Finding the Centerline for Weight Distribution

[Wyatt]

As is common with most RV’s, I’m close to maxed out on my GAW for the rear, and have plenty of headroom for more weight on the front axle, but I don’t know where the delineation is between the two axles.
So I want to see of I can redistribute some weight from the rear to the front.
Q. Does anyone know how to find the centerline in an RV, the point at which the RV would balance if hypothetically placed on a fulcrum, like a teeter totter.
Is it exactly half way between the two axles? I don’t think it’s that simple, but then again that’s why I’m asking—I don’t know. Thanks in advance for your ideas/help.

[l1v3fr33ord1]

Wyatt-

I would think the center of gravity would be located approximately two-thirds of the wheelbase aft of the front axle.

     14k              7k
      |  x  |    2x    |

     rear   ^        front
            |
        C.G. here

[Wyatt]

Originally Posted by l1v3fr33ord1

Wyatt-

I would think the center of gravity would be located approximately two-thirds of the wheelbase aft of the front axle.

     14k              7k
      |  x  |    2x    |

     rear   ^        front
            |
        C.G. here

I see, so you’re using the front and rear axle weight ratings to determine a mid point. Interesting. Thanks!

[backtrack15]

I’m not sure the center of gravity matters for your purposes. Anything placed between the axles will have its weight split between them. Directly over the front axle, 100% goes on front axle. Directly over the rear axle, 100% goes on the rear. Half way between, split 50/50. In front of front axle will actually add more than 100% to front axle (as rear axle loses a bit to balance the torque, which is then supported by the front). Same goes for anything loaded behind the rear axle (loads rear slightly more than the item’s weight).

Just do a simple moment balance using either axle as the pivot point. Couple that with a static force balance to solve for all the loads.

LoadOnFront= ItemWeight*DistanceFromRearAxle/Wheelbase
LoadOnRear=ItemWeight-LoadOnFront

*DistanceFromRearAxle is positive if item is in front of the rear axle.

[Wayne M]

Weigh each tire/dually and you will find which is above max load and which is lower than max load redistribute items to balance it out. is what I do. There isn't much of a compartment in front or behind axles. My thinks!

[Wyatt]

Originally Posted by Wayne M

Weigh each tire/dually and you will find which is above max load and which is lower than max load redistribute items to balance it out. is what I do. There isn't much of a compartment in front or behind axles. My thinks!

That would solve a left to right issue, but not front to back. On a side note,
I’d love to go weigh all four corners, but I know of nowhere around me that has that capability.

[Wyatt]

Originally Posted by backtrack15

I’m not sure the center of gravity matters for your purposes. Anything placed between the axles will have its weight split between them. Directly over the front axle, 100% goes on front axle. Directly over the rear axle, 100% goes on the rear. Half way between, split 50/50. In front of front axle will actually add more than 100% to front axle (as rear axle loses a bit to balance the torque, which is then supported by the front). Same goes for anything loaded behind the rear axle (loads rear slightly more than the item’s weight).

Just do a simple moment balance using either axle as the pivot point. Couple that with a static force balance to solve for all the loads.

LoadOnFront= ItemWeight*DistanceFromRearAxle/Wheelbase
LoadOnRear=ItemWeight-LoadOnFront

*DistanceFromRearAxle is positive if item is in front of the rear axle.

Here’s my line of thinking, which could be flawed. Imagine like I said, a teeter totter in a park.
You can slide all of the weight from the heavier downhill side that you want, but until it gets over the centerline of the fulcrum, it’s still weight on the downhill side.
So I can move all the weight I want forward, but until it’s over the centerline of the coach, it’s still going to be counted as weight on the rear axle.
Now I imagine that the centerline is somewhere near 2/3rds of the way towards the rear. Commerical trucks have to have this demarcation in order to load their trucks, so I assume that the same principle would apply here?

[backtrack15]

I understand the analogy, but your RV has two axles (not a single fulcrum). It isn’t a simple matter of getting over the fulcrum. The equations I provided can be used to predict the change in axle loads for any item you move. Use them for the item’s current location. Then use them for the item’s new location. See how the weight put on each axle changes by moving the item.

For example:

Assume you have an item weighing 50 lbs located 2 feet behind your rear axle and you have a wheelbase of 20 feet.

LoadOnFront = (50)*(-2)/(20) = -5 lbs.
LoadOnRear = 50 - (-5) = 55 lbs.

Now move the item to halfway between the axles

LoadOnFront = (50)*(10)/(20) = 25 lbs.
LoadOnRear = 50 - 25 = 25 lbs.

Moving the item removed 30 lbs from the rear axle and added 30 lbs to the front axle.

