“Combined” Weighbridge weight correct or wrong?

[Saxo Appeal]

Correct,

Which is why we decoupled the caravan to weight on its own.

We are going with what is allowed regarding the weight plate.

2000kg is the plate weight and which we are sticking to or under

Keeps us legal if ever pulled

 

[Sam Vimes]

The biggest confusion seems to relate to some people being under the false impression that by taking the load off the jockey wheel and placing it on the towball, the caravan suddenly becomes lighter. Strange logic!

Yes....and when I stand on one leg I only weigh half as much.😉

 

[Saxo Appeal]

Yes....and when I stand on one leg I only weigh half as much.😉

Wait …does that mean I’m double the weight 😏

 

[Lutz]

One problem with using the uncoupled weight is the axle weight will be different. The front will now use the jockey wheel which is further back so weight will differ from the hitch and the hitch will probably be higher than the weight when hitched adding a further difference. Only by using a hitch weight gadget can you get the hitch weight.

Or, as I said, you weigh the car first when it's hitched to the caravan and again after it's unhitched.
The difference between the load on the towball and that on the jockey wheel is minor and can, for all intents and purposes be neglected as it's within the accuracy of the weighbridge.

 

[Buckman]

Just to be pedantic, MTPLM could also variable as according to earlier posts as there are two different weight plates on the caravan. One is mandatory and the other isn't. However one probably refers to axle loads and the other to MTPLM?

 

[Lutz]

Just to be pedantic, MTPLM could also variable as according to earlier posts as there are two different weight plates on the caravan. One is mandatory and the other isn't. However one probably refers to axle loads and the other to MTPLM?

One never refers to axle loads because they have their own limits on the same plate. An MTPLM is an MTPLM and an axle load an axle load. Don't confuse the two.

 

[Buckman]

One never refers to axle loads because they have their own limits on the same plate. An MTPLM is an MTPLM and an axle load an axle load. Don't confuse the two.

Isn't the MTPLM based on the axle load?

 

[Lutz]

Isn't the MTPLM based on the axle load?

The MTPLM can never be greater than the maximum permissible axle load, that's true. That's a safeguard to prevent the noseweight to be used as a margin of error, but the MTPLM can be less than the sum of the permissible axle loads, and it often is. That is to allow for a limited variation in weight distribution between axles.

 

[GUMPY]

Check hitch weight before I leave home 80kg
Weigh car and caravan 3200 kg
Drive forward until car is off the weighbridge
Weigh caravan 1420kg
Caravan total weight = 1500kg. (1420 +80)
Car weight when towing 1780kg (3200-1420)

 

[Mr Plodd]

Isn't the MTPLM based on the axle load?

NO, no and no again! Axle weight and gross weight are 2 totally separate things!!!

Axle weight is the maximum weight that can be transmitted by the wheels on that axle onto the road surface.
MTPLM is the maximum weight permissable for the entire caravan when supported on its wheels AND jockey wheel.

A hypothetical situation........

A caravan has an MTPLM of 2,000Kg It's put onto a weighbridge so just the wheels are on the weighing plate and it reads exactly 2,000kg which is what the plated maximum axle weight is. So far so good no apparent offence.
The tow hitch is exerting 150kg onto the tow hitch which the towhitch is designed to carry. Still all well and good.
The caravan is then unhitched and both the axle and jockey wheel are now on the weighing plate. The weight registered will now be 2000 PLUS 150kg.

So the caravan on its own, actually weighs 2150Kg (2000 axle loading plus 150kg jockey wheel loading), so, despite it still being legal on its axle weight (2000kg) it is now OVER ON ITS GROSS WEIGHT by 150kg.

As I keep saying, axle weight and gross weight are NOT the same thing.
I spent 30 years dealing with vehicle weights on a regular basis, so I DO know what I am talking about here., and have had to stand up in court to prove it when questioned by some smart alec solicitors!

 

[ProfJohnL]

You have selectively taken a quote out of context with the rest of my post. It’s not the adherence to the MTPLM I was querying but the fact that it seems that it’s regularly subject to spurious queries about its relevance and legality, if transfer of load to the towbar allows a increase in weight carried in the caravan, etc, and whether the police can enforce it. In some respects my post was TIC. I’m sorry you took it so seriously. As a professional engineer I take safety seriously and I would have thought by now that would have come across.

I cut down your post to keep it succinct, but if the post was tounge in cheek, it wasn't obvious.

