Center of gravity...how high is too high?

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Center of gravity...how high is too high?

Postby Patrio » Tue Feb 11, 2020 11:04 am

Hi.

Some of you might remember my Thing that I'm building.

http://www.tnttt.com/viewtopic.php?f=40&t=72216

Aesthetics take a back seat. This is all about function. I used Skechup to make a scale model of it, and from that, superimposed it over a pic of my trailer and re-sized it to get the correct proportions.

Image

The Thing is going to be pretty lightweight, considering it is all wood construction. My initial estimates were around 600 lbs. I did this by calculating the weight of individual panels and so far, I'm under the projected weight on most.

I wanted a "standy-pod", but I'm starting to get leery about CG and tipping point, especially considering I'm planning on taking this over some unimproved farm roads with some off-camber, tilty sections.

I estimated the weight of my trailer at 350 lbs, based on others that I have seen that were similar. It is a homebuilt trailer, and very rugged, so if anything, that's a low estimate. Since the trailer and the Thing will be tethered together as one piece, I estimated the weights as such. The whole thing, once mounted, will be right at 1000 lbs, before I put my gear in.

The total height, roughly, from the floor of the trailer to the roof of the camper, will be right at 6 feet or so. Overhangs on each side extend past the wheel approximately five inches beyond the wheels of the trailer.

I started analyzing the weight of my panels and doing calculations about where the weight was distributed. Here's the first analysis. The line between the green/red is just under 4 feet off the ground.

Image

I'm looking at a 66/33% bottom/top weight distribution, which is encouraging. Adding 100 lbs of gear (food, coolers, water, etc.) to be placed in the floor while hauling brings the bottom/top distribution to a very practical 70/30 ratio.

It's when you start breaking it down further that my concerns start. The roof panel is going to be just over 100 lbs. Which means that, of the top third will weight a not-so-hot 255 lbs.

Here's the breakdown:

Image

Should I panic now?
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Re: Center of gravity...how high is too high?

Postby Woodbutcher » Tue Feb 11, 2020 12:02 pm

I think you should be OK. My concern is, does the overhang in the front, interfere with the tow vehicle when making turns?
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Re: Center of gravity...how high is too high?

Postby tony.latham » Tue Feb 11, 2020 12:09 pm

I'm starting to get leery about CG and tipping point, especially considering I'm planning on taking this over some unimproved farm roads with some off-camber, tilty sections.


"Tilty" is a rather subjective description.

We were headed to Chaco Canyon last spring and checked the forecast. Gusts to 60. We turned around and hunkered in a secure campspot. I think we would have been okay but what's the fun in that?

Image

I'd be more concerned about side wind loads but you've got a lot of weight down low.

Tony
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Re: Center of gravity...how high is too high?

Postby Modstock » Tue Feb 11, 2020 12:19 pm

As with most vehicle related things, you would want all your heavy gear as low as possible.
I.e. ) 6 gallon water jug is 50lbs filled.

I recently sold my teardrop cause all my gear was at the tongue and rear of the vehicle.
Wanting better weight distribution, Im doing a 6x10 cargo trailer conversion.
Plus with room to stand, it's a plus.
Still I need to build lightweight cause it adds up fast.

It's a good idea you have drawn it out before building and can change things before it's too late. 1/8" Luan wood is best for things like inside walls, ceiling, etc.




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Re: Center of gravity...how high is too high?

Postby John61CT » Tue Feb 11, 2020 12:34 pm

Any batteries, water tanks, propane, locate centered near axle low as possible.

Take it easy cornering, even lane changes

Go slower when windy, pull over when it really blows.

Consider a wider wheel base (track width) and longer tongue.
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Re: Center of gravity...how high is too high?

Postby Patrio » Tue Feb 11, 2020 2:52 pm

Woodbutcher wrote:I think you should be OK. My concern is, does the overhang in the front, interfere with the tow vehicle when making turns?


My hitch comes back another foot or more off the bumper. Tow vehicle is an Xterra.

This pic is for diagrammatic purposes only. Don't ask for an explanation.

Image

Let me just leave it at, when that adorable little cousin of yours asks you nicely to be the shuttle vehicle for his river float, make sure you know exactly what you will be hauling before you agree to do it.
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Re: Center of gravity...how high is too high?

