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Am I calculating this hitch force correctly?


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My Carriage trailer has the "normal" rv upper 5th wheel plate and king pin location stuck out ahead of where it mounts to the frame of the trailer for pickup towing.

The king pin centerline is 13-3/4 inches ahead of the center of the frame extending down from the trailer that the "pin box" bolts to. 

The pin weight road ready is 3400lbs

Is this correct:  (13.75/12) x 3400 = 3895.8 lbs-ft of torsion on the mounting frame ?   Plus 3400lbs vertical? 

Does the draw pin force - that is the force applied through the pin to move the trailer or decelerate the trailer - remain equal whether the pin is forward or straigth down? 

The frame is no doubt designed to withstand these forces...

But when I smash over a bridge deck or hit a crossways hole/dip at speed (see Interstate 5 north of Grapevine, CA)  I imagine in my pea brain those forces are being multiplied by the lever the king pin is on the end of...

I have room to use a straight up and down pin set up.  My reason for changing would be for stress relief for the trailer frame.  The 13-3/4" of length change wouldn't matter much either way. 

p.s.  yep - I know what suspension 5th wheels (hitches) do.  I'm asking about leveraged force (or not) on the trailer frame. 




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So what happens to the applied forces if you simply “reverse” an extended pin box. By reverse I mean instead of extending forward you flip it 180 degrees and have it closer to the trailer. Sure would “tighten it up to the truck!  I seem to remember someone doing that a while back. Probably wouldn’t work for everyone due to “spacing” issues. 

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but do paulted cats land on dere feet?

but - if you build a rig aiming for 65 ft 120 inches overall length, 

and you place a suspension hitch 9 feet back of the singled axle what falls all the way into every dip and hole

does the acceleration moment imparted to the trailer on the end of the lever upswing increase by the dababanel factor of the lever length?

does placing the hitch over the rear axle reduce the moment of vertical  acceleration/travel of the hitch to only what the frame of tow vehicle travels at the axle. Sorry "axel" in internet spelling? 

I'm going to a bike shop not to ponder the above ^. 

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I was a Vice President once in a company full of engineers who liked to have "fun" Being able to steer funding to "fun" projects" helped too.


Here's a $1,500 fifth wheel pin with an internal strain gauge measuring the forces on that pin. We built several of these.


And here's the graph of the forces on the pin while braking pretty aggressively. What the pinbox sees is not very pretty, these are the back and forth forces in addition to the vertical pin weight. With trailer brakes working (the above graph) the maximum peak force was 2,420 pounds on a 8,000 lb fifth (32 footer) or approximately 1/3G

With trailer brakes disconnected (on purpose) we saw this.


3,450 pounds on a 8,000 lb fifth or almost 1/2G

So the pinbox is likely to see lot more forces from the braking and tugging forces that from the vertical gravity force.

1/2 a G on 18K trailer is 9,000 pounds, 1/3 G is 6,000 pounds. I have a Carriage and they were built superbly and their frames were not Lippert. So I wouldn't sweat the pros or cons on the frame, their welding was good, so if you want less length it's a good option.

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phoenix that is great info.

You do know if you brought that pin to market with an interface to dash screen #4 real time it would sell to rv’rsz...

I left the Mor/Ryde pin I installed from won ton towing times on the trailer. It reduced chuckling of the ton truck by a large factor. Now that the towmobile is much closer in weight to the trailer I’m thinking it may be detracting from ride quality. 

My truck had a simple brake controller when I got it. There were some interesting dynamics going on during braking events between air brake take up and power and T controller deciding whether or not to apply trailer brakes...the flexible pin added to the fun according to the slider tracks in the dust on the pin box..

The Hayes control has settled things down to way more like normal...

The fifth wheel “hitch” is solid mount at the moment. 

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noteven, been there done that.




The pin was communicating to a pretty impressive piece of electronics also mounted in the fifth which you can see in the advertisement. You can also see that there was a dash mounted control unit which would communicate to the fifth's system over the RV yellow wire (auxiliary line) to check on the system's performance, calibrations and sensitivity. The system was quiescent (ignoring the pin output) until it saw brake light, then it would "go to work". There was probably $50K of software written for the system (few months) but it was for "fun", meaning we were producing other products that "really" made money. One of the software challenges was how do you deal with "this".


This is the "mess" that occurs as a result of interaction between the truck and trailer and the interaction of two different weights (truck GVW and trailer GVW), two different wheelbases and two different suspensions, pretty stiff spring suspension (with a helper spring) on the truck, and a fairly soft suspension (rubber torsion bars) on the fifth. The "smart" engineers I had working on this came with this assessment, remember, this was done with brake controller on the fifth disabled, aggressive stop from about 50 mph to dead stop (the time interval was about four second, each "tick" on the bottom line is a second) :

1. Truck brakes are applied, the trailer comes forward and applies forward force to the pin, pretty sharp shock force of about 2,000 pounds.

2. At the peak the truck springs rebound and send the force backwards into the trailer suspension.

3. The torsion bar suspension on the trailer says "no, I've had enough" and send it back forward.

4. The truck suspension sends it back and then there are couple more interactions

5. Truck and trailer are slowing down and the force diminishes.

6. Truck stops there is one more peak as all the springs relax and the force goes away.

Process this so far and in the next post I'll tell you how we dealt with it (if you are curious)?

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Phoenix- next chapter please - I was the guy who would spoil the bs session about how wonderful the big Cat or ISX or Deeetroit was pullin by asking stupid chit like “If your 700hp tuner engine accelerated your rig to 60mph in 2/3 of a mile, how many hp does the brake system have to stop your rig in 400 feet? 20,000?” 

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OK, those of you who "studied" calculus should remember the terms, derivatives, integral, area under the curve, etc.

Seeing how non-linear the forces that govern the interaction between the truck and trailer are and how "nasty" they were, we realized that this was like holding a tiger by the tail and trying to "influence" it's behavior. We noticed that you could see 2,000 pounds of force in the forward (positive) direction and couple hundred milliseconds later almost as much tugging backwards (negative vector). The system was certainly fast enough to react to whatever it saw from the strain gauge in the pin, certainly we didn't wanted it to react to all those spikes on and off, we needed "solid and linear" output. Since it was all on the positive side of the y axis all we needed to do was to "smooth it out". Hence the calculus which averaged all the areas under the curve and came out with a descending line representing average of all this "nonsense". It was an interesting algorithm, not much of a challenge to the guys with MIT degrees we had working for us.

The next challenge was this. What happens when you stop and the force on the pin goes away and yet you want to have brakes to keep things from rolling forward or back. As long as the brake light was on we supplied a "holding force" to the trailer brakes. Then "how much of that force"? The power electronics portion of the circuitry was capable of amps but over extended period  would overheat once the heat sinks were no longer "heat sinking" and needed to cool down. Yes we solved that too.

The bottom line it was an incredible exercise, but this braking system was going to cost $2,000, not a very competitive situation against cheap ass stuff from Tekonsha for under hundred bucks.

But things did "evolve further".   

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