I certainly do not consider discussions about weight transfer to be spurious. It's a real phenomena, and in some limited circumstances it can make some car and caravan matches possible, all-be-it marginal and requires great care to understand and achieve it.

The subject usually arrises when someone erroneously equates the tow vehicles Maximum Braked Towed Mass spcifiction as the MTPLM limit for a trailer it can tow.

Where I have proposed it, I have taken great care to explain how it works, but I have never suggested it increases a caravans MTPLM, or allows you to exceed any other statutory or safety limits.

 

[ProfJohnL]

One problem with using the uncoupled weight is the axle weight will be different. The front will now use the jockey wheel which is further back so weight will differ from the hitch and the hitch will probably be higher than the weight when hitched adding a further difference. Only by using a hitch weight gadget can you get the hitch weight.

I presume you can agree that if the same trailer is coupled or uncoupled its total mass is the same, it doesn't change.

Consequently if a coupled out fit pulls onto weigh bridge and continues to roll forward so only the trailers road wheels are on the weighbridge you will get a reading that represents the weight of the trailer minus the nose load acting on the tow vehicle. If the outfit is now uncoupled and trailer sits with its road wheels and Jockey wheel on the weighbridge, the result will be the total weight of the trailer.

The difference between the total weight and the coupled axle load must be the nose load the trailer produces on the tow vehicle.

 

[ProfJohnL]

Surely weighing the caravan on its own allows the MTPLM to be measured. Weight transfer to the tow car isn’t relevant if you need to obtain the weight of the caravan. The weight transfer to the towball can be a variable. But MTPLM isn’t.

Strictly you cannot measure the MTPLM - its a mass limit not a mass.

 

[otherclive]

I presume you can agree that if the same trailer is coupled or uncoupled its total mass is the same, it doesn't change.

Consequently if a coupled out fit pulls onto weigh bridge and continues to roll forward so only the trailers road wheels are on the weighbridge you will get a reading that represents the weight of the trailer minus the nose load acting on the tow vehicle. If the outfit is now uncoupled and trailer sits with its road wheels and Jockey wheel on the weighbridge, the result will be the total weight of the trailer.

The difference between the total weight and the coupled axle load must be the nose load the trailer produces on the tow vehicle.

Gumpy at #59 gave a Dummies guide to using a weighbridge. Very succinct.

 

[otherclive]

Strictly you cannot measure the MTPLM - it’s a mass limit not a mass.

I really needed that little gem.
As a mechanical engineer I’m quite aware of mass cf weight. I was bought up on poundals, slugs, and SI equivalents, but now you are being pedantic for no benefit to the wider readership. Weight is more widely understood by the majority of people who really don’t give a jot for gravity’s input.

 

[ProfJohnL]

I really needed that little gem.
As a mechanical engineer I’m quite aware of mass cf weight. I was bought up on poundals, slugs, and SI equivalents, but now you are being pedantic for no benefit to the wider readership. Weight is more widely understood by the majority of people who really don’t give a jot for gravity’s input.

But that is one of the problems, using the same name or acronym to describe a limit and a measured weight has led to all sorts misunderstandings in the arena of tow matching.

 

[otherclive]

But that is one of the problems, using the same name or acronym to describe a limit and a measured weight has led to all sorts misunderstandings in the arena of tow matching.

Must remember when the nurse asks me what my weight is I reply “540 newtons” and then watch her reaction. 🙈

 

[Mel]

Once again an interesting discussion about weights is starting to take an accusatory turn. Please stick to the debate and leave personal comments aside.
Thank you
Mel

 

[Buckman]

NO, no and no again! Axle weight and gross weight are 2 totally separate things!!!

Axle weight is the maximum weight that can be transmitted by the wheels on that axle onto the road surface.
MTPLM is the maximum weight permissable for the entire caravan when supported on its wheels AND jockey wheel.

A hypothetical situation........

A caravan has an MTPLM of 2,000Kg It's put onto a weighbridge so just the wheels are on the weighing plate and it reads exactly 2,000kg which is what the plated maximum axle weight is. So far so good no apparent offence.
The tow hitch is exerting 150kg onto the tow hitch which the towhitch is designed to carry. Still all well and good.
The caravan is then unhitched and both the axle and jockey wheel are now on the weighing plate. The weight registered will now be 2000 PLUS 150kg.