Postby Patrio » Tue Feb 11, 2020 2:55 pm

tony.latham wrote:
I'm starting to get leery about CG and tipping point, especially considering I'm planning on taking this over some unimproved farm roads with some off-camber, tilty sections.


"Tilty" is a rather subjective description.

We were headed to Chaco Canyon last spring and checked the forecast. Gusts to 60. We turned around and hunkered in a secure campspot. I think we would have been okay but what's the fun in that?

I'd be more concerned about side wind loads but you've got a lot of weight down low.

Tony


I plan to use this mostly within 45 minutes of my house, so if I have to travel on a bad day, I won't have to go far. WV doesn't get extremely windy often unless you're on top of a mountain. It's considered windy here when we get gusts above 25 mph or so. With enough weight, I think the wind part will be OK.
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Re: Center of gravity...how high is too high?

Postby tac422 » Tue Feb 11, 2020 3:36 pm

Have you thought about using 2" foam for the roof ?
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Re: Center of gravity...how high is too high?

Postby Shadow Catcher » Tue Feb 11, 2020 3:47 pm

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Re: Center of gravity...how high is too high?

Postby Patrio » Tue Feb 11, 2020 5:48 pm

Shadow Catcher wrote:CR has a Very low CG There are formula for that,
https://www.raeng.org.uk/publications/other/forces-centre-of-gravity-reactions-and-stability


I sent an email to my dad, who was an engineer in his non-retired life.

He said this:

Think of this. you hang a plumb bob inside from the center of the roof. On level ground, the total weight (less some on the hitch) is balanced across both tires. Now, as one side is raised, the plumb bob moves toward the lower wheel. The total weight also shifts toward the lower wheel and it shifts proportional to the distance from dead center when the trailer was level. I.E., if weight on the wheels is 1000 lbs, and the incline moves the plumb bob 6",( 5' tire center), the weight shifted to the lower side is 6"/60 = x/1000 = 100 lbs shifted to the lower wheel.

The trailer will not turn over unless the plumb bob moves to being directly over the lower wheel. This will be hard to do unless you get it rockin' pretty hard and shift the total weight to one wheel.


Which is basically what your post stated, although with more precision. This, of course, doesn't account for wind, but there's always checking the forecast and pulling over if necessary :D

Doing this, I mocked up a scale drawing of my Thing from the back, taking into account total height, axle width, etc. I then put a guide line down the middle and rotated the drawing around an imaginary center point on the roof until the guide line went through one of the wheels. I came up with this:

Image

21 degrees of tilt before there's a danger of tipping. I'm more than happy with that.

I went back and looked at screencaps from my dashcam from once when we went to my BIL's farm and I was following another vehicle. This is where I hope to use the camper during hunting season, etc.

The measurement is not exact, because my vehicle wasn't always level when I took the screencaps, but nowhere did the Jeep ever approach a 20 degree angle. In fact, it didn't even reach a 10 degree angle. This was the steepest angle on the trail.

Image

Yes, we're driving in the creek...lol.

Anyway, this looks good. I think I shouldn't have any issues, and that is undoubtedly the worst road I will travel.
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Re: Center of gravity...how high is too high?

Postby John61CT » Tue Feb 11, 2020 7:58 pm

That's why the main warnings are about fast cornering, heavy winds and excessively fast highway speeds.
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Re: Center of gravity...how high is too high?

Postby GPW » Wed Feb 12, 2020 6:03 am

This is the only instance we know of a blow over … But generally we don’t build higher than wide … :o
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There’s no place like Foam !
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Re: Center of gravity...how high is too high?

Postby John61CT » Wed Feb 12, 2020 6:25 am

The higher you go, the heavier the (low-down) weight needs to be.

So if the goal is super-light, need to stay low, maybe do a lifting roof design of some sort.
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Re: Center of gravity...how high is too high?

Postby Modstock » Wed Feb 12, 2020 1:19 pm

I remember that. So sad, never want to see that happen to a fellow builder.

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Re: Center of gravity...how high is too high?

Postby working on it » Thu Feb 13, 2020 11:51 am

Patrio wrote:...
1) Aesthetics take a back seat. This is all about function...

2) I estimated the weight of my trailer at 350 lbs, based on others that I have seen that were similar. It is a homebuilt trailer, and very rugged, so if anything, that's a low estimate. Since the trailer and the Thing will be tethered together as one piece, I estimated the weights as such. The whole thing, once mounted, will be right at 1000 lbs, before I put my gear in.

