“Combined” Weighbridge weight correct or wrong?

[Hutch]

There are ramps available for the Reich, which stop it from tilting too much, and leaves the unit proud by about 1 cm. They are not made by Reich.
Whilst this is fine for a single axle, there are no ramps designed as far as I can find so suit what Reich are suggesting to do.


Maybe these ones might work though for a twin axle.

 

[Saxo Appeal]

There are ramps available for the Reich, which stop it from tilting too much, and leaves the unit proud by about 1 cm. They are not made by Reich.
Whilst this is fine for a single axle, there are no ramps designed as far as I can find so suit what Reich are suggesting to do.
View attachment 7003

Maybe these ones might work though for a twin axle.

@Hutch …Now your just toying with my emotions

😂

 

[Hutch]

Don't do it I have thought about it myself. The Duo one , means the you put the first one with the unit on , and the other behind. It doesn't work.
Send the Reich back.
It is really,only good for single axle units or motorhomes / cars.

 

[ProfJohnL]

45mm isn't going to shift the Centre of Gravity that much to make a difference worth considering.

As Roger L has written each caravan will be different , as its the height of the Cog which also important. The greater the height, the greater the horizontal shift when a wheel is lifted. The height of the COg will vary as every caravanner will use a different loading scheme.

I don't think any one can adequately predict the variation without some quite complex calculations. Its the same effect that causes the change in nose load if the hitch is not weighed at the same operating height.

The simplest solution is to ensure a similar lift height is applied to the other wheel on the axle.

Because there is no consistent offset or dynamic correlation, it would be impossible for Reich to make the systems auto compensate for the effect as suggested in #258

 

[Buckman]

I wonder how accurate the scales are that DVSA or police use to check trailers? Maybe they have a tolerance of about 2-3%?

 

[Lutz]

If 2-3% makes the difference between “go” and “no-go” I would say that you are sailing a bit too close to the wind in the first place.

 

[Buckman]

If 2-3% makes the difference between “go” and “no-go” I would say that you are sailing a bit too close to the wind in the first place.

I was referring to the scales and not the weight of the unit being weighed. How accurate are the scales?

 

[Saxo Appeal]

Reich now returned back to Amazon….

Can’t be bothered now with all the faffing about with ramps, and I’m pretty sure it’s not been calibrated properly.

It should have also weighed me at 95kg give or take 1-2 either way, but weighed me at 114-115kg after 5 attempts at testing., that just leads me to it’s not calibrated properly.

 

[Hutch]

Reich now returned back to Amazon….

Can’t be bothered now with all the faffing about with ramps, and I’m pretty sure it’s not been calibrated properly.

It should have also weighed me at 95kg give or take 1-2 either way, but weighed me at 114-115kg after 5 attempts at testing., that just leads me to it’s not calibrated properly.

I think you have done the sensible thing, It is not suited for a twin Axle.

 

[Saxo Appeal]

Soon as I noticed it didn’t give the correct reading when I stood on it, I was very dubious but trying to make it work!

I also asked the MFTR regarding the calibration certificate, but not heard any reply yet.

 

[AndrewAventura]

You can shift wheel load on a twin axle from one to the other without altering the CoG.

For example, assume the spring rates are 10 kg per mm and that the front wheel is on a 20mm block while the rear wheel is in a 20mm hole so the caravan is level across the axles - the load on the front wheel will increase by 20 x 10 = 200 kg and the load on the rear wheel will decrease by the same - any measurement of the front wheel will be 200 kg more than normal - if you then change the configuration so the rear is higher while it's being measured it will equally read too high - thus adding the two figures together will overstate the total weight on that side of the caravan.

The only way to use the Reich accurately is to make up planks to match the height of the Reich measuring pad.

I think my assumed spring rates and wheel deflection are of similar order to Saxo Appeal's testing and that his computed weight is over-stated significantly.

For what it's worth, RogerL is absolutely right in this analysis.