So the caravan on its own, actually weighs 2150Kg (2000 axle loading plus 150kg jockey wheel loading), so, despite it still being legal on its axle weight (2000kg) it is now OVER ON ITS GROSS WEIGHT by 150kg.

As I keep saying, axle weight and gross weight are NOT the same thing.
I spent 30 years dealing with vehicle weights on a regular basis, so I DO know what I am talking about here., and have had to stand up in court to prove it when questioned by some smart alec solicitors!

This is where it gets confusing as when hitched to the towing vehicle no offence is being committed as all well and good, but when unhitched and on it s own and exceeding the gross weight how can an offence occur?

 

[otherclive]

This is where it gets confusing as when hitched to the towing vehicle no offence is being committed as all well and good, but when unhitched and on it s own and exceeding the gross weight how can an offence occur?

Surely the caravans mass at MTPLM is a fixed entity so the fact that some weight is being carried by the tow hitch is irrelevant. The caravan is still exceeding its MaximumTPLMass. Once uncoupled from the towcar a simple weighing will show it as being in excess of MTPLM. I don’t know how DVSA check caravans these days, but mine was uncoupled from the car and weighed with road wheels and jockey wheel on the weighbridge.

 

[Lutz]

This is where it gets confusing as when hitched to the towing vehicle no offence is being committed as all well and good, but when unhitched and on it s own and exceeding the gross weight how can an offence occur?

Now you've really got me. Just because one cannot check whether the MTPLM of the caravan has been exceeded or not without unhitching from the car doesn't mean that an offence cannot have been committed. Even without unhitching, it would be possible that maximum axle loads or the gross train weight limit could be exceeded and that would also be an offence.

 

[Mr Plodd]

This is where it gets confusing as when hitched to the towing vehicle no offence is being committed as all well and good, but when unhitched and on it s own and exceeding the gross weight how can an offence occur?

No it doesn't, Both Other Clive and Lutz are 100% correct!

A caravan has a maximum permitted weight when used on the Road and that is shown on the plate. If its over weight sitting on your drive then when hitched to the car the weight of the caravan does not reduce, but some of it is borne by the tow hitch, ! So the offence of excess weight is STILL committed even if the axle weight is within its limit (as per my previous example above)

The offence of being over (gross) weight can only be detected by detaching the caravan and weighing it on its own. Maximum gross weight is the weight of the caravan when not attached to anything!

Yet again stop confusing the maximum axle weight with the maximum gross weight, they are not the same thing.

 

[Buckman]

Surely the caravans mass at MTPLM is a fixed entity so the fact that some weight is being carried by the tow hitch is irrelevant. The caravan is still exceeding its MaximumTPLMass. Once uncoupled from the towcar a simple weighing will show it as being in excess of MTPLM. I don’t know how DVSA check caravans these days, but mine was uncoupled from the car and weighed with road wheels and jockey wheel on the weighbridge.

However the caravan as per Mr Plod's post cannot be on the road unless hitched to a car and if hitched it is perfectly legal. When it is weighed on its own, it is no longer on the road and not being towed so surely no offence is being committed?

I am not suggesting that any one overload their caravan at any time and it is alos best to stick within the MTPLM.

 

[Buckman]

No it doesn't, Both Other Clive and Lutz are 100% correct!

A caravan has a maximum permitted weight when used on the Road and that is shown on the plate. If its over weight sitting on your drive then when hitched to the car the weight of the caravan does not reduce, but some of it is borne by the tow hitch, ! So the offence of excess weight is STILL committed even if the axle weight is within its limit (as per my previous example above)

The offence of being over (gross) weight can only be detected by detaching the caravan and weighing it on its own. Maximum gross weight is the weight of the caravan when not attached to anything!

Yet again stop confusing the maximum axle weight with the maximum gross weight, they are not the same thing.

I never mentioned axle weight or MTPLM and I am no longer confused about them. I am confused about how there can it be an offence if the caravan is off the road and not hitched to the towing vehicle. but when on the road is perfectly legal?

 

[RogerL]

I never mentioned axle weight or MTPLM and I am no longer confused about them. I am confused about how there can it be an offence if the caravan is off the road and not hitched to the towing vehicle. but when on the road is perfectly legal?

Your assumption that it's "perfectly legal" when on the road is incorrect - if it's mass/weight on a weighbridge detached from a car exceeds it's limit then it's still exceeding the limit when hitched to a car as the mass/weight doesn't change.