3) The total height, roughly, from the floor of the trailer to the roof of the camper, will be right at 6 feet or so. Overhangs on each side extend past the wheel approximately five inches beyond the wheels of the trailer.

4) I started analyzing the weight of my panels and doing calculations about where the weight was distributed.....


*
  • 1) agreed, function over form has been my goal-to change whatever is needed to make the trailer work better for me.
  • 2+4) same here, until I started weighing all components and calculating their effect on trailer balance. With just the trailer, floor, wheels & fenders completed, it weighed in at 400 lbs, so that's where my calculations started. I also aimed for 1000 lbs, but quickly surpassed that figure: 1438 lbs on the first trip (1280 lbs unloaded). Now, it has gone beyond 2100 lbs.
  • 3) trailer is 72" high (before the upper racks were installed), and the track width is 73.25" (adding 63.75" axle hubface+1" wheel offset + 8.5" tread centerline-to-centerline), so the width is sufficient to counter the overall height. Plus, I've recently added storage above the fenders, so overall width and lower-half weight has increased.

* I also worried about the center of gravity on my trailer, too, and used the trailer balance worksheet formerly on this website in Excel, but now lost to me along with hundreds of other files, during initial construction, and for a few years afterwards during modifications, to keep track of where the weight/balance was at, and with some adaptations to the worksheet, also to locate the center of gravity.

* After confirming that the COG was low enough (and located in front of the axle), by actual off-camber road-testing over the years, I started to add heavy storage racks well above center-line onto the trailer. About 300+ lbs has been added on the front rack and rooftop, which increased the tongue weight considerably (to over 12%) and was sill favorably placed in regards to COG, being mostly forward of the axle (though high up), where previously there had been only about 120 lbs located in the top third of the trailer.

* If I can ever find my lost files, I'll integrate the current configuration to revise the COG calculations. In any case, on the road, the increase in tongue weight percentage, and heavy -weight additions to the lower third of the trailer (reinforced framerails, beefy 3500 lb axle set-up, larger/wider/heavier tires) all have countered any ill effects from adding weight to the topside. Also lowering the former nose-up towing position to 100% level also helped. The heavier the trailer became, the more stable the ride became also.

from another thread
working on it wrote:* Though I intended to build a lightweight trailer at first, 1000 lbs or under, I ended up going far in the other direction. I used 3/4" plywood for most of it, and 1" and 1/2" oak for interior bracing and fitments, and steel reinforcements everywhere. I load it to the max, with onboard A/C and generator, and storage boxes filled with supplies, so it's now over 2150 lbs (on my last trip). I usually tow it using a Weight Distribution hitch (especially on long trips) behind a heavy-duty pickup ('04 Chevy 2500HD), that I beefed up to haul my racecar trailer. Wind is not a problem.

* In Texas, the speed limits are 75 mph, and the best way to drive without incident here is to keep up with traffic, so I usually go 70-75 mph (but have been known to go faster, while passing in some situations). I keep my vehicles in top condition (trailer, too), and use LT tires on all, so typical "may-pop" trailer tire worries are now gone. I've towed larger trailers thousands of miles, so my small 4x8 is almost negligible behind my truck, semi's passing isn't noticed, and towing it is a breeze. I really don't think that I'd like a lightweight trailer back there, now. And didn't one TTT forum member have their foamie blow over once?
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centers of gravity, old & new.jpg
a long time ago, I kept track of everything on the trailer; now, not quite as accurately
centers of gravity, old & new.jpg (118.6 KiB) Viewed 703 times
2013 HHRv "squareback/squaredrop", rugged, 4x8 TTT, 2220 lbs
  • *3500 lb Dexter EZ-Lube braked axle, 3000 lb.springs, active-progressive bumpstop suspension
  • *27 x 8.5-14LT AT tires (x 3) *Weight Distribution system for single-beam tongue
  • *100% LED's & GFCI outlets, 3x fans, AM/FM/CD/Aux. *A/C & heat, Optima AGM, inverter & charger(s)
  • *extended-run, on-board, 2500w generator *Coleman dual-fuel stove & lantern, Ikea grill, vintage skillet
  • *zinc/stainless front & side racks *98"L x 6" diameter rod & reel carrier tube on roof
156215157958148599
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