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Semi Based Trailer - Hitch placement question


les_garten

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I am looking at the Volvos like the 780 and the Semi Trailer based builds from Forks and Spacecraft.

If you are going to do a smart Car Hauler 780 with the Semi Trailer Kingpin placement.

How far back(Feet) do you have to move the hitch for the smart car, considering the Semi Trailer kingpin?

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The pins are the same and the builder has much more to consider in the placement to ensure there is balance. There is a spread sheet somewhere on the site that allows you to put in numbers to determine the balance. My pin is much closer to the front of my trailer than standard and I have a long stinger behind my axle. I have no problems with my setup, but have plans to decrease the length of the stinger in the near future. There will be a learning curve again finding the correct lines and how the trailer will turn. I will keep the front of the trailer to the back of my drom box at least 60 inches apart. I think that's pretty close to the magic number. (it's also listed somewhere on the site).

 

Good luck with your search and builds

 

Rod

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The kingpins on the Spacecraft I have looked at had a 3 foot setback.

That is correct and the usual placement. Wyatt was able to alter mine to accommodate my hitch location on the truck when the trailer was built. With the Spacecraft you ask for something and they are usually able to make it happen.

 

Rod

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use a squared plus b squared = c squared to calculate the corner radius of the trailer. This is the distance from king pin to the corner of the trailer. This will tell you the arc of clearance the trailer needs from the pin as it swings over the frame ahead of the fifth wheel aka "hitch".

 

Depending on the shape of the nose of the trailer you will also need to add some distance for "dip clearance" as the tractor and trailer bend in the horizontal plane going through dips and ditches and the World's Steepest Approaches at fuel stations. The taller the object on the truck (Smart Car, drom box, sleeper,) the more dip clearance you need.

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As the others have said, Spacecraft can position the kingpin with a great deal of latitude. But it must be PLANNED. Weight distribution must be maintained. You are looking for 25% on the pin, +/-. To get that, axle position and knowing what is in the trailer is important. It is totally a different process that a "normal semi-trailer". You must design for it. In all cases, the pin will be straight down, unlike a conventional RV pin. It will always be under the nose and never forward extending. I seem to recall we could get it within 14" of the front nose, but only Marsha and Wyatt could really say.

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Others will know more about this as well, But an ET hitch might be able to be used still. Rod uses one with his semi RV, and there are several with pin weights close to or above the 8k mark. With what we talked about with Space Craft, we would be 47', 26k lbs and have a 7k pin on a regular 5th wheel build. These are rough numbers but something to think about.

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Very interesting replies from guys who know, thanx!

 

If I were to dream up a rig, it would be:

 

Volvo 780 >> Mini Cooper Lengthwise on he truck >> 48-49' Semi trailer

 

My conundrums are how to get the mini up and over the Hitch, and how to get it as short as possible all things considered.

 

I was thinking leaving the tractor tandom'd

 

I was hoping the Kingpin could go forward a few feet

 

How doable would that setup be?

 

I'm wondering how a deck works with a Semi type Trailer since the box is so close?

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Very interesting replies from guys who know, thanx!

 

If I were to dream up a rig, it would be:

 

Volvo 780 >> Mini Cooper Lengthwise on he truck >> 48-49' Semi trailer

 

My conundrums are how to get the mini up and over the Hitch, and how to get it as short as possible all things considered.

 

I was thinking leaving the tractor tandom'd

 

I was hoping the Kingpin could go forward a few feet

 

How doable would that setup be?

 

I'm wondering how a deck works with a Semi type Trailer since the box is so close?

Conundrum solved we had a marine anchor hauler KW with a folding knuckle-boom crane right behind the cab and the driver would tow-bar tow a company one ton pickup behind the truck when it had a large anchor(s) on the bed but as soon as the anchors were unloaded he would just hoist the spreader-bar over the pickup and eye-sling the four corners of the pickup and fly-cast the pickup on the truck bed , strap the pickup and spreader to the bed fold the boom behind the cab and then finish his cigar....no sweat.

 

Or......just drive the mini into the back garage of your custom toy hauler....forget loading to truck with anything but Dr Pepper.....no sweat.

 

This RV thing is a piece of cake....just bring some $$$$...?

 

Drive on....(Load with care....)

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I'll add that especially for your configuration, I would be adamant to get a tractor with a minimum 13,300 # front axle and to avoid the Volvo D16 engine for its very high weight and generally being more problem prone. The standard 780 wheelbase of 229" will serve you well carrying the nose heavy MINI.

 

Thanx for that info!

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Can you guys provide some numbers to help me mull this over.

 

On a Volvo 780:

 

1) What is the distance from the edge of the rear fairings to the average 5er Hitch when it is between the rear Tandem axles?

 

2) What is the distance from the Back of the Cab(Drom area or depth) to the Hitch when it is between the rear axles?

 

3) What is the minimum distance needed from a New Type Smart car to the pin of say an ET hitch for most folks with 5ers.

 

I figure some of you guys will have these right on the tip of your tongues.

 

I'll most likely end up with a Smart car, but I'm trying to figure out all my options and compromises that need to be done.

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You've fallen into the same trap as many of us. We want to take it all with us, and still have a large enough living space.

 

As mentioned above, a toy hauler solves many issues, but takes a chunk of your living area, especially if the car is on board.

