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What if ....we move the, Axle, Hitch, Both,,,,,or Whatever...


Dollytrolley

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Lately I have seemed to have gotten my long johns wrapped around the axle so to speak with my thread about the methods I use to keep my odd RV combonations in some semblance of Weight and Balance.

 

Much of my abused younger days I seemed to bumble into "enterprises" that demanded calculating and maintaining certain Weights and Balance within certain limits so calculating Weight and Balance was pretty much a daily event.

 

Of course one of the things I Thought about retirement was that I would not be anymore Weight and Balance .......well it appears old habits die hard.

 

As retirement allowed longer RV trips with Dolly-the-paint-horse along for the ride it appears that my RV cargo load increased by a few TONS and then a few TONS more.....

 

So.....a odd thing about my retirement is that I seem to have fallen into a Very ODD Hobby of build-it-yourself-Truck-and-Trailer building....

 

Not to worry, it seems that a lot of RV folks partake of the same Odd Hobby.

 

What gives me pause is how ......lucky....some folks seem to be able to put their thumb up in the air and seem to "guesstimate" where to move the drive axle, hitch, Smart, Drombox, etc .....without any real world calculations of the consequences......

 

Now I am surely pretty dirty amoung the "unwashed" as Jeff has pointed out and some of the times my "thumb-engineering" I was barely able to escape with my mangy hide.....obviously others have better "thumbs".....

 

As my odd RV HDT has evolved I have resorted to the old habit of calculating my RV loads mostly to try to offload the Toy hauler loads onto the Freightshaker and then evenly distribute the truck loads on the truck to mostly improve the suspension dynamics.

 

In vehicle design the big brain geeks tend to talk about two fields of study, first is "statics" and the other is "dynamics".

 

For the most part Weight and Balance is a "static" once our RV starts down the road we enter into the complex world of...."dynamics".

 

What tends to make me less bold these days is recalling sitting in the back of the room full of BIG-Brain engineers hashing out the Battle of getting the vehicle "statics" correct enough so that the test vehicle "dynamics" would not kill the "test-lab-rat" (me).

 

Now in my old age for the most part I seem to be all three geeks rolled into one....I am the "statics-geek"...I am the "dynamics-geek"....and I am still a lab-rat......

 

Maybe this subject is a bit like talking about how often you beat your wife but....sometimes it's best to come clean and see where we are....

 

Rather than posting the too many numbers in the calculations of a HDT "what-if-study" ......perhaps it might be more informative to just post some examples of....what if we change the truck wheel base from this to this.....or if we move the hitch from this location to this location ....or.....if we move Both the hitch AND axle at the same time.

 

Maybe a few examples could be food for thought....

 

Drive on.......(static often becomes...dynamic...)

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Some people just can't seem to get this whole idea of "Static" The more they try, the more "dynamic" spills out. Maybe the medical people could label this DSRS (Dynamically Static Retirement Syndrome)? Unfortunately, a lot of you supposedly static people on here seem to have this!. :lol: With 15 years until I get to try my hand at this....I'm not sure if I'm jeally or just tired! :(

 

I for one am glad that said engineers got it mostly right...ei said lab-rat is still around to share the dynamically explored static balance relationships to dynamic operation of HDT usage. :D:D

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Just posted this on the other thread as well:

 

Most of us didn't have to rely on being lucky in trying to figure out how to set up out trucks for towing purposes. In 2008, there was a great presentation from Mark Shelly done at the National Rally, in Hutch, explaining what we were doing and why.

You can view his Power Point file, and the PDF file by clicking here and scroll about a third of the way down looking for the 2 sections called, "How to build your own bed".

 

This will give you a great look at what is being discussed. Unfortunately, there is no audio on the Power Point presentation, but it is very informative.

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Just posted this on the other thread as well:

 

Most of us didn't have to rely on being lucky in trying to figure out how to set up out trucks for towing purposes. In 2008, there was a great presentation from Mark Shelly done at the National Rally, in Hutch, explaining what we were doing and why.

You can view his Power Point file, and the PDF file by clicking here and scroll about a third of the way down looking for the 2 sections called, "How to build your own bed".

 

This will give you a great look at what is being discussed. Unfortunately, there is no audio on the Power Point presentation, but it is very informative.

Indeed Hero.....you bring up a great point, the "unwashed" versus the "Washed" classes of HDT folks ......these classes are alluded to by HDT sage Jeff.

 

So....the "Washed" look at the presentation you have linked and simply do the math......sorta

 

The "Unwashed" look at the presentation you have linked and......the math looks too complex....

 

The graphics in your linked presentation looks ok to me....I have looked at tons of weight and balance graphics for too many decades and I have seen many "dear in the headlights glaze over looks" from "Washed" and "Unwashed" alike.

