King Pin Load for Semi-Trailer

[mechengineer18]

What is the typical hitch weight (percentage of trailer weight) placed on the king pin of a semi-trailer? I know gooseneck trailers use around 20% but it seems like semis must carry more based on how far rearward their axles are.

Thanks for the help,
Mike

 

[TenPenny]

I think the typical goal is 45% on hitch pin, 55% on trailer wheels. But obviously, that's not the going to happen all the time.

 

[minerk]

It depends on a lot of factors, but the biggest concern is getting the axle weight distribution right. You can put more weight on the tractor than you can on the trailer, so unless you've got an unusually heavy tractor the split is going to go the other way (55% hitch, 45% trailer). Most states allow 12,000 lbs on a steering axle and 34,000 lbs on a tandem. So with an 80,000 lb gross weight, the tractor would have to have 12k + 34k = 46k gross, while the trailer would have the remaining 34k. When I ran a hopper bottom we would usually split the net load into 28k in the front hopper and 26k in the rear hopper.

 

[zeusfaber]

Two rules of thumb for stepframe trailers:

1. When fully laden, you can expect the fifth wheel to bear roughly the unladen weight of the trailer, and the trailer axles to bear roughly the weight of the load (of course, each contributes to both, but the totals come out about that way).

2. It's incredibly hard to get a load far enough forward to exploit the full weightbearing capacity of the fifth wheel. You will almost always exceed the trailer axle weight well before you reach the theoretical maximum gross combination weight. In various parts of the world, I've seen armoured vehicles driven insane distances up trailer swan-necks to try and beat this one.

For other semis, it depends a lot on the load. The vast majority of those that run around the UK have bulked out long before they reach any weight limit.

A.

 

[rmw]

Minerk,

Good explanation but your weights are loads to the ground not king pin loads. If your tractor weighed 12K (to pick an easy number to work with) then the king pin load would be 34K which is the same as the trailer load to the rear suspension (neglecting the unsprung weight of the axles, etc.)

ME18, it is not on a percentage basis as tractor and/or trailer configurations can change drastically. Minerk's explanation is changed just by choosing a trailer with a 10 ft 4 in spread rear axle which allows 20K to the ground per axle in most states. That doesn't allow any more gross weight, just more trailer weight, less king pin weight for an almost identical rig. It is often almost impossible to get enough weight on a king pin to fully load the tractor so that it will axle out and the spread tandem trailer allows more weight on the trailer so that the tractor is not so critical.

Add axles to either the tractor or trailer or add jeeps or boosters, it all changes.

Bottom line, it has to be figured on a rig specific basis.

 

[zeusfaber]

Have a look at:

http://www.broshuis.nl/pages/uk/ukhome.html

and click on the bottom button on the left hand side.

A.

 

[mechengineer18]

All,

Thank you very much for the valuable insight.

Mike

 

[ccw]

From RMW: "Bottom line, it has to be figured on a rig specific basis."

Another important consideration is to keep heavy down force on the fifth-wheel and tractor bogies at all times. It is required to develop towing traction and directional control. This is particularly important in off-road scenarios. High CG trailer loads on steep inclines can cause problems, otherwise.

 

[rmw]

The scariest vehicle in the world to drive is a bobtail tractor in traffic on a rain slicked road. Believe me.

rmw