Pondering the COG of a camper mounted on a Mitsu or Isuzu

[Jfet]

Perhaps I was a bit hasty. I ordered four 270 watt solar panels for our Isuzu NRR flatbed truck camper build. They are pretty massive but fit easily on the flat roof because there is nothing else up there.

I started thinking about making the roof rack to hold the panels, and perhaps making it able to tilt (manually) to get a better angle when parked. I then started worrying about the COG of our truck + camper (vertically) because the panels + rack could easily be 300 pounds and that would be 12 feet off the ground.

We have a tall camper (8 feet of headroom) but we have tried to keep the weight down low both in frame construction and placement of heavy items like water/holding tanks and batteries. I am not really sure how I might accurately determine the current COG though. Possibly I could go back through the bill of materials and try to estimate.

What would you do? I don't have experience with Mitsubishi or Isuzu trucks to know how they handle sway when loaded. I guess things could be done (placing heavy storage compartments under the flatbed) to bring down the COG. Do you think it is enough of a concern that I should hold off adding the solar panel system until I get the camper mounted on the truck and take it for some test drives?

Here is a model of what it will look like on the truck (with solar panels):

 

[Jfet]

Nevermind, I pondered it using the body builder's guide and it looks like we are nowhere close to exceeding the maximum vertical CoG distance for a NRR which is 63"

I documented my calculations here if you are interested:

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/26735683/gotomsg/27630338/p/1.cfm#27630338

 

[LeishaShannon]

Excellent work! We have 4 x 300W panels weighing 16.5Kg each and I was concerned about COG, I'm somewhat less concerned after your calculations

 

[gait]

be careful, its not just COG!

Its also about the distribution of mass. Moment of inertia is mass x distance from COG squared.

So, a heavy weight at roof height, compensated by a heavy weight low down, will roll much more than with the same weights concentrated at COG height.

Similar occurs when we put heavy spare wheels hanging off the extreme of the rear or on the bull bar, we get more pitch, and (less noticeable) yaw.

Interesting twist is to have COG and Roll Centre aligned. Helps stability in high side winds, reduces roll steer (with rigid front axle roll causes steer).

Having said that I have 600w of solar on the roof, 3m above ground, simple flat mounts, and vehicle around 4.5mt is stable though tending to uncomfortable at speeds greater than signed warning speed for corners. I take care when no or adverse camber. I normally drive conservatively. I don't notice any difference between empty/full water/fuel tanks - about 450kg low down weight.

Easier to visualise the above is maybe watch an ice skater in a spin. They move arms and leg in to spin faster, extend arms and leg to spin slower. Mass and COG unchanged, moments of inertia (and thus angular momentum) changed.

 

[dlh62c]

Have you considered sourcing different panels?

Global Solar sells panels that are attached to the roof using adhesive, there's no massive external holding frame required. I think the 100 watt panels weight around 8 lbs each. Last I heard, ATW USA is going to install these panels on the roofs of their Alpha Camper builds.

 

[Jfet]

be careful, its not just COG!

Its also about the distribution of mass. Moment of inertia is mass x distance from COG squared.

So, a heavy weight at roof height, compensated by a heavy weight low down, will roll much more than with the same weights concentrated at COG height.

Gait, thanks for the warning. I took a course in statics and dynamics about 20 years ago as part of my EE coursework but things are a bit fuzzy

I do hope the body builder's guide issued by GM/Isuzu takes into account dynamic forces when they give the 63" maximum vertical CoG. We should be well under that (I would guestimate 50" ). They do show in the pictures things like reefer units, which are probably several hundred pounds right at the top of the reefer box. I see what you mean though about the increased roll with a large mass up high but this would be a factor at speed, correct? If you were moving slow on a side hill then the dynamic force would not come into play and it would be more like what I calculated?

