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Old 03-11-2011, 05:31 PM   #1 (permalink)
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Default air bag pressure to lbs

Does anybody have the formula to convert air bag pressure to lbs. I have look at the intraax load scale for out super bees, and I can load overall to withing a couple hundred kgs no problem, however when I look at their chart I am not sure why the lbs per axle is not a linear relationship. For example at 16 psi their load shows 149kgs/psi but at 95psi the load shows 110kgs/psi, which is a fair bit of difference, so obviously not a linear relationship. Just wondering if anyone has a formula so that it would be easy to determine the psi to achieve a certain load.

This must not be very difficult because you can get air scales for trucks that are supposed to be accurate to within 300lbs, so obviously their must be a non linear formula used but I have not found it.

Any thoughts

Thanks
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Old 03-11-2011, 07:03 PM   #2 (permalink)
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I honestly have no idea, but am curious about this.

I think the partyou have to consider is that if you double the pressure, the same kg/psi means you doubled your load - which is untrue. They're probably going off of documented values, which vary because of the weight of the trailer, # of airbags, etc.

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Old 03-11-2011, 07:55 PM   #3 (permalink)
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A rather very strange mixed set of relationships between the USA imperial measurement and the Metric measurements you claim to have there.
Are you sure that the KGs are being measured as "KG's Per Square Inch" [ Kg / PSI ]

To provide clarity here could you provide the load scale decals number or describe the decal heading or provide an identification so that the scales literature you are using can be checked; EG;

INTRAAX® AAEDT 30K / AAEDL 30K
PSI / LOAD SCALE


The Intraax scale decals I have brought up on the net as the example above, all use the air bag gauge Lbs /PSI to calculate the axle loads.

The Air bag gauge Lbs / Psi readings are in the LH column,
The axle loads that result from those Gauge PSI readings are in "Lbs" in the centre column.
And the exact same axle loads are in kilograms [ Kg's ] in the RH column.

Only Lbs / PSI are used in the airbag gauge readings.


Ps; If you want a good converter from Imperial to metric, after having researched a lot of converters I use the MConvert exclusively as it is almost the only converter that lists all the conversions at once so that you can see all the alternative conversions
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Old 03-11-2011, 08:07 PM   #4 (permalink)
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Quote:
Originally Posted by puddles View Post
Does anybody have the formula to convert air bag pressure to lbs. I have look at the intraax load scale for out super bees, and I can load overall to withing a couple hundred kgs no problem, however when I look at their chart I am not sure why the lbs per axle is not a linear relationship. For example at 16 psi their load shows 149kgs/psi but at 95psi the load shows 110kgs/psi, which is a fair bit of difference, so obviously not a linear relationship. Just wondering if anyone has a formula so that it would be easy to determine the psi to achieve a certain load.

This must not be very difficult because you can get air scales for trucks that are supposed to be accurate to within 300lbs, so obviously their must be a non linear formula used but I have not found it.

Any thoughts

Thanks
There are many variables that affect the ability to convert bag air pressure to weight of load.

Bag diameter, type and make of suspension and number of axles are three of the biggest. You can have the same air bag in two different suspensions and you will get two totally different pressure readings with the exact same weight.

Ride height of the suspension can also affect the bag pressure to weight ratio. Fifth wheel height on different tractors can make a difference on the trailer readings.

I'm sure lots on here will be quick to say I don't know what I'm talking about and that you can accurately measure weight with air bag pressure, I've never seen a system that was able to come within 300 pounds consistently, in over twenty years of trucking and fixing trucks in the oil patch.

Just a couple of weeks ago I shipped a load of barley that was over three tonne more than what the driver thought he had, I knew there was no way he could have been legal. but I kept loading him till he said stop. His gauges told him he was legal....wrong!!

Inquire with the manufacturer of your suspension.
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Old 03-11-2011, 08:55 PM   #5 (permalink)
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Quote:
Originally Posted by puddles View Post
Does anybody have the formula to convert air bag pressure to lbs. I have look at the intraax load scale for out super bees, and I can load overall to withing a couple hundred kgs no problem, however when I look at their chart I am not sure why the lbs per axle is not a linear relationship. For example at 16 psi their load shows 149kgs/psi but at 95psi the load shows 110kgs/psi, which is a fair bit of difference, so obviously not a linear relationship. Just wondering if anyone has a formula so that it would be easy to determine the psi to achieve a certain load.