 

What I propose here is more than simple ....."what are my biggest loads numbers?"......instead what I propose is a series of layout configuration calculations of many RV configurations to attempt to obtain a good "Compromise" .......as Jack often points out.....RV's are a study in compromise.......

 

Drive on.......(washing can feel ok....)

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I have composed a sample generic HDT RV truck and trailer hitched combonation in the attached Weight and Balance listing attached as a PDF to this post.

 

The example truck is Baseline at:

Hitch Pin 00000 Lbs

Steer axle 12,160 Lbs

Drive axle 7,800 Lbs

Wheelbase 283 inches

Baseline C. G, 110.59 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Drive axle to hitch Pin= 52 inches aft

 

Loading Example: A-1

 

Hitch Pin 5,357 Lbs

Steer axle is 13,048 Lbs

Drive axle is 17,290 Lbs

Wheelbase 283 inches

Loaded C. G, 161.29 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Total Loaded Wt 30,337 Lbs

 

Looking at the example above what loading changes might we expect if we were to move the drive axle forward 36 inches to a mid position of 247 inch wheelbase?

 

Loading Example: A-2

At the new 247 inch wheel base the new loading is:

 

Hitch Pin 5,357 Lbs

Steer axle is 11,665 Lbs

Drive axle is 18,673 Lbs

Wheelbase 247 inches

Loaded C. G, 152.03 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

 

Total Loaded Wt 30,337 Lbs

Drive axle to hitch Pin= 88 inches aft

 

By moving the drive axle to a more mid position we have seen 1,383 pounds shifted from the steer axle to the drive axle, so if the truck is equipped with a 12K steer axle it is below rated weight and if the drive axle is 19K rated it also is below rated limit.

 

What might we change next?

 

Maybe move hitch?

 

Pin load.....more?.....less?

 

Move axle ......?

 

Ideas?

 

Drive on......(whats the next what if?)

 

 

 

 

HDT_WT_BAL_H_01052016.pdf

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something doesn't look rite, A1 if weight was added 52" behind the rear axle the front axle should not increase in weight unless I'm missing something

Shorts, good point.

 

What needs to be considered and likely is potentially confusing is the fact that the very first data example is for the Baseline truck without any loads added....

 

It's natural for our minds to try to fixate on the LARGE hitch load fairly far aft of the rear axle and then not "see" the "hidden" loads such as driver and passenger, fuel, tools, and don't forget the Smart in front of the drive axle but here is where our minds play tricks on us and that is the various distances (arm) that the various loads are from the drive axle fulcrum point.....you we fixate on the weight value that we are well equipped by nature to "feel" but the various "arms" tend to drive more variables into the loading than we are equipped to organize in our subconscious minds.

 

You bring up a couple of good points ......perhaps we could calculate two example loadings to better understand what's going on here....

Maybe just unhitched the trailer but leave all other loads as they are on the truck and see what our axle loads are.....

 

Perhaps we should just unload the the truck back to the baseline condition and then just hitch up the trailer and then see what the axle weights are.

 

Might be a good couple of examples......

 

I will sneak away from my stable boy chores to cipher a couple loadings so hang on.....

 

Drive on.......(Dolly eats a lot of hay....)

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I have looked and studied your PDF, Dolly, but I still can't figure out exactly what the MOMENT number does for the calculation. Any way to simplify it for my simple mind so I can understand it?

Mntom , good question so I'll try to not bumble the answer too much......maybe....

 

The MOMENT value is a composite number derived by multiplying the item weight by the distance from a given Datum point....so that's about as clear as mud so.......perhaps you might consider another example that we tend more naturally able wrap our Minds around

.......a torque wrench.......you see when we use a torque wrench the bolt head center is the datum point, the length of the wrench handle is the "arm" (lever) and the pressure that we apply on the wrench handle is the "load" (force) and we express the desired value as a moment composite number of say 10 ft pounds ......that represents the applied torque to the bolt head by a one foot lever with ten pounds of force twisting the bolt head.......

 

Now engineers may use inch pounds or metric or other values to confuse us "unwashed folks"'but that's just the lack of social graces that come with a engineering sheep spin....

 

So how bad did I mangle the........Moment?

 

Drive on.......( so how many inches and pounds are in that moment?)

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You use the moment column to know if you need to sleeve your rail when extending wheelbase or adding tail frame to put a hitch way way back there. So you use that column to know if what you are about to build meets the factor of safety you want. 90's highway trucks with 5/16 frames only have a 1,776,000 RBM rail, so it cuts pretty deep towards the 80% I saw mentioned in one of the threads.

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Thank you..... Now I understand.

