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Old 04-29-2013, 02:02 PM   #1
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Weight & Balance - Location of Items

OK...I'm hoping someone with some engineering smarts can help me a little bit on determining how to balance my MH weights. We are in the process of moving into our MH and I'm playing with some configurations but trying to avoid multiple weigh sessions just to get it right. So, I want to ballpark our storage locations based on what I have learned from previous weighing. Where this comes in to play is that once we get it loaded, we have one last decision to make and that is adding either a combo W/D or stacked.

As some background, I am a pilot familiar with CG and arm calculations and I *THINK* things should be similar but I'm not positive that a solid set of beams connecting front and rear axles might change things. BTW, I know I have tag axle that may have a minor impact on things but for general purposes, let's ignore that for now.

So my question is this. Does weight added between the axles get equally distributed between the axles or is it proportional to location between them?

As an example, if I load a 100# item half way between them (say 24' apart for simple numbers) it should put 50# on each axle. If I move it so that it 16' from the steerer and 8' from the drive would that mean I now have added 33.3# (33%) to the steerer and 66.7# (67%) to the drive?

So...in real life here is the situation and my working theory:

The location for the center of the space where a W/D will go is about 4' in front of the drive axle. That puts it 19' behind the steerer axle. A stacked set will run about 225# with all the hardware. The Combo will be about 150#. If the load balance between those axle is roughly proportional to location between the drive and steer axles then that would mean 82% of the weight would be on the drive axle and 18% on the steerer. For the stacked that would be about 40# on the steerer and 185# on the drive. BTW, the W/D location is centered between the left and right side so that won't have any factor on L/R load balance.

Now, if my theory is all wrong and the load is shared closer to 50/50, I'm gonna have to go on a diet. LOL Before we started loading for FT use we only have about 620# to put on the front axle but at the last weighing we did shift everything in the basement to the front to intentionally load up the front axle. There will not be any problem with the rear and tag weights.

What we find out will help us adapt as needed and choose our W/D. THANKS IN ADVANCE!
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Old 04-29-2013, 03:37 PM   #2
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If you're not familiar with it, I think you'd find Bailey of Bristol's "Caravan Stability Studies" very interesting and informative. They've been studying this very thing for many years...here's a link to their site.
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Old 04-30-2013, 03:18 AM   #3
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I think you are correct.

Basic physics still applies.

Probably some vector calculations like flying in a cross wind...(not a pilot but have read some things)

You have a long lever with two fulcrums along its length.

On top of this lever at different points are amounts of downward force that interact with each other.

Loads applied at the back end behind the wheels tend to lessen the force towards the ground on the front wheels by transfering it to the rear wheels.

In your aircraft you only have the wing as a focal point, with the MH you have 2 main via fron and rear axles, add a tag and it gets more complicated.

The total force towards the ground stays constant.

So adding 100 pounds exactly 1/2 way between the front and rear wheels may not load equally between them depending on how the weight beyond the wheels effects the loading in the middle.

There must be a CG someplace, one could take a few "heavy things" and play with it at the scales if they would let you.

Water, fuel and other things also may change it.

For what you are doing you should be fine, most of the weight should stay on the back, again think about the levers, the load being close to the rear wheels will impact it the most, the distance from the front means little impact.

You could place some concrete blocks in place and weigh it to be sure.

I am not a mechanical engineer, but tinker enough to get by...

There will be others who may add to this, we all will learn.
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Old 04-30-2013, 08:11 AM   #4
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Thanks TQ...I found a couple resources that helped me confirm my general thoughts along the line of what you have said. We are definitely loading the closet with as much as possible. The heavier it is, the further back in the basement too. I have thought about picking up a couple hundred pounds of cinder blocks to place them where the W/D will go to double check. I always weigh with full water, fuel and LP. I also try to have about 10% in both the black and grey tanks.

For now I will run the drive wheels at least 10 PSI above the tag wheels. My working theory is that I want to drive wheels to act more like a fulcrum to allow the weight of the coach behind it to off-load some of the weight on the front axle. I will probably fine tune that a bit. My goal would be to ensure that my drive wheel loading is a little lower per PSI than the tag. If I went by my current 6 corner weight:

1. Without the manufacturer's minimum PSI, the tag would need about 70 PSI but the minimum is 75 PSI. The tag will never get enough weight on it to require much more than 75 PSI.