I plan to drive very carefully also

 

[Jfet]

dlh62c,

I already have the 1080 watts of monocrystaline panels on hand so there is that. Also, the CIGS cells, while efficient, are not really close to the efficiency of monocrystaline. My panels have a overall efficiency of 17.2% vs 11% for the powerflex. I am not sure of the price but I believe they are around $5 a watt compared to the $0.73 a watt I paid. They do have advantage of lower weight but it isn't as drastically lower as you might imagine. I would need 11 panels to get the same output I have now. That is 88 pounds vs 170 pounds for my four panels. Figure 70 pounds for support structure and I am at 240 pounds. Ok it is quite a bit lighter But I think it would cost $3,000+ vs $1000 and I am not sure the sticky panels would adhere to the truck bed liner I coated our aluminum roof with.

The park in any direction is somewhat BS. I have a 26 watt CIGS panel I use hiking and the output goes from 100% to about 20% as you tilt it away from the sun. There is definitely a benefit from angling a panel toward the sun, even if it is a CIGS.

 

[mog]

This would be more on the timeline of after completion, but Doug did a lot of work calculating the C.G. /roll center of his Fuso:HERE

 

[Jfet]

This would be more on the timeline of after completion, but Doug did a lot of work calculating the C.G. /roll center of his Fuso:HERE

Yikes, his vertical distance to C.G. is 58.92" and his rig is quite a bit shorter than ours.

Ah the Mitsubishi he uses (2007 Fuso FG 140 4x4) has a vertical C.G. distance of 28.3" where my Isuzu NRR is 18.9" (Isuzu wins! heh heh)

I wonder if 58.92" fits within the recommendations by Mitsubishi for total vehicle C.G. I didn't see their recommendation but Isuzu it is 63" for a NRR

 

[pugslyyy]

Yikes, his vertical distance to C.G. is 58.92" and his rig is quite a bit shorter than ours.

Ah the Mitsubishi he uses (2007 Fuso FG 140 4x4) has a vertical C.G. distance of 28.3" where my Isuzu NRR is 18.9" (Isuzu wins! heh heh)

I wonder if 58.92" fits within the recommendations by Mitsubishi for total vehicle C.G. I didn't see their recommendation but Isuzu it is 63" for a NRR

View attachment 218993

The stepped frame of the FG to accomodate the front axle is what makes the CG a bit taller than a 4x2, but that NPR is a lot better.

Also, since you sit so high in the FG it really magnifies the sensation of 'tipping over'. I'd say that the 30+ degrees feels about right - but the pucker factor kicks in quite a bit earlier.

 

[gait]

very much dynamic, not static. I guess the vehicle wouldn't be much use if it didn't move.

Depends where its going. Perhaps more of a "lurch" than a "roll" but crossing a rough creek bed provides some interesting movement.

I guess I've become used to it.

30 degrees is roughly equivalent to "beam me up Scotty". Only been there once. When we fell of the road.

But basically, the further away from COG the individual masses are the harder to start moving and the harder to stop.

 

[Iain_U1250]

I was surprised how far a Unimog can go on the static test - 38 degrees side slope.

Trying to drive on a side slope is one thing, but the more likely scenario is more dynamic like getting bogged in a desert by just driving slightly off the hard track.

 

[whatcharterboat]

I was surprised how far a Unimog can go on the static test - 38 degrees side slope.

Hi Iain,

38degrees is what we get out of a Japanese chassis chassis with a high bus module on the rear. Like you say, that's a crazy angle.

I have a pic somewhere.


The Ivecos will go a long way too. 45 is not the tilt angle indicated but you can see the body is leaning a long way over towards that angle.


Regards John.

 

[whatcharterboat]

Trying to drive on a side slope is one thing, but the more likely scenario is more dynamic like getting bogged in a desert by just driving slightly off the hard track.

Yeah that's a crazy angle too.

Here's a canter in a similar situation.



BTW He was able to drive out after a cold one.

 

[gait]

1933 double deck London buses would get to about 35 degrees,

Canters do a bit more ...... just a moment's loss of concentration,