In my younger homeless days I used to crash in a spare room of a very large home of a chief engineer of a outfit that made large flying metal doohickeys in Seattle.....some huge brain engineers would stop by for the cocktail hour and sometimes they would "talk-shop"..... Sometimes I would mutter......."oh yes I think I understand"......all the geezer engineers would stop....and then stare at me.....then the chief would grin and say......."well kid hang onto that thought because in a minute.....we will get way up over yor pay grade....hang on, kid...."

 

Ya know in just a few words of "engineer-speak" I was once again clueless..

 

But you do bring up a interesting subject about how truck structures are modified on the population of HDT RV units.....here we go ....

 

"Hang on kid".....

 

Drive on .......( next thing ya know some engineer-geezer will mutter something about .......kips an new-tons an such..)

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Just an FYI (like you didn't know it already) it won't take much to get above my pay grade..........

 

.... Sometimes I would mutter......."oh yes I think I understand"......all the geezer engineers would stop....and then stare at me.....then the chief would grin and say......."well kid hang onto that thought because in a minute.....we will get way up over yor pay grade....hang on, kid...."

Ya know in just a few words of "engineer-speak" I was once again clueless..

But you do bring up a interesting subject about how truck structures are modified on the population of HDT RV units.....here we go ....

"Hang on kid".....

Drive on .......( next thing ya know some engineer-geezer will mutter something about .......kips an new-tons an such..)

Kid..... Ha ha, that's something people quit calling me more than a few days ago! Thanks for the compliment just don't pay any attention to the grey hair!!

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The example truck is Baseline at:

 

Hitch Pin 00000 Lbs

 

Steer axle 12,160 Lbs

 

Drive axle 7,800 Lbs

 

Wheelbase 283 inches

 

Baseline C. G, 110.59 inches

 

Fifth Hitch 335 inches

 

Datum 0 Center of Steer axle

 

Drive axle to hitch Pin= 52 inches aft

 

 

 

Loading Example: A-1

 

 

 

Hitch Pin 5,357 Lbs

 

Steer axle is 13,048 Lbs

 

Drive axle is 17,290 Lbs

 

Wheelbase 283 inches

 

Loaded C. G, 161.29 inches

 

Fifth Hitch 335 inches

 

Datum 0 Center of Steer axle

 

Total Loaded Wt 30,337 Lbs

 

Load A-1 is detailed in the attached weight and balance attached PDF file.

 

If we leave the trailer hitched up but unload the truck to baseline condition with only the 5,357 pound pin load what would are ne weight and balance be?

 

Answer, Load B-1:

 

Hitch Pin 5,357 Lbs

 

Steer axle is 11,136 Lbs

 

Drive axle is 14,142 Lbs

 

Wheelbase 283 inches

 

Loaded C. G, 158.08 inches

 

Fifth Hitch 335 inches

 

Datum 0 Center of Steer axle

 

Total Loaded Wt 25,317 Lbs

 

Now if we were to reload the original loading of the truck and then unhitch the trailer and lose the 5,357 pound hitch weight our weight and balance would be:

 

Load C-1:

 

 

Hitch Pin 0 Lbs

 

Steer axle is 14,032 Lbs

 

Drive axle is 10,948 Lbs

 

Wheelbase 283 inches

 

Loaded C. G, 124.03 inches

 

Fifth Hitch 335 inches

 

Datum 0 Center of Steer axle

 

Total Loaded Wt 24,980 Lbs

 

Please note that without the trailer pin weight on the truck hitch much weight is transfered to the steer axle to overload it.

 

This is a case where unhitching the trailer from the truck has actually resulted in overloading the steer axle.

 

Drive on.......(Unloading can sometimes cause ....Overloading...)

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The example truck is Baseline at:

Hitch Pin 00000 Lbs

Steer axle 12,160 Lbs

Drive axle 7,800 Lbs

Wheelbase 283 inches

Baseline C. G, 110.59 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Drive axle to hitch Pin= 52 inches aft

Loading Example: A-1

Hitch Pin 5,357 Lbs

Steer axle is 13,048 Lbs

Drive axle is 17,290 Lbs

Wheelbase 283 inches

Loaded C. G, 161.29 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Total Loaded Wt 30,337 Lbs

Load A-1 is detailed in the attached weight and balance attached PDF file.

If we leave the trailer hitched up but unload the truck to baseline condition with only the 5,357 pound pin load what would are ne weight and balance be?