I nearly embarrassed myself by adding a very similar post, but fortunately checked back a page or two first!
It is also consistent with the reply given by Reich themselves. This kind of load cell is entirely appropriate for adding up wheel loads to get the overall load, but if you have more than 3 contact points / wheels then the technique you use to measure the load at one wheel without influencing the reaction at the others is maybe a bit difficult to achieve, certainly not obvious. Without that extra plank compensation, I figure that the Reich device would overestimate the weight supported at each wheel by about 50kg, so give a result for the caravan about 200kg more than it actually weighs.

Here's a little experiment for you to try at home to demo the problem. Find yourself a 4 -legged chair and place it with one leg on your bathroom scales. Sit on it, and you're going to get a result of about 50kg (depending on how corpulent you are. And how heavy your chair). Do the same thing for all 4 legs. Add up the weight recorded. Then, weigh yourself, standing on the scales, holding your chair. That number will be much less.

Have fun, and be careful!

 

[Saxo Appeal]

For what it's worth, RogerL is absolutely right in this analysis.

I nearly embarrassed myself by adding a very similar post, but fortunately checked back a page or two first!
It is also consistent with the reply given by Reich themselves. This kind of load cell is entirely appropriate for adding up wheel loads to get the overall load, but if you have more than 3 contact points / wheels then the technique you use to measure the load at one wheel without influencing the reaction at the others is maybe a bit difficult to achieve, certainly not obvious. Without that extra plank compensation, I figure that the Reich device would overestimate the weight supported at each wheel by about 50kg, so give a result for the caravan about 200kg more than it actually weighs.

Here's a little experiment for you to try at home to demo the problem. Find yourself a 4 -legged chair and place it with one leg on your bathroom scales. Sit on it, and you're going to get a result of about 50kg (depending on how corpulent you are. And how heavy your chair). Do the same thing for all 4 legs. Add up the weight recorded. Then, weigh yourself, standing on the scales, holding your chair. That number will be much less.

Have fun, and be careful!

Other half thought I was mad but tried this test using chair and bathroom scale

1st leg = 42.55
2nd leg = 34.65
3rd leg = 34.65
4th leg = 41.45
= 153.30 kg

Standing on the scale with the chair = 103.20 kg

 

[ProfJohnL]

...Here's a little experiment for you to try at home to demo the problem. Find yourself a 4 -legged chair and place it with one leg on your bathroom scales. Sit on it, and you're going to get a result of about 50kg (depending on how corpulent you are. And how heavy your chair). Do the same thing for all 4 legs. Add up the weight recorded. Then, weigh yourself, standing on the scales, holding your chair. That number will be much less...

That is a very poor example, for three reasons. Firstly a chair is usually a ridged structure, and raising one leg enough to sit on a set of scale's would mean the chair is only standing on three legs. where as a TA caravan has a compliant suspension that will allow all four wheels to remain actively supporting the caravan.

The second reason is the chair would be sharing the actual load through only three points of contact. For the sake of demonstration if we assume the load on chair is shared equally on each supporting leg, then the measurement you take will be 1/3 of the total, so if you measured all four legs and summed the result it would be 4/3 of the real load on the chair.

But from a practical point of view, the third reason: if you try your suggestion on a chair , I doubt anyone could guarantee you could keep exactly the same position on the chair each time the legs were changed for measuring. and it would be subject to point two as well.

 

[Lutz]

I was referring to the scales and not the weight of the unit being weighed. How accurate are the scales?

I appreciate that, but the same applies to the accuracy of the scales. If 2-3% inaccuracy is going to be a major stumbling block, then you would also be sailing too close to the wind.

 

[Buckman]

I appreciate that, but the same applies to the accuracy of the scales. If 2-3% inaccuracy is going to be a major stumbling block, then you would also be sailing too close to the wind.

I don't understand what you are trying to say as it seems to be totally different and no relation to the context of my post?

 

[Saxo Appeal]

That is a very poor example, for three reasons. Firstly a chair is usually a ridged structure, and raising one leg enough to sit on a set of scale's would mean the chair is only standing on three legs. where as a TA caravan has a compliant suspension that will allow all four wheels to remain actively supporting the caravan.

