Fifth Wheel Balance Question

I klnow that if a weight is placed behind the axle, the frame becomes a lever and reduces the pin weight - I also have a formula that calculates the effect on the pin weight depending on how far back the weight is placed. This question involves the effect of weight placed IN FRONT of the axle.

If I place a 100 lbs in the front of my fifth wheel (say in the bedroom), is that weight distributed between the axles and the pin? In other words, is the effect on the pin 50 lbs and is 50 lbs distributed to the coach axles? I know the towed weight increases by the actual weight but I am just interested in the effect on pin weight.

Interesting question, not sure i can help with the answer. But at this point I'm more curious about the "reasons behind" the question...

Think see-saw,,, exactly midpoint between axles and 5th wheel pin and equal weight is distributed to pin and axles, move forward and you get more on pin... move to rear and you get more on axles.

When I had a pin weight problem (induced by the adding of a generator and 2 T1275 Trojan batts and a Xantrex inverter/charger, I needed to know where the pin weight was coming from and how to load the rig. I did the math and determined the weight distribution in percentage at each foot behind the pin.

IMHO here is how it works. anything you put above the pin, or in a closet above and forward of the pin is added pound for pound to the pin load. In my rig, anything in the garage or battery compartment yields about 70% of its total weight to the pin and 30% to the rig, items in the front of the basement yield 65%. Toward the back of the basement the yield is about 55%, then less and less as you go to the middle of your axle assembly, where of course it is 0% contribution.

The answer to your question is; above the pin and forward, such as a closet, is 100% pin weight, plus an infinitesimally small %. This could only be calculated utilizing known weights with multiple significant figures beyond the decimal point, and could only be demonstrated in a laboratory.

I think the weight of the rear over hang is in play with weight add between the pin and the center of the axle. If the axles were at the very back then the increase would be quite linear. However with fixed weight behind the axles, the weight behind them effects the results to some extent. The formula should be axle based not just the space between the pin and axle center.

Now that we all are confused, it is time for dinner! Bye!

Chris

You are correct, it is simple leverage. Make an assumption that the pivot point is midway between the axles. Measure the distance of the pin from the center between the axles, call pd. Measure the distance of added stuff to the center between the axles, call it sd. The portion of weight added to or subtracted from the pin load is sd/pd times the weight of the added stuff. It does not matter how high or low the elevation of the added weight is.

SNOKING said:

I think the weight of the rear over hang is in play with weight add between the pin and the center of the axle. If the axles were at the very back then the increase would be quite linear. However with fixed weight behind the axles, the weight behind them effects the results to some extent. The formula should be axle based not just the space between the pin and axle center.

Now that we all are confused, it is time for dinner! Bye!

Chris

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My comment is based on the center distance of the axles.

Jim.Allison said:

My comment is based on the center distance of the axles.

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However does not account for the extension behind the axles. I believe that if you put 100 lbs half way between the axle center and the pin it will not add 50 lbs to the pin, as some of that 100 is used to over coming the teeter tarter effect of of the rear extension. Maybe that is what you are saying? Chris

Any mechanical/civil engineers in the audience? Sounds like Jim.Allison comes the closest because it sounds like he had a "trial and error" scenario. Still, what others are saying makes sense.

Makes me wish I'd paid attention in school ...

It is truly depends on individual trailer configuration...I've had lots and it really varies...my triple axle th is super pin heavy from the get go...and designed so since the garage (behind the triple axles) holds 3k lbs of cargo...

But generally anything on or above the pin is direct pin weight...lb for lb...as it moves back its less...

But if I have 500lbs above the pin and 2800 in garage I net about zero...my old single axle trailer was more sensitive to weigh (pendulum effect) and my old double less so...anymore I don't ponder...I just hit the cat scale...it's like 7$...lol

sent from space via an invisible beam from a flying metal dish

This is a question that will make many brains hurt but really is not bad. It is better shown in a diagram.

Here my treat. So far I have only made equations for front of the axle. I have not done for the whole trailer yet. It may not be perfect but I will work on the spreadsheet for you guys. It is only as accurate as the measurement but everyone should get a good idea of the added weights if I did everything right. It is in excel format

View attachment Weight calc front of axles.xlsx

Here is a site that has a calculator for trailers. Maybe does not address this issue however.

http://changingears.com/rv-sec-calc-trailer-weight-fw.shtml

From Escapees.

https://www.escapees.com/smartweigh/182-SmartWeighPamphlet-WEB-0415.pdf

Chris

To offset the weight of the gen and batteries, put the Beer on the couch. You should be good! LOL

Carleen said:

To offset the weight of the gen and batteries, put the Beer on the couch. You should be good! LOL

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You have the winning answer to this problem. I knew I shouldn't have put the 10 cases of beer in the front storage compartment!

BigGuy82 said:

You have the winning answer to this problem. I knew I shouldn't have put the 10 cases of beer in the front storage compartment!

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It's really no problem as long as the kegerator goes in the garage behind the axles??

sent from space via an invisible beam from a flying metal dish