 

A 47'-48' trailer will only be legal in a handful of states, and difficult to get into campgrounds.

 

Eventually, you will need to prioritise, dividing things into a "want" columm and a "need" columm. Then adjust to fit how far you want to open your wallet.

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For some numbers on a 780 you can look at the build album of my truck.It has some measures on the second photo. LINK.

 

An ET set up properly can handle some of the Spacecraft semi's. "it just depends". But you can run in the 8-9K area without an issue. At 25% on the pin that allows you a 32-36K lb trailer. Anything much over that and you are really outside of the optimum for the ET.

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You've fallen into the same trap as many of us. We want to take it all with us, and still have a large enough living space.

 

As mentioned above, a toy hauler solves many issues, but takes a chunk of your living area, especially if the car is on board.

 

A 47'-48' trailer will only be legal in a handful of states, and difficult to get into campgrounds.

 

Eventually, you will need to prioritise, dividing things into a "want" columm and a "need" columm. Then adjust to fit how far you want to open your wallet.

 

I realize a lot of that, but I'm starting large and whittling from there. I need the base numbers to start doing drawings though and mulling things over.

 

I would quote RVHaulers here as well.

 

" Most of our customers are Over length"

 

Campgrounds will be limited for sure.

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For some numbers on a 780 you can look at the build album of my truck.It has some measures on the second photo. LINK.

 

An ET set up properly can handle some of the Spacecraft semi's. "it just depends". But you can run in the 8-9K area without an issue. At 25% on the pin that allows you a 32-36K lb trailer. Anything much over that and you are really outside of the optimum for the ET.

 

 

Thanx for the link, don't suppose you know the numbers for the Volvo before the bed?

 

I just used the ET as an example because I thought more people might know the base numbers. I expect to be I might end up heavy enough I would have to stay with the commercial hitch.

 

If I can get the baseline numbers I would be able to see where I'll end up with my 2 options.

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Unfortunately Volvo 780's come in multiple wheel base depending on how they were spec. While a lot might be the same, there can be differences. This is the same for other brands too. This makes giving you an exact answer to some of your questions hard to do.

 

Regarding hitch pin placement on the trailer, you will get more trailer length by putting the pin farther back from the nose of the trailer.

 

Most typical 5th wheels have an extended pin box which moves the pin out in front of the trailer. When basing bed design on this type of pin, you basically allow 51" (1/2" of the typical 102" trailer width) for swing of trailer to 90 degrees plus what ever clearance you want to have. With the Semi king pin say set back 3' from nose you have to allow 62-1/2" (see formula in Noteven's post). So you have to allow just under 1 foot extra space but get around 3 more feet of trailer. If your pin weight is excessive where you have to use the commercial hitch, then you may be limited on how far back behind the axles you can place the hitch. Better to start with a longer wheel base truck and get a heavier rated front axle.

 

Dave

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Also regarding the mini cooper, there was someone on the forum a while back that had one that was building on bed for it on a long nose freightliner. I think he was also trying to put the car at an angle to shorten it up some but was having trouble finding the maximum angle that was allowed for the motor. I believe he had some health issues and sold the HDT before it was finished. You might find the postings by doing a search.

 

With a low sitting car like a mini, you also have to watch your break over clearance which might mean longer ramps and then where do you store them.

 

You can build a bed that will allow you to drive over the hitch but then it won't work with a typical semi pin arrangement on the trailer. I know because I am having that issue with the Space craft trailer we are designing.

 

Dave

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Also regarding the mini cooper, there was someone on the forum a while back that had one that was building on bed for it on a long nose freightliner. I think he was also trying to put the car at an angle to shorten it up some but was having trouble finding the maximum angle that was allowed for the motor. I believe he had some health issues and sold the HDT before it was finished. You might find the postings by doing a search.

 

With a low sitting car like a mini, you also have to watch your break over clearance which might mean longer ramps and then where do you store them.

 

You can build a bed that will allow you to drive over the hitch but then it won't work with a typical semi pin arrangement on the trailer. I know because I am having that issue with the Space craft trailer we are designing.

 

Dave

 

 

I know I'll end up with a Smart Car, because I've been doing all these ruminations for a while, but I keep "wanting" to make it work out somehow with a bit larger vehicle!

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You can actually build a 48' trailer and be legal IF you forgo a deck. The average length limit used is 65 feet, a 48' trailer gives you 17' to work with. Most trucks have a published BBC measurement. That is the distance in inches from the front bumper to the imaginary back of a day cab or the end of the sleeper. A Volvo 780 is 189" that leaves you with a very close 15", perhaps a 47 0r even 47.5' trailer would be a better fit with that cab. A deeper pin 48-50" will give you a trailer with little to no nose swing. In theory a 16" gap would be enough but, you would be right at cab extensions and be real careful on driveways. There are a number of trucks running tight kin pin settings where the trailer is just a few inches behind the cab extenders. You would also has a lower drag profile and get better mileage. The trucks with close coupled trailers generally run 22-24" gaps, that gives you enough wiggle room in a high angle ramp. If you are really set on a 48' trailer a air slide fifth wheel with a forward stop at 65' would solve the problem. You slide the trailer forward if you come across a situation where length may be an issue?

 

Steve

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