Answer, Load B-1:

Hitch Pin 5,357 Lbs

Steer axle is 11,136 Lbs

Drive axle is 14,142 Lbs

Wheelbase 283 inches

Loaded C. G, 158.08 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Total Loaded Wt 25,317 Lbs

Now if we were to reload the original loading of the truck and then unhitch the trailer and lose the 5,357 pound hitch weight our weight and balance would be:

Load C-1:

Hitch Pin 0 Lbs

Steer axle is 14,032 Lbs

Drive axle is 10,948 Lbs

Wheelbase 283 inches

Loaded C. G, 124.03 inches

Fifth Hitch 335 inches

Datum 0 Center of Steer axle

Total Loaded Wt 24,980 Lbs

Please note that without the trailer pin weight on the truck hitch much weight is transfered to the steer axle to overload it.

This is a case where unhitching the trailer from the truck has actually resulted in overloading the steer axle.

Drive on.......(Unloading can sometimes cause ....Overloading...)

Any ideas ?

 

In airplanes we often limit fuel loads to skinny by on weight and balance limits.....most aircraft burn fuel at high hourly rates so adjusting fuel loads from mostly empty tanks is common. Fuel often lingers longer in most HDT RV units.

 

Maybe offload the Smart and have someone drive it.....?

 

Offload the passenger and cargo....?

 

Maybe the +14,000 steer load is within the safety margin of the axle and tires......and maybe the overload is not too large?

 

Maybe install a Heavy capacity steer axle , wheels and tires?

 

Maybe move the drive axle from long to mid or.....maybe short wheelbase??

 

What to do?

 

Suggestions?

 

Drive on......(feeling a bit nose heavy....)

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Maybe I can add a couple thoughts from an unwashed engineer, pilot and now glass shop owner on the topic of weight, arm and moment. Moment is a made up number, an engineering tool used to figure these calculations. It isn't a graduation on a scale or measuring stick.

 

Think of it in terms of balancing a teeter-totter.

 

If there is equal weight on both sides, it will balance. Everyone has fun. (50# kid, 100" away from center on both sides.)

 

If the fat kid (100#) and the skinny kid (50#) want to ride the teeter-totter, what has to happen? The fat kid needs to be closer to the fulcrum (hinge point) to make it balance. This is where "arm" comes into play.

 

The 50# kid is still 100" out, where does the 100# kid have to be? Here is the calculation:

 

50# (weight) times 100" (arm) equals 5,000 (moment). This is the known skinny kid side.

 

To calculate the fat kid side, take that moment, 5,000, divide it by the weight of the fat kid, 100#, and you get the arm length, 50". Therefore if you place the fat kid at 50" from the fulcrum everyone has fun, again.

 

 

 

 

Now to transition this to loading a vehicle. You use the weigh times the arm for the moment or derivatives of this equation to calculate weights or locations of weights to make things work on your vehicle.

 

Where it gets a little more abstract is on your vehicle, if you add up the weights taken from sum of the known arm locations you can then calculate the center of gravity (balance point) of the vehicle:

 

Take the total weight at front axle (arm 0) and total weight at rear axle (x arm) added together divided by the total of the moments and you get the arm length of the center of gravity.

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Maybe I can add a couple thoughts from an unwashed engineer, pilot and now glass shop owner on the topic of weight, arm and moment. Moment is a made up number, an engineering tool used to figure these calculations. It isn't a graduation on a scale or measuring stick.

 

Think of it in terms of balancing a teeter-totter.

 

If there is equal weight on both sides, it will balance. Everyone has fun. (50# kid, 100" away from center on both sides.)

 

If the fat kid (100#) and the skinny kid (50#) want to ride the teeter-totter, what has to happen? The fat kid needs to be closer to the fulcrum (hinge point) to make it balance. This is where "arm" comes into play.

 

The 50# kid is still 100" out, where does the 100# kid have to be? Here is the calculation:

 

50# (weight) times 100" (arm) equals 5,000 (moment). This is the known skinny kid side.

 

To calculate the fat kid side, take that moment, 5,000, divide it by the weight of the fat kid, 100#, and you get the arm length, 50". Therefore if you place the fat kid at 50" from the fulcrum everyone has fun, again.

 

 

 

 

Now to transition this to loading a vehicle. You use the weigh times the arm for the moment or derivatives of this equation to calculate weights or locations of weights to make things work on your vehicle.

 

Where it gets a little more abstract is on your vehicle, if you add up the weights taken from sum of the known arm locations you can then calculate the center of gravity (balance point) of the vehicle:

 

Take the total weight at front axle (arm 0) and total weight at rear axle (x arm) added together divided by the total of the moments and you get the arm length of the center of gravity.

 

 

 

OK Av-ate..........just how s k i n n y or ......FAT .....was you as a kid?

 

Dollymomma looked at the weight listings and said....."looks like the driver could lose a bunch of weight by......cleaning Dolly-the-paint-horse stall more often"......shucks she did NOT use any math......

 

Av.......nice story.....fly on....

 

Drive on......(open to more ....what-if's...)

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