Ramped weighbridge calculation suggestion. (1 Viewer)

[nicholsong]

It is known that ideally one should use a flat weighbridge to get accurate individual axle figures.

Some of us do not have access to a flat one, only one with approach and exit ramps.

With a ramped weighbridge if one has an axle just on the plate the other axle is down the ramp and taking some of the weight off the axle being weighed, so inaccuracies creep in.

In order to try to compensate for the inaccuracies I have thought of this method, which may have been suggested before.

THIS METHOD FOR COMMENTS PLEASE

On a flat weighbridge the two axle weights should add up to the GVW.

On a ramped weighbridge they will almost certainly fall short of the GVW.

So for example the GVW is weighed at 3800kg but the total of the axle weights shows 3600kg. Therefore the inaccuracies caused by the ramps equal 200kg.

The 200kg needs to be added to the axle weights to compensate for the inaccuracies, but adding a straight 100kg to each axle weight would not take account of the usually unequal distribution of weight between front and rear axles.

To compensate for this unequal distribution I propose that one calculates the ratio of distribution by front weight /rear weight and apply this ratio to the 200kg and then add each portion to the weighed axle weights.

This formula may not result in an totally accurate figure compared with a flat weighbridge but should give a good approximation.


COMMENTS PLEASE

 

[funflair]

Looks close enough to me Geoff but getting the off bridge wheels as close as possible to the bridge should minimise any discrepancies.

 

[DavidG58]

Quite often the measured weight is visible from the cab, it would be interesting to compare different positions on the slope to see the actual difference

But as above once the wheels are off the weighing plate the weight should be right for the remaining axle

 

[Barti Ddu]

As funflair says, getting the non-weighed axle wheels as close to the top of the ramp as possible, but just shy of the weight plate, is going to minimise the error, but it's not ideal.

Taking several goes to check repeatability, and perhaps average out the various axle readings obtained may be useful too.

It really depends on how short and steep the ramps are. If they are very steep then the axle weight readings may be well out.

Allocating the resulting errors according to the ratio of reading obtained as you suggest makes sense and will probably provide reasonable reassurance that you are legal providing you're not sailing too close to the wind weight wise already.

It's not going to help you - but we're in Norway at the moment and there are loads of weighbridges provided by the authorities for enforcement purposes in lay-bys at the side of the road. I've not seen any being manned yet, but they are turned on 24/7. I've jumped on a couple myself to check axles and gross weight. Plates are short, but just long enough to fit a PVC or short wheelbase motorhome on, and the approaches are nice and level to get good axle weight readouts too. Shame there's not more in the UK (or in your case, Poland).

 

[donkey]

It is known that ideally one should use a flat weighbridge to get accurate individual axle figures.

Some of us do not have access to a flat one, only one with approach and exit ramps.

With a ramped weighbridge if one has an axle just on the plate the other axle is down the ramp and taking some of the weight off the axle being weighed, so inaccuracies creep in.

In order to try to compensate for the inaccuracies I have thought of this method, which may have been suggested before.

THIS METHOD FOR COMMENTS PLEASE

On a flat weighbridge the two axle weights should add up to the GVW.

On a ramped weighbridge they will almost certainly fall short of the GVW.

So for example the GVW is weighed at 3800kg but the total of the axle weights shows 3600kg. Therefore the inaccuracies caused by the ramps equal 200kg.

The 200kg needs to be added to the axle weights to compensate for the inaccuracies, but adding a straight 100kg to each axle weight would not take account of the usually unequal distribution of weight between front and rear axles.

To compensate for this unequal distribution I propose that one calculates the ratio of distribution by front weight /rear weight and apply this ratio to the 200kg and then add each portion to the weighed axle weights.

This formula may not result in an totally accurate figure compared with a flat weighbridge but should give a good approximation.


COMMENTS PLEASE

I understand your wishing for accurate figures, but most of the commercial weigh bridges round here are only accurate to plus or minus 10kg. So there is always a bit of guess work involved.

Geoff

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[cyberyacht]

With mine, the difference on the sum of the individual axles and the total weight was only 20Kg so is it really anything to worry about unduly?

 

[nicholsong]

With mine, the difference on the sum of the individual axles and the total weight was only 20Kg so is it really anything to worry about unduly?

Well I suppose the difference will vary according to the steepness of the ramp. I have only just thought up this method of calculation, so will put it to the test in the next days, when I visit the weighbridge after loading to full touring trim.

 

[Barti Ddu]

Well I suppose the difference will vary according to the steepness of the ramp. I have only just thought up this method of calculation, so will put it to the test in the next days, when I visit the weighbridge after loading to full touring trim.

I'd be interested to see your actual obtained figures. Perhaps take a photo too of the weighbridge and ramps to add to the discussion.

 

[John-Laika]

Ours local one has a on and off ramp the DVLA have stopped them from weighing motorhomes for this reason

 

[nicholsong]

Ours local one has a on and off ramp the DVLA have stopped them from weighing motorhomes for this reason

I presume you mean DVSA(ex-VOSA).

DVLA personnel mostly would probably not know a weighbridge if they were standing on it !

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[John-Laika]

Yes now you mention it your right I will put my error down to old age I put most things down to that

 

[dunnah01]

If I'd stayed awake long enough during lectures I'd know it was all to do with vectors. A quick fag packet calc tells me that at 4.3m wheelbase on a 500mm ramp height our fully loaded 2400kg axle would read under by 12kg at 2380kg

 

[Cush]

getting the off bridge wheels as close as possible to the bridge should minimise any discrepancies.

Used this method today, axle weights 20kg out for overall GVW

 

[Scotch mist]

COMMENTS PLEASE

I think you think too much

Must have too much time on your hands

 

[barryd]

I bet it doesn't make much difference otherwise why have a weighbridge with a ramp? Surely it's used all the time for weighing axles and not just for motorhomes. I think it will be near enough Geoff and your calculations sound about right.

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[Barti Ddu]

I bet it doesn't make much difference otherwise why have a weighbridge with a ramp? Surely it's used all the time for weighing axles and not just for motorhomes. I think it will be near enough Geoff and your calculations sound about right.

Weighbridges with ramps are much cheaper to install than those within a pit, and much easier to maintain with easier access to load cells. They also don't suffer the issue of the pit filling with water and pumps having to be installed.

Weighbridges are used primarily for commercial transactions so it's only the entire truck weights that matter. Weighed twice, once empty for the tare weight, and then full for the gross weight. Subtract the former from the latter to give the net weight of product being transported.

Vehicle body designers, commercial operators and drivers have a much better understanding of axle weight limits than the typical motorhome owner (MHF members being an exception)!. A truck for sand and aggregate, or a road tanker for petroleum products will be designed so that if loaded correctly and within gross limits, then it's axle weights won't be an issue. Contrast this with a motorhome, where no two users will load the same make and model of motorhome the same - some running pretty empty, others with a motorbike in the garage, or e-bikes hanging off the back, making the possibility of overloading the rear axle very real.

 

[Guigsy]

If I'd stayed awake long enough during lectures I'd know it was all to do with vectors. A quick fag packet calc tells me that at 4.3m wheelbase on a 500mm ramp height our fully loaded 2400kg axle would read under by 12kg at 2380kg

It'll be a little more as fluid moves in the tanks and the suspension says and transfers a bit more load. But yeah, it ain't gonna be a lot.