The second reason is the chair would be sharing the actual load through only three points of contact. For the sake of demonstration if we assume the load on chair is shared equally on each supporting leg, then the measurement you take will be 1/3 of the total, so if you measured all four legs and summed the result it would be 4/3 of the real load on the chair.

But from a practical point of view, the third reason: if you try your suggestion on a chair , I doubt anyone could guarantee you could keep exactly the same position on the chair each time the legs were changed for measuring. and it would be subject to point two as well.

Well now I feel stooooopid 😂

 

[AndrewAventura]

That is a very poor example, for three reasons. Firstly a chair is usually a ridged structure, and raising one leg enough to sit on a set of scale's would mean the chair is only standing on three legs. where as a TA caravan has a compliant suspension that will allow all four wheels to remain actively supporting the caravan.

The second reason is the chair would be sharing the actual load through only three points of contact. For the sake of demonstration if we assume the load on chair is shared equally on each supporting leg, then the measurement you take will be 1/3 of the total, so if you measured all four legs and summed the result it would be 4/3 of the real load on the chair.

But from a practical point of view, the third reason: if you try your suggestion on a chair , I doubt anyone could guarantee you could keep exactly the same position on the chair each time the legs were changed for measuring. and it would be subject to point two as well.

It's an extreme illustration, but that doesn't really make it a poor example.

I see from some of your earlier posts that you were thinking of the impact of height difference on cofg, but that really will be negligible on this caravan weighing example. Much more significant is the weight transfer due to the compliance at each of the four reaction points on a TA caravan, as I can see has been commented by Beardy and RogerL, maybe others. Caravan suspension is so stiff, even 2cm at one wheel will have a significant effect on the measured load. Balancing with a similar thickness plank at the adjacent wheel compensates for the error.

 

[AndrewAventura]

Well now I feel stooooopid 😂

No, really no need for that! It wasn't a trick or anything, just a very extreme case of what is happening with load transfer.

Admittedly it would be more relevant if using a chair with suspension so that all four legs maintained contact with the floor. Probably easier to find someone with a TA caravan and Reich scales and do the measurements described, with or without the compensating pad.......

 

[otherclive]

Easier

No, really no need for that! It wasn't a trick or anything, just a very extreme case of what is happening with load transfer.

Admittedly it would be more relevant if using a chair with suspension so that all four legs maintained contact with the floor. Probably easier to find someone with a TA caravan and Reich scales and do the measurements described, with or without the compensating pad.......

Easier still to find a weighbridge and there after be disciplined in what you carry in the caravan. Something in: something out.

 

[Saxo Appeal]

No, really no need for that! It wasn't a trick or anything, just a very extreme case of what is happening with load transfer.

Admittedly it would be more relevant if using a chair with suspension so that all four legs maintained contact with the floor. Probably easier to find someone with a TA caravan and Reich scales and do the measurements described, with or without the compensating pad.......

I know, just messing

If it worked it worked but hey ho!

 

[Saxo Appeal]

Easier

Easier still to find a weighbridge and there after be disciplined in what you carry in the caravan. Something in: something out.

We will be going to one next time which reads in KG and will weigh the caravan on its own much like when we bought it 😊

 

[Lutz]

I don't understand what you are trying to say as it seems to be totally different and no relation to the context of my post?

What I meant to say was that if, say, 30 or 40kg underreading actual weight is going to be a problem, regardless of whether it’s due to inaccuracy or inadequate resolution of the measuring equipment or operator error, then you’re sailing darned close to the wind.

 

[Buckman]

Caravan suspension is so stiff, even 2cm at one wheel will have a significant effect on the measured load. Balancing with a similar thickness plank at the adjacent wheel compensates for the error.

Would it make any difference if the caravan had shock absorbers fitted?

 

[RogerL]

Would it make any difference if the caravan had shock absorbers fitted?

No - shock absorbers / dampers don't affect the static spring rates.

 

[otherclive]

Would it make any difference if the caravan had shock absorbers fitted?

No as they only act dynamically.