Twin axle noseweight

[Lutz]

Even an idiot knows that a level twin axle caravan will have no noseweight.

Eh?
Why on earth should a twin axle caravan not have any noseweight? Depending on how the load is distributed inside it can't be that well balanced.

 

[xtrailman]

My towball sits at 19" from ground level.

As off-road 4x4s have the Type-Approval height requirement removed that 19" isn't wrong as such - but you might well get a better tow if you use a drop plate to get it within the 350-420mm (14-16") required for all other vehicles.

When the caravan is coupled up to the vehicle, the noseweight of the caravan pushes down on the towball taking it to between 14 - 16 " which should then give it a good tow.
Prior to leaving the site after our last road trip we removed all the linen including the duvert and pillows from the rear island bed and this had a detrimental effect on the nose weight increasing it by about 30kgs even though the linen removed was only about 10 kgs!
Main reason why we decided to purchase a more reliable easy to read nose weight guage as the ride back was not very comfortable due to light steering. Luckily it was less then 10 miles back to storage on an A road. Lesson learnt!

Are you seriously saying the tow car drops 3" when hitched up?
I think you are having a laugh.

 

[Surfer]

Even an idiot knows that a level twin axle caravan will have no noseweight.

Eh?
Why on earth should a twin axle caravan not have any noseweight? Depending on how the load is distributed inside it can't be that well balanced.

Lutz noseweight only occurs when there is a downward pressure so if the caravan is level how can there be a downward pressure? Nose weight only occurs when the caravan hitch puts downward pressure on the towball. What is so difficult to understand about that? It is simple physics.

 

[RogerL]

That's defying the laws of physics - taking all twin axles together there will be huge variation in the way that they're loaded, from very front heavy to very tail heavy - with no car hitched, no jockey or steadies down they'll all sit at different angles - getting them level will involve force at the hitch which will be exactly equal and opposite to the noseweight.

 

[Surfer]

My towball sits at 19" from ground level.

As off-road 4x4s have the Type-Approval height requirement removed that 19" isn't wrong as such - but you might well get a better tow if you use a drop plate to get it within the 350-420mm (14-16") required for all other vehicles.

When the caravan is coupled up to the vehicle, the noseweight of the caravan pushes down on the towball taking it to between 14 - 16 " which should then give it a good tow.
Prior to leaving the site after our last road trip we removed all the linen including the duvert and pillows from the rear island bed and this had a detrimental effect on the nose weight increasing it by about 30kgs even though the linen removed was only about 10 kgs!
Main reason why we decided to purchase a more reliable easy to read nose weight guage as the ride back was not very comfortable due to light steering. Luckily it was less then 10 miles back to storage on an A road. Lesson learnt!

Are you seriously saying the tow car drops 3" when hitched up?
I think you are having a laugh.

The guidance is for between 15 and 18 inches so if the vehicle drops by an inch it is legal. Lunar states that coupling height should be 17". It does drop about two inches, but have not checked the nose weight with the caravan hitched up yet which is why we purchased the TLC. Once I have had the opportunity to do measurements with the correct noseweight at about 100kgs I can report back.

 

[Martin_E]

It seem to me that some are trying to hard to be exact, one thing springs to mind when you have measured the ride height of the car then weighed the hitch head at that height is.........what happens when say the 70lts of fuel in the car has all been used up, all your heights go out out the window as would your nose weight as surely the tow hich will rise by a fair amount??

 

[RogerL]

The guidance is for between 15 and 18 inches so if the vehicle drops by an inch it is legal. Lunar states that coupling height should be 17". It does drop about two inches, but have not checked the nose weight with the caravan hitched up yet which is why we purchased the TLC. Once I have had the opportunity to do measurements with the correct noseweight at about 100kgs I can report back.

It's not guidance, it's a legal requirement under Type-Approval BUT it doesn't apply to off-road 4x4s as I believe you have.
The limits are 350-420mm which rounds to 14-16 inches - NOT 18"
Caravan manufacturers simply quote the height of the hitch when the caravan is level - it's not a recommendation to tow at that height.

