider runnig down the road with the pin slid back.
Any suggestions?
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BILL P.

2004, 2500 SLT Quad Cab, Dodge Ram,
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wrote: <snip>

hey Wild Bill... how ya been??
We decided not to get a fiver, but researched them for quite awhile... most folks said that the extra $100 or so for the sliding hitch was "cheap insurance" for when they had a tight spot to get in or out of... Most had never needed it, but used it a few times to see how it worked..
A lot depends on the truck config.... the longer the bed, the less chance of needing one, etc...
mac
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mac davis wrote:

Life is great how is it with you?

Mac, you are correct here, but I got what I got and it would be cheaper to get the hitch slider than a new truck, and I would hope that I would be like the rest of them and not need it.

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BILL P.

2004, 2500 SLT Quad Cab, Dodge Ram,
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wrote: <snip>

We're doing really well.. no major illness, still married, getting ready for an rv trip to the delta today.. (Delta Bay)

Well, we all know that the longer the wheel base, the better the TV is supposed to be... The trade off is that if you get a 2500 or 3500 quad cab with long bed, you need some place to park it.. *g*
Our rv buddies with an f250 4 door and 36' fiver are semi-full time, and have only needed to slide the hitch back once... he figured that he could get it out without sliding, but it was going to take a lot of cutting back and forth to get around the lug nut that pulled in across from him during the night and left his trailer hitched with the truck sticking out in the street..
mac
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Greetings,
Different vehicle manufacturers (as well as the hitch manufacturers) will have varying recommendations. You are going to have to read your owner's manual as well as the help and installation pages from the hitch manufacturer's web sites for the best information for the set-up you have.
As for handling, the farther forward the king pin is in front of the rear axle, the more weight is transfered forward to the front wheels. Admittedly 6" may not sound like a whole lot, but the only way to determine what feels best to you when towing your particular combination is trial and error, which is much easier with a sliding set-up.. A change of an inch or two could make your steering feel heavier or lighter, or give you too much rear end "squirm". If you go to a newsgroup like alt.rv I can almost guarantee that the folks there will have more than a few things to say about this!
Depending on the length of the bed it most likely effect your turning radius as well. This is especially important with a short-bed truck. The last thing you want is to have the front corner of the trailer crush your back window and back edge of your roof while you are taking a tight turn. This is why you see so many forward-offset kingpin setups on the larger 5th wheel trailers these days.
Cheers - Jonathan

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My hitch is directly over the rear axle. Vehicle is a '99 F350 CC 4x4 8' bed. Hitch weight is 2300 lbs. Handling is transparent. With the 5th wheel attached, the truck is level. 5th wheel axles have been reversed to match the truck.
: A fifth-wheel trailer is one that has a hitch that connects to the : towing vehicle at a point above the rear axle of the towing vehicle - : not at a point on the rear of the vehicle as with a pull-behind trailer. : For example, this is a 5th wheel: :
http://www.theironworker.com/Fifth%20Wheel%20Trailer.jpg
: : My question concerns position of the point of attachment, let's say, the : position of the king pin. For normal pulling of the trailer, where is : the best point to have the king pin? Should it be directly over the axle : of the towing vehicle or should it be ahead of the axle? If ahead, by : how much? In what ways does handling change if the position of the king : pin is changed? : : This page http://www.etrailer.com/faq/fifthwheelfaq.asp discusses using : a slider arrangement to move the king pin for maneuverability. It doesn' : t discuss the optimum position for towing. : : Instructions that came with a hitch say to mount 0 to 6 inches ahead of : the axle, but provides no further information. : : -- : |||||||||||||||| Nehmo Sergheyev |||||||||||||||| :
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I towed a 30' fifth wheel with a RAM 2500 with an 8' bed for ten years. The hitch was positioned so that the king pin was directly over the rear axle and the handling was great. The trailer weight was ~8,000 lbs.
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PcolaPhil


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There is a formula which can be used to accurately calculate the position of the fifth wheel, but you need to have a few details about the trailer and tow vehicle.
You need to know how much weight is transferred onto the fifth wheel when laden by the trailer.
You need to know the unladen weight of the towing vehicle and the unladen weight on each axle. You also need to know the maximum weight allowed on each axle.
You need to know the wheelbase of the towing vehicle.
Loadbase is the distance from the fifth wheel to the front axle
The formula is
weight transferred onto front axle (wheelbase / loadbase)* weight transferred onto the fifth wheel
You can the play around with the "loadbase" number to achieve the optimum weight distribution onto the front and rear axles. Ideally you want to have weight transferred onto the front and rear axles in the same ratio as the manufacturer's maximum gross weights. eg max weight on steer of 3 tonnes and 6 tonnes on the rear indicates a 1:2 ratio.
We used extensions of this basic formula to achieve maximum loading on B Double fuel tankers with 8 compartments with varying fuel density to ensure we max loaded to legal limts every load.
If you use pounds then use inches, if you use kilogrammes use centimetres. Dont mix your avoirdupois with your metrics.

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