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Open Roads Forum  >  Technology Corner

 > Automatic RV weighing?

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Popsie

Livingston, TX, USA

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Posted: 05/08/12 07:47pm Link  |  Print  |  Notify Moderator

One of my neighbors drives a long haul semi-truck.

His setup has a system that calculates the weight of the trailer by sensing the air pressure in the trailer suspension air bags.

I believe that it's a simple calculation of summing the suspension air pressure (psi) times the surface area of the bottom of all the air bags.

It seems like it would be fairly simple to make the same sort of calculation for the front and rear axle weights of an RV with air suspension.

Any thoughts?

dbbls

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Posted: 05/08/12 07:55pm Link  |  Print  |  Notify Moderator

I don't see haw that could work. The area of the air bag base does not change. The air pressure can vary greatly regardless of the weight. It will not be a simple calculation. There has to be some kind of a sensor in the air bag of your neighbors semi trailer.


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Eycom

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Posted: 05/08/12 10:24pm Link  |  Print  |  Notify Moderator

I seem to recall reading about the RV Safety & Education Foundation providing wheel by wheel RV weighing services. Made sense to me to weigh a rig in that fashion as opposed to using a truck scale to get a GVWR and/or axle weight. While the truck scale approach indicates a modicum of sanity, I'd have to question the accuracy of a scale that's calibrated for much, much heavier loads.

I would imagine that RVSEF is on to something. Portable scales, properly calibrated, would provide more accuracy and probably save a few tires. IIRC, their service also includes weighing the towables and the tow vehicle separately to get accurate pin weight. Maybe a Google search will provide more information for anyone interested.


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fla-gypsy

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Posted: 05/08/12 08:43pm Link  |  Print  |  Notify Moderator

I am aware of this technology but I do not know how it works. A lot of flatbed semi trailers use it now. I also understand it is only accurate to a degree, does not give an exact weight but rather a calculated weight.


This member is not responsible for opinions that are inaccurate due to faulty information provided by the original poster. Use them at your own discretion.

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donn0128

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Posted: 05/08/12 08:37pm Link  |  Print  |  Notify Moderator

On board scales are real common on log and chip trucks. However they are notorious for being inaccurate. Besides I just don't see a need for them. There are several DOT highway scales on my way out of town. And I simply stop at one that is closed. I have been within 100 pounds every time I stop, so That is close enough for me.


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gijoecam

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Posted: 05/09/12 07:48am Link  |  Print  |  Notify Moderator

It's going to take a certain amount of pressure to support a certain amount of load at a ceertain distance of extension for the airbags, or ride height. Add more weight, the bags compress, so they add more air to restore the ride height. Dump the trailer, the ride height climbs, so you drop the air pressure to reduce the ride height.

If you know how much pressure it takes near the top of the capacity, and you know how much pressure it takes when empty, you can approximate the weight of the load based on the air pressure as it's an almost linear relationship. For example, if it takes 10PSI empty, and 110PSI with a load of 50,000 lbs on it to set the same ride height, if you throw an unknown load in there, and it takes 60PSI to establish that ride height, you know you have roughly 25,000 lbs in it, give or take a bit.

I'm sure there are more accurate ways to estimate loads, using more accurate load sensors on key components, but that's air pressure alone makes a decent ball-park guesstimate.

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Posted: 05/09/12 04:58pm Link  |  Print  |  Notify Moderator

We have load cells on all our grain carts. They're electrical, not air. Very accurate. The difference between what we read on the load cell and when taking it across the certified scales doesn't vary more than a few pounds. It would be simple enough to mount them on an RV type trailer however I don't think it would be cost effective.
If interested in doing such a mount then just do a websearch of "load cell". Plenty of choices.


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tatest

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Posted: 05/09/12 04:14pm Link  |  Print  |  Notify Moderator

It would have to be the air pressure at a particular ride height. Or it could measure the ride height at a particular pressure, the way some aircraft can weigh themselves. What you actually need to know is the internal area of the air bag, but for a sloppy system you could make some assumptions about the changes in other dimensions and measure height only.

You could also calculate the weight from tire pressure needed to maintain a particular contact patch. Or just as well, at a given tire pressure you could calculate the load being supported by measuring the contact patch. Either way of calculating it, you need a way to accurately measure the contact patch, just as you need a way to measure the internal area of an air spring.

For that matter, on a vehicle with steel springs, you can measure weight by measuring spring deflection. That's what was going on inside the old spring scale weight/force measurement systems.


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greende

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Posted: 05/09/12 05:39pm Link  |  Print  |  Notify Moderator

Wadcutter wrote:

We have load cells on all our grain carts. They're electrical, not air. Very accurate. The difference between what we read on the load cell and when taking it across the certified scales doesn't vary more than a few pounds. It would be simple enough to mount them on an RV type trailer however I don't think it would be cost effective.
If interested in doing such a mount then just do a websearch of "load cell". Plenty of choices.

X2 probably a load cell. The CAT Scales etc are now load cells.


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johntichy

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Posted: 05/09/12 01:12pm Link  |  Print  |  Notify Moderator

Suspension air pressure? Right!!!!!!

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