 

[RogerL]

It seem to me that some are trying to hard to be exact, one thing springs to mind when you have measured the ride height of the car then weighed the hitch head at that height is.........what happens when say the 70lts of fuel in the car has all been used up, all your heights go out out the window as would your nose weight as surely the tow hich will rise by a fair amount??

The specified height of 350-420mm is with the car fully laden - obviously it may be different with less load.

 

[Lutz]

Lutz noseweight only occurs when there is a downward pressure so if the caravan is level how can there be a downward pressure? Nose weight only occurs when the caravan hitch puts downward pressure on the towball. What is so difficult to understand about that? It is simple physics.

But why is the caravan level? Because it is held level and the load to hold like that is the noseweight. Anything else would defy the laws of physics.

Caravan manufacturers simply quote the height of the hitch when the caravan is level - it's not a recommendation to tow at that height.

The height of the hitch when the caravan is level is also covered by regulations. It must be between 395 and 465mm when the caravan is fully laden.
Note that this is not the same as the towball height (which has been correctly quoted above as being between 350 and 420mm).

 

[Dustydog]

John I am astounded that you offer such misleading information especially when you claim to be a prof. Even an idiot knows that a level twin axle caravan will have no noseweight. Perhaps you could enlighten us with your definition of what is nose weight? I define it as "This is the amount of weight that the trailer puts in a downwards direction on the tow bar or the rear of the vehicle. ".
I really think you are in la la land with this one, but thanks for responding any way as it made me laugh on this miserable day.

Surfer
You can't be serious?

Please be my guest and come and see my TA which I can assure you came from the factory with a "built in" nose weight. Empty. I'll buy you a beer too.
Do you have a TA?? Honestly in reality it is not as you describe.
Loading the TA requires just as much care as a SA and whilst I have doubts about my Milenco NW guage I do know it is probably within 15kgs either way accuracy, compared to the bathroom scales.
Look I'm a big dog and I can walk to the rear of the Wyoming, steadies wound up and it does NOT tip backwards!
It's all to do with where the axles are in relateion to the Cof G .

 

[Parksy]

My T/A caravan wouldn't remain level when unhitched either but for most of the time it is stored loaded with the things that we need ready to go, so the noseweight is already adjusted.
The only slight variable would be the amount carried in the car.

 

[ProfJohnL]

Just as surfer is a nickname, so is my Prof John L

 

[ProfJohnL]

Well!

I have always stated that the dynamics of a TA's noseweight is far more complex than that of a SA, and this debate has certainly shown how much the subject is still miss-understood by some.

I think the combined wisdom of several of the contributors have shown there is a fundamental flaw with all the noseweight gauges the public can buy because none of them allow the hitch height to be adjusted to hitches actual ride height on the tow vehicle, so none of them actually measure the real noseweight, unless by some coincidence the loaded length of the gauge allows the hitch to settle at the ride height.

The most basic and reliable method is the bathroom scales with a stick to set the hitch ride height.

As for the necessity for accuracy, I do hold a contentious but accurate view that the limit is absolute, you should make allowances for any inaccuracy in the measurement equipment or method by reducing the applied load so any measurement error will still be within the absolute limit.

There is little more to be said on this topic.

 

[Surfer]

Here is an interesting read which states that twin axle trailers should be level when towing. The suggested method is to sue bathroom scales.
When I next hitch up I will measure height of coupling head and whether caravan is level. Then check noseweight using Reich TLC and check level of caravan. When hitched up measure height of towball/coupling head and also check if caravan is level. Hopefully I will remember to resurect this post again to update my findings.
As John says we all have our own ideas some may be right some may be wrong.

 

[RogerL]

Here is an interesting read which states that twin axle trailers should be level when towing. The suggested method is to sue bathroom scales.
When I next hitch up I will measure height of coupling head and whether caravan is level. Then check noseweight using Reich TLC and check level of caravan. When hitched up measure height of towball/coupling head and also check if caravan is level. Hopefully I will remember to resurect this post again to update my findings.
As John says we all have our own ideas some may be right some may be wrong.