This must not be very difficult because you can get air scales for trucks that are supposed to be accurate to within 300lbs, so obviously their must be a non linear formula used but I have not found it.

Any thoughts

Thanks
Good luck!
I used to pull super B's in the winters and found these air guages to be consistant from load to load. However, I found that if you wanted to load a different amount of weight from the norm by using these guages and the provided charts, I was SOL. I asked the other drivers who were full time year round, and the owner who has trucked all his life, and they could not tell me either. Different trailers of the same make and model had different readings for the same weights. For obvious reasons, uneven or sloped terrain affected readings, and if load time was fast you were guaranteed to be heavy as the guages were slow to respond.
Hopefully someone smarter and more experienced than I can help you answer this one.

Last edited by irrigationfarmer; 03-11-2011 at 10:50 PM.
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Old 03-11-2011, 09:55 PM   #6 (permalink)
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albertabuck, your right on, there are a pile of variables, but once you know your unit you should be able to get quite close consistently.
irrigationfarmer, heres what I did when I was hauling grain.
When loading always try not to set your brakes, this leaves your axles and wheels free to rotate and move as required.
I would try to rock the unit a bit especially on soft ground or let it coast to a stop when moving while loading.
Another thing is to take multiple readings, if your close to loaded, stop loading take a reading, move the truck and take another. then average your readings.
Always take your readings while the unit is straight so your not side loading your axles and affecting readings.
And of course smooth, hard, level ground.
If your truck has Hendricson or International air ride with the slipper springs you almost have to move it a bit because of the friction between the pads and springs.

With my unit I was accurate within 300kg gross weight 80% of the time and never got an over weight ticket.
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Old 03-11-2011, 10:03 PM   #7 (permalink)
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Seems like I get the weights figured out for a truck and trailer combo and they switch me out to a different set or change trailers etc.... and you are at square one again. Seems to take me 2-3 loads to get it figured out. Last week I loaded 62500 and was a tonne over on the tri, glad I didn't get stopped at the scales, that's why i like to drive at night, the scales are rarely open at night.
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Old 03-11-2011, 11:06 PM   #8 (permalink)
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Yes if you pay attention you can learn what works and what the numbers are for a particular combination of truck and trailer, but like Wheatking said some wonderful dispatcher can screw that up in a heartbeat!

I was lead hand at Raydan Transport out of Nisku for seven years. Yes, this is the company that invented the Raydan Air Link suspension. The original prototype was an 1988 Mack winch tractor, unit 214. My foreman and I installed this suspension into almost all 12 of our older Mack 8 ton picker units and all 6 new 2000 Western Stars.

Once we had everything set and ready, with the help of portable platform scales we tried in vain to determine pressure numbers for each unit for a given maximum allowable weight. The biggest problem was constantly changing truck and trailer combinations, but even if we removed the load and replaced it while still sitting on the scales we got different readings. we gave up on coming up with accurate numbers for each truck and would simply post a sticker on the dash with an approximate reading for the driver to use to help get it close. I remember some being several pounds different.

We were running tandem and tri axle highboys and we ran into the same trouble again. No two trailers rode at exactly the same height. We had trailers of different makes. And very few of our trucks had a consistent fifth wheel height which really seemed to make a lot of difference on the trailer end. Anything that played with the geometery of the suspension made for different readings.

So the moral of the story is, yes, if you know your unit, you can use the air pressure gauges as an approximate means of determining your weight, but like offroad said, uneven ground, brakes applied and a multitude of other factors can render the readings on those gauges useless and aint no scale master I know who will buy that as an excuse for an overweight.
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Old 03-26-2011, 06:52 PM   #9 (permalink)
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LOAD WEIGHT GAUGE with BLUE or RED DISPLAYS - Truck Gauges - Teltek U.S.A INC.
These guys make a gauge that converts PSI to LBS.
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Old 03-27-2011, 04:50 PM   #10 (permalink)
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Default digital scale

This is a digital scale for all axles which compensates for slopes, but i guess its no good if you are swapping trailers.
Air-Weigh On-Board Scales for Air-Ride Trucks and Trailers
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