[backtrack15]

I personally haven’t noticed the loading marks you mention on commercial trucks. Many semi-trailers have “lift here” marks for lifting via crane onto trains/ships. Maybe that’s what you’re thinking of? Many also have the ability to slide the trailer axles forward or rearward a bit to adjust the weight on the axles. Forward adjustment puts more weight on the trailer and less on the truck. Rearward adjustment does the reverse.

[Wayne M]

Originally Posted by Wyatt

That would solve a left to right issue, but not front to back. On a side note,
I’d love to go weigh all four corners, but I know of nowhere around me that has that capability.

I was thinking out loud, one of my problems. I was assuming that if the load weight on all the corners were withing Mfg specifications there should not be a problem regarding center of gravity.


Jump on up to Oregon and go over the scales that are not attended or close for the day. The leave the scales on. I went over one with all wheels and got the weight on all the axles, then the next one I went over with only one side of the MH and calculated the weights. Again, thinking out loud.

[catmandoo62]

if you have any good size truck stops in your area they have scales you could at least get a front axle weight,full weight and rear axle weight.they are usually set up on elevated platforms.which limits your ability to weigh each wheel.i'm in iowa and my local grain elevator has a full flat concrete driveway.so i can get weight per corner as i can stradle over the edge of the scale.and i can weight both sides so i can make sure what weight i have is centered front to back and side to side.

[backtrack15]

If you really are focused on finding the center of gravity (lengthwise), you just need the scaled values for your front and rear axles along with the wheelbase of your RV.

F = scaled weight for front axle
R = scaled weight for rear axle
W = wheelbase (distance between centers of front and rear axles)
X = distance to center of gravity, measured from center of rear axle.

X = (F*W)/(F+R)

Again, there is no special significance to this point in terms of your original stated need. Any forward movement of stored items will help transfer weight to the front axle. The more stuff you move and the farther forward you move it, the more weight you will shift.

[Wyatt]

Originally Posted by backtrack15

If you really are focused on finding the center of gravity (lengthwise), you just need the scaled values for your front and rear axles along with the wheelbase of your RV.

F = scaled weight for front axle
R = scaled weight for rear axle
W = wheelbase (distance between centers of front and rear axles)
X = distance to center of gravity, measured from center of rear axle.

X = (F*W)/(F+R)

Again, there is no special significance to this point in terms of your original stated need. Any forward movement of stored items will help transfer weight to the front axle. The more stuff you move and the farther forward you move it, the more weight you will shift.

Thank you for all of your replies. I actually didn’t think I was going to get any definitive answers.
I see now what you say makes sense. Again, using the teeter totter analogy, if the person who is heaviest moves towards the centerline where the fulcrum is, it can balance the teeter totter, even though they weigh more, just by shifting the weight towards the center, not necessarily over it.
Your algebra will definitely help.
This is where I read about the whole centerline thing…
https://www.worktruckonline.com/1478...load-made-easy

[Wyatt]

Originally Posted by catmandoo62

if you have any good size truck stops in your area they have scales you could at least get a front axle weight,full weight and rear axle weight.they are usually set up on elevated platforms.which limits your ability to weigh each wheel.i'm in iowa and my local grain elevator has a full flat concrete driveway.so i can get weight per corner as i can stradle over the edge of the scale.and i can weight both sides so i can make sure what weight i have is centered front to back and side to side.

Thanks. I do have the front and rear axle weights, that’s how I know I need to make an adjustment. I thought about your idea of driving into the platform with just one side of the RV on the scales, to get a left front and left rear value, but like you said, the scales I’ve found are up on a concrete pad, which would tilt the RV skewing the results. I’ll keep looking…

[Wyatt]

Originally Posted by Wayne M

I was thinking out loud, one of my problems. I was assuming that if the load weight on all the corners were withing Mfg specifications there should not be a problem regarding center of gravity.


Jump on up to Oregon and go over the scales that are not attended or close for the day. The leave the scales on. I went over one with all wheels and got the weight on all the axles, then the next one I went over with only one side of the MH and calculated the weights. Again, thinking out loud.

Oregon is not really a hop skip and a jump from my home in CA, but if you have an address where you do such a thing, I’ll definitely add it to my itinerary. Thanks!

[Wayne M]

Originally Posted by Wyatt

Oregon is not really a hop skip and a jump from my home in CA, but if you have an address where you do such a thing, I’ll definitely add it to my itinerary. Thanks!

We were just going up the 101 when I ran into it.

Just some info

Stay away from the fires!!

[Wyatt]

Thank you to everyone who weighed in on this issue, (pun intended).
You’ve all been a great resource and I deeply appreciate the time you all devoted to pondering my dilemma.