That article is about twin-axle goods trailers which don't have the same aerodynamic issues as caravans.
The reason that caravans are recommended to be hitched slightly nose down, when stationary, is that the aerodynamics at towing speed lifts the nose a little to bring the caravan approximately level. If you start from level the aerodynamic lift will tilt the caravan backwards at speed making it less stable particularly due to air pressure disturbance (bow wave) from other vehicles. In this respect, there's no reason that twin-axles are any different to single-axles.

 

[Lutz]

Aerodynamics are only indirectly responsible for making a caravan less stable when being towed nose-up. Roger is correct in saying that the front end of the caravan will tend to lift with increasing speed. This is due to the enormous aerodynamic pressure acting on the frontal area of the caravan, particularly above roof level of the towcar where the caravan is not shielded in any way by the car ahead. By raising the front end, the noseweight is reduced and that is what makes the outfit less stable, not any turbulence from other vehicles. If a way could be found to compensate for this reduction in noseweight, the outfit would be equally stable, whether nose-up or nose-down. The effect is less pronounced the longer the caravan because the change in attitude is less, given the same frontal force. That's why longer caravans tend to be more docile when towing.
Another practical reason for trying to avoid a nose-up attitude is that one loses ground clearance at the back end of the caravan. This could cause problems when negotiating speed bumps, for example.

 

[TonyG]

John I am astounded that you offer such misleading information especially when you claim to be a prof. Even an idiot knows that a level twin axle caravan will have no noseweight. Perhaps you could enlighten us with your definition of what is nose weight? I define it as "This is the amount of weight that the trailer puts in a downwards direction on the tow bar or the rear of the vehicle. ".
I really think you are in la la land with this one, but thanks for responding any way as it made me laugh on this miserable day.

Surfer
You can't be serious?

Please be my guest and come and see my TA which I can assure you came from the factory with a "built in" nose weight. Empty. I'll buy you a beer too.
Do you have a TA?? Honestly in reality it is not as you describe.
Loading the TA requires just as much care as a SA and whilst I have doubts about my Milenco NW guage I do know it is probably within 15kgs either way accuracy, compared to the bathroom scales.
Look I'm a big dog and I can walk to the rear of the Wyoming, steadies wound up and it does NOT tip backwards!
It's all to do with where the axles are in relateion to the Cof G .

My Swift Conqueror 645, had a 72kg noseweight ex factory..... Completeley empty, Verified by Swift when i asked them for the ex factory noseweight

 

[Lutz]

My Swift Conqueror 645, had a 72kg noseweight ex factory..... Completeley empty, Verified by Swift when i asked them for the ex factory noseweight

The ex factory noseweight is really only of interest when picking the caravan up from the factory. As soon as any form of payload is added the noseweight will have to be adjusted to suit anyway. After that, the ex factory data is of no further use to anyone.

 

[Surfer]

Aerodynamics are only indirectly responsible for making a caravan less stable when being towed nose-up. Roger is correct in saying that the front end of the caravan will tend to lift with increasing speed. This is due to the enormous aerodynamic pressure acting on the frontal area of the caravan, particularly above roof level of the towcar where the caravan is not shielded in any way by the car ahead. By raising the front end, the noseweight is reduced and that is what makes the outfit less stable, not any turbulence from other vehicles. If a way could be found to compensate for this reduction in noseweight, the outfit would be equally stable, whether nose-up or nose-down. The effect is less pronounced the longer the caravan because the change in attitude is less, given the same frontal force. That's why longer caravans tend to be more docile when towing.
Another practical reason for trying to avoid a nose-up attitude is that one loses ground clearance at the back end of the caravan. This could cause problems when negotiating speed bumps, for example.

Lutz not disputing the above as it is an interesting theory, but if you are towing a twin axle behind a 4 x 4 at 60mph on a motorway with a noseweight of i.e. 100kg what sort of lift would one expect and would it be of any consequence? A single axle may be a totally different kettle of fish.
Generally as the front of the caravan is sloping downwards, surely the ait flowing over the top of the car is creating air pressure forcing it downwards and maybe this would compensate for any sort of lift by turbulence under the front of the caravan? swhich would be the greater?

