Surely this can't be correct - or is it?
When a friend of mine bought his last caravan, a Bailey Arizona, he was told by the salesman that the weight of a motor mover does not have any impact on the MTPLM "Because it is below the axle line". This seems totally illogical to me. Whether any weight is above or below the axle makes no difference in my mind, or have I got it wrong? I can accept the argument that the nose weight of the 'van is carried by the car and therefore might not be considered to constitute part of the MTPLM but is this an accepted argument if the police did a roadside weight check?
I may be wrong but I think it was the salesman just wanting to make a sale as the MTPLM of the caravan represents 89% of his car's kerb weight and a 33kg motor mover would eat well into the available payload. By his own admission, my friend is not a confident tower so anything over the 85% guide lines, is in my mind, unwise. He doesn't know what weight he is actually carrying in the caravan, " it can't be very much I don't think" so he could actually be above the MTPLM.
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of course anything bolted to the caravan (anywhere) will add to its weight....
it wont affect the MTPLM as that is a specific number of kilos that mustnt be exceeded, but it will affect the actual weight, which in turn affects whats left as 'payload'.
im no expert on towing weights, but, when calculating towing ratios, wouldnt one use the 'maximum weight' of the caravan...ie, the MTPLM?
in this case, the calculation is unchanged.
however, the amount of 'other stuff' that can be put into the caravan prior to reaching the maximum, will have dropped by 33kg....the weight of the mover.
edit.....snap, ET
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As said above the salesman is talking utter rubbish.
next he'll be saying that if you load 33kg of birds inside your caravan and keep them flying all the time it won't have any effect on your payload
Yes its the MTPLM BB, an empty van's weight is the mass in running order MIRO, MTPLM - MIRO = payload
BTW technically, mass and weight are two different things but most people use them as the same thing.
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It probably means that you should take someone who understands caravans when buying. The mover reduces the potential payload by its weight and in some cases this might be a problem. The weight on or below the axle line is good for stability but that is all. The salesman seems to have been talking rubbish to sell the caravan.
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It's a salesman - it is rubbish!
It was absolutely pouring when I was about to leave my local caravan shop so I waited a few minutes at the door and got chatting to one of these salesmen about the Calor Lite withdrawal. He told me it was because people were dropping the cylinders and they were cracking (believable). This was due to them being made of aluminium (believable).
I told him there were steel and proved this by taking a magnetic sign off their display and sticking it to the Lite cylinder they used as a door stop. His answer was that they were "magnetic aluminium".
You can't even have a casual conversation with one of these people without them making up stuff.
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[I told him they were steel and proved this by taking a magnetic sign off their display and sticking it to the Lite cylinder they used as a door stop. His answer was that they were "magnetic aluminium".
You can't even have a casual conversation with one of these people without them making up stuff.]
He didn't make it up Nav, he just misspoke.
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A poke is what these people need - and I for one would not miss.
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"His answer was that they were "magnetic aluminium"."
It was as "ferrous" he was concerned
Sorry, couldn't resist that
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If you want to help your friend with his towing confidence I suggest you pressure him to attend a Club towing course. In reality it is impossible to tell the difference between 85 and 89% and 85% in itself doesn't make a caravan outfit safe and 89% unsafe.
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Of course the noseweight is carried by the car, but that doesn't affect the MTPLM because that includes the noseweight. However, the car is not towing the MTPLM but only the MTPLM minus the noseweight. It is carrying the noseweight, as you correctly state, but not towing it.
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I'm not sure I agree with your last statement at all. The noseweight is acting vertically downwards (just as the remainder weight on the two wheels) The towing force from the engine needed to keep the two at a constant velocity or accelerating the caravan is parallel to the road and therefore at right angles to the nose and other weights weights. Forces at rights do not affect each other. therefore the car has to tow the weight of the van. What you describe just doesn't fit with Newtons laws. According to you then if you could have a higher nose weight (perhaps increase it to half the caravan weight then the car is towing only half the weight now? It doesn't matter where the weight is distributed as the weight acts vertically downwards, while the towing/engine forces are at right angles to this.
perhaps you could explain your self ?
see here: http://www.schoolphysics.co.uk/age16-19/Mechanics/Dynamics/text/Car_towing_a_caravan/index.html
or http://www.schoolphysics.co.uk/age16-19/Mechanics/Dynamics/text/Newton%27s_third_law/index.html
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It sounds as though either the salesman was talking bilge, or the buyer misunderstood what the salesman said. I wonder if the BE test includes any requirement to know these simple facts?
