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2 + 2 = 5??

4357 Views 15 Replies 9 Participants Last post by  Biglol
Can anyone answer this conundrum. I've looked at it every which way, but still can't work it out.

We recently took our MH to a nearby weighbridge (fully loaded for a week away) to check we were all legal.

As it turned out we were well in but the results were interesting.

Front axle = 1620kg

Rear axle = 1660kg

Total weight = 3340kg

Now I would have expected the total weight to be the combination of the front and rear weights but it's 60kg more. I was eating a Mars bar at the time but even so!!!

Any ideas?
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It cant be more than the front and back weights put together.

I'm guessing you had the same problem i used to have driving tankers, if you want to weigh something with a moving load you have to put the axle on and wait a couple minutes for the load, to settle. If you drove on and they snapped the weight, your fuel, water and waste tanks would have still been moving hence the diffrence in figures.

Still its good to know your under weight anyway.
Plus if the weighbridge is not level with the surrounding road you will have a small error when weighing the single axles.
Read paragraph 2 at

Individual limits do not need to add up to the total permissable mass.

Dave P
sorry that takes you to the first page of the site.

easier way
google axle weight limits.
scroll down to Van legislation.

then away you go

Dave P

Thinking about it as I do occasionally. :lol: :lol:
I feel it would make a difference depending on just how far you placed the wheels on the weighbridge.

I would guess it would have a different reading with an axle only just on the scale as against fully on with the other axle only just off.

Does this make sense? Cos I also got slightly differing totals from weighing my three axles separately and two together.

Hi Ray
should not make a difference how far you drive onto the weighbridge, the weight on the axles does not change.

Dave p

Dave P That depends

Have you ever stood on the edge of bathroom scales?

Also I think your earlier point about them not adding up to the same is not about weighing but about the difference between MAM and Max per axle where the total of adding both axles can be a very different from MAM dependant upon the sort of vehicle eg a downplated MAM will still have the same axle ratings.
Yes Frank and the scales weigh less, but my weight is the same as standing in the middle.
As long as you are on the weighbridge in the position indicated by the operator you are ok.
i am speaking from experienc of being pulled up "suspected overweight" and w/bridge opertator explained it to me.

Dave P

Many thanks for the replies, at least I know I'm not losing it (yet) and 2+2 should =4.

From your comments it could be down to a couple of things. As this was a quarry weighbridge, used to weighing big lorries, then maybe the positioning on the bridge might give errors, particularly at the lower end of the scale, where it's weighing a single axle. There was also a bit of a ramp up/ramp down so maybe (as Frank suggested) this would also have caused errors.

If I take the worse case scenario and add the "missing" 60kgs to either axle weight then they are both still within limits and my overall weight still gives me 160kg to bring some wine back from Europe in the summer!!

:lol: :lol: :lol:
I can't see there's a mystery. Inevitably there must be an element of double-counting the part of the load between the two axles.

Hi Dave,
I can't argue the technical reasons why there should be any difference how far on the scale you are positioned but just like a dragster the weight distribution must make a difference.

Anyway, the sum of my three axles ends up more than the total weight.

Hi ray its a bit early in the morn but basicly it depends on where the suspension points are on the weighbridge. near the end = you do not have to drive on too far. Nearer the centre = lower reading at the edge.
tried it out at one of my customers who way 40 footers.

Dave P

As this was a quarry weighbridge, used to weighing big lorries, then maybe the positioning on the bridge might give errors, particularly at the lower end of the scale, where it's weighing a single axle.
The fact that this weighbridge is normally used for weighing trucks is immaterial - if the weighbridge is used for trade purposes (i.e. a charge is made for the load carried, which is sold by weight), and is a plate weighbridge (from your description, it is) then it must have been calibrated by Trading Standards and will therefore be accurate.

However, not all operators check their plates regularly, being a quarry, there could be bits of stone or whatever lodged at the edges of the plate, preventing it from "floating" freely, this will cause variations in the weight of anything weighed. 8O

As far as the weighing itself goes (i.e. split weighing) - firstly I would remind you that the approach to and exit from the plate must be level.

To weigh the first axle, you should drive the vehicle onto the plate until the rear axle is just clear of the plate, then take the reading. The second (or in the case of a 'tag' axle, the second and third axles) axle weight is obtained by driving the vehicle forward until the first axle is just clear of the plate, then take the reading. The sum of the axle weights is the gross vehicle weight (so 2+2 does equal 4) - this is always the case and has been for as long as I've been weighing vehicles for enforcement purposes (which is longer than I care to remember). :roll:

Also remember, with a tag axle, the two "closely spaced" axles are treated as one axle for weighing/enforcement purposes so should be weighed together, as long as you do not exceed the sum of the two 'plated' weights (e.g. each axle plated at 1500 kg so combined weight is 3000 kg). It is not easy to weigh each axle of a tag arrangement separately on a plate bridge, the only accurate way to do it is on a "dynamic" axle weigher or axle weight plate bridge, and there aren't too many of those about.


Keith (Sprokit)
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If the weighbridge is for weighing quarry lorries; at how many tonnes!?
then what's likely to be it's 'resolution'; it must have an accuracy to the nearest...
-and just because it's calibrated and the display shows smaller units, it's not necessarily within it's accuracy.

(eg with a tolerance of +-2; then 2+2 could be anything between 2 & 6!)

Could it tell the difference if you were on it or not too, let alone a mars-bar?!
2 + 2 does = 5 if you use the large 2's

Biglol :)
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