 

[Lutz]

Lutz not disputing the above as it is an interesting theory, but if you are towing a twin axle behind a 4 x 4 at 60mph on a motorway with a noseweight of i.e. 100kg what sort of lift would one expect and would it be of any consequence? A single axle may be a totally different kettle of fish.
Generally as the front of the caravan is sloping downwards, surely the ait flowing over the top of the car is creating air pressure forcing it downwards and maybe this would compensate for any sort of lift by turbulence under the front of the caravan? swhich would be the greater?

The lift created by the aerodynamic pressure is dependent only on the speed and the frontal area and has nothing to do with whether the caravan has one or two axles. I don't know whether anyone has any data on the forces involved, but I would expect them to be quite considerable.
The changes in attitude between nose-up and nose-down are far too small to create any sort of aerofoil effect at the speeds we are talking about. (I presume that it is an aerofoil effect that you are referring too). After all, caravans aren't usually towed at the speeds of flying aircraft.

 

[RogerL]

The lift created by the aerodynamic pressure is dependent only on the speed and the frontal area and has nothing to do with whether the caravan has one or two axles. I don't know whether anyone has any data on the forces involved, but I would expect them to be quite considerable.
The changes in attitude between nose-up and nose-down are far too small to create any sort of aerofoil effect at the speeds we are talking about. (I presume that it is an aerofoil effect that you are referring too). After all, caravans aren't usually towed at the speeds of flying aircraft.

The Swifttalk website has a section on fluid dynamics which Swift have done with one of their caravans and 3 vehicles (saloon, estate and SUV) and includes colour-coded pressure maps of the outfits under tow. There are no numbers and I feel sure they deliberately aren't giving anything away to other manufacturers but it does give a good representation where the high/low pressure areas are.
On the subject of aerofoils, it's perfectly possible to design a wing that gives lift at walking pace - indeed the solar-powered round the world aircraft has so much lift it can't be landed properly - it has to come in at walking pace at about a foot off the ground and then stall out. This is compounded by ground effect but then so are caravans.

 

[Lutz]

I'm not denying that there is some aerofoil effect, Roger. I'm just saying that this effect is negligible compared with the enormous forces acting on the frontal area of the caravan. Just try to hold up a piece of board, say 10 square ft in size, against a force 8 gale and you will appreciate what I mean.

 

[Surfer]

I'm not denying that there is some aerofoil effect, Roger. I'm just saying that this effect is negligible compared with the enormous forces acting on the frontal area of the caravan. Just try to hold up a piece of board, say 10 square ft in size, against a force 8 gale and you will appreciate what I mean.

I would imagine that all manufacturers would do some wind tunnel tests or is that expecting too much as after all it could have a major effect on your nose weight going either up or down and affecting your safety.

 

[RogerL]

In the 1970's Swift and some others did some basic wind tunnel testing but concluded that the airflow around the towcar varied so much for different models and didn't go any further Their recent computer simulated fluid dynamics study wasn't backed up by real test in a wind tunnel - but then it focusses on fuel saving rather than stability.

 

[ProfJohnL]

So the thread has moved on from nose weight gauges to towing angles!

We have been through the aerodynamics debate before, and my own thoughts on the matter agree with Roger L's comments regarding the Swift airflow study, basically the issue is so full of different variables depending tow vehicle types, such that little value can be extracted that would significantly influence caravan design.

We have also been through the caravan nose down level and up debate before. Technically, provided the tow hitch rides within the permitted 350 to 420mm height this will result in an angular range of less than 2 Degrees for caravans. Caravan designers are aware of this height range and will design their caravans accordingly.

It is my considered generalised opinion and experience that any aerodynamic differences that may arise from the small change in the permitted attack angle will be swamped by other variables, and thus the attack angle will have no discernible affect on towing.