2. According to the weight prior to our weekend loading the drive axle only required about 78 PSI. Under those conditions I still used 85 PSI under my working theory. Even after we load it for FT I doubt the drive wheels should require more than 90 PSI and that might require a bit of a L/R imbalance to happen.

3. At the last weighing the L/R balance for all my axles was darn close. As long as I maintain that kind of balance, even at 14,600# my front pressures will only require 110 PSI. If we find the FT loading is a bit less balanced then I don't see needing more that 115 PSI. I am expecting a little bit more of an imbalance once we load the refer and galley since all that stuff is on the curb side and there isn't much storage above the basement that can be moved to the driver's side. I also expect the basement to be a tad heavier on the curb side because that is where all the good stuff (chairs, grill and other popular stuff) will be.

I know...I'm a tad bit of a techie.
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Old 05-01-2013, 12:34 PM   #5
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As a follow up...

I got it weighed last night after we added a bunch of stuff. However I didn't get a chance to fill the water tank nor did I have my GSDs in their crates like we normally do. Still, the numbers were encouraging. The first numbers are previous and the second one is last night.

Steerer - 13980 - 13840
Drive - 14560 - 14700
Tag - 6920 - 6880

Now, these are pretty curious to me but perhaps I don't know were the water actually sits. After the amount of loading we have done:

1. I would have expected an increase of weight on the tag! That was a huge surprise but perhaps since I am running 10 PSI higher on the drive over the tag PSI I am lifting some weight off of the tag? Something to think about.

2. I would expect the drive wheels will take the brunt (80%) of the missing 500# of dogs and water.

3. I was expecting some mixed info on the steerer situation. I intentionally forward loaded all the "heavy" stuff I had to the very front of the basement but for the latest I moved it all back to it's probable location. I was also a little bit over 1/2 LP compared to full LP last time. So, I think adding dogs, full water, some more groceries, and other odds-n-ends forward of the drive will put, maybe, 200-300# more on the steerer.

The L/R balance is awesomely well distributed. Both the tag and drive wheels are nearly a perfect 50/50 while the steerer is about 49/51.

We have one more major loading/moving in stage over the next couple weeks and will do another weighing to see how it all shakes out. I expect we will not have any problems from a weight stand point of stacked vs. combo W/D after going through this exercise so far.
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Old 05-01-2013, 05:03 PM   #6
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More relevant as to how those weights compare with the maximum allowable loads on the three axles because if you are so close that worrying about where a relatively light washing machine is to be located, you may have other things to worry about. For instance, it would be quite possible to be travelling with fresh water tanks full and at least equivalent to the grey water tank full and both of these tanks could be mounted off the centre line, yet you are not concerned with including them at this stage. Passenger and driver, food, fuel, propane, toys all need to be in travelling configuration and then you have a legitimate point to start calculating the effect of adding more weight.
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Old 05-01-2013, 05:23 PM   #7
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Skyboxes the mfg probably plumbed for the washer and dryer. So they considered the best location but the weight is minimal as suggested above. Consider the function of the tag it's designed to relieve the drive axel. Loading heavy to the rear allows you to increase the air pressure on the tag which in turn transfers the weight to the steer axel. Tire pressure has no effect on balance. After weighing the units 4 corners look at the tire mfg load chart and set the tire pressure buy their specifications. This will eliminate tire issues down the road. You are on the wright track, Happy Days.
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Old 05-02-2013, 07:09 AM   #8
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Quote:
Originally Posted by CHCam42 View Post
Skyboxes the mfg probably plumbed for the washer and dryer. So they considered the best location but the weight is minimal as suggested above. Consider the function of the tag it's designed to relieve the drive axel. Loading heavy to the rear allows you to increase the air pressure on the tag which in turn transfers the weight to the steer axel. Tire pressure has no effect on balance. After weighing the units 4 corners look at the tire mfg load chart and set the tire pressure buy their specifications. This will eliminate tire issues down the road. You are on the wright track, Happy Days.
On the bold, this is what I was thinking as well. Load up the rear. Even if you have to adjust the ride height setting to load the tag a little more, so be it! That's where you can gain some capacity if required?
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