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even when folk 'understand' weights, how many actually go to the weighbridge fully loaded 'just to be sure'?
i had my actual van weighed by my dealer prior to deliver so that i knew 'exactly' what my (proposed, stock) van weighed and, as i had weighed everything i proposed to carry in the van, i knew exactly whether i could manage the van at 3500kg.
ive seen a few MHers posting their actual weights in the 'tyre pressure' threads....cant say ive ever seen a caravanner post their van's weight on CT.....does anyone ever do this?
surely everyone should with such tiny marginal payloads (less than 200kg) in most caravans.
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The noseweight counts as part of the towing vehicle's overall weight. In other words, the noseweight is part of the car's payload. What is left to tow is the axle load of the trailer, not its overall weight. Otherwise one would be counting the noseweight twice.
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No sorry you are mistaken, and it's a common mistake. The car has to tow ALL the weight of the caravan or trailer. And it's not just me in IMHO it's Newton's laws which have been successfully been used in these situations for over 300 years. We modelled situations on paper and then like this at university using to-scale trailers. These had movable jockey wheels and you could removed them completely. The force supporting the front is a vertical force. the force needed to move the van is horizontal. they cannot help each other. We tried various positions of the jockey wheel and removed it. the force required to move the van remained constant.
Easily proved. Get someone to hold up your van at the front. This involves a vertical force, Now get behind the van and push your van till its moves. You will have to push the whole van's weight won't you, not just the bit at the back axle. Mathematically applying a force at the front is the same as the back The person at the front is just supporting the front vertically , you have to push all the weight not just the bit left over the axle. You can try pulling from the front but the person will be in the way.
Or, you could try and lift the van yourself, then when you have lifted it with your legs straight, what must you do to move the van? you then have to pull the van with an horizontal force and once again you are pulling the whole weight. yes its feels like more effort because you are supplying the vertical force as well as horizontal.
Whether the jockey wheels holds the hitch up or you do, you have to push or pull the the whole van's weight. It's the same with a car holding up the hitch. There cannot be different rules for a car holding it up or you.
is the payload of an articulated lorry just the parts over the back axle or the weight over all of it?
It is fairly simple to write the equations:
Driving force - resistive forces of the car - resistive forces of the trailer = mass x acceleration
The tensions is the towbar cancel out. If the velocity is constant, the acceleration is zero, the driving force = the total resistive forces. No mention of weight just mass, but of course the main resisitive forces are made up of air resistance (which is based on velocity not weight) and of course friction. Friction is based, for two given surfaces, on the forces pressing down on the road through the wheels of the car and van, so the extra weight of the van on the car will makes these contact forces higher hence causing more friction on the car.
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Ok this morning I've phoned three people with the above 'problem'
One engineering lecturer from Hull University (a former flatmate)
'the car is towing the full weight of the caravan, if the caravan wasn't there the car would tow nothing, as it is there it has to two the full weight, where the pivot points are have no bearing'
a engineering lecturer from a college of FE
'the nose weight is completely in the wrong direction to the towing force. it will have no effect, the frictional forces will increase on the car though'
A manager of HGV firm,
the tractor unit has to tow the weight of the trailer, usually 44 tons. some of that weight is on the tractor unit, but it still has to tow 44 tows
Suggest you get your own independent experts?
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While the car "tows" the trailer it does not tow the full weight as some of it is supported by the trailer wheels. It is not like a towbar bike rack (no trailer in use) where all the weight is on the towbar.
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Sorry to go on, but that is incorrect. Whether it is supported by wheels or not the weight acts vertically downwards at all times and remains constant no matter if the trailer is moving or not. All that wheels do is share out the weight at the points where they wheels touch the ground, and of course having wheels affect the frictional forces. Putting a box on wheels is easier to push or pull.
The mass of the trailer converts into weight and weights are vertical forces. These weights affect the friction between the trailer and the road. The full weight of the trailer has to be towed (dragged pulled) by the car. It doesn't matter if the hitch is outside the car or even if you could rest it inside the boot. While the car supports weight of the trailer it still has to tow the full weight.
Another engineer (aerospace) quote . The weight and towing forces are vectorially at right angles, they cannot affect each other. When towing an airplane the tractor attaches to a the plane by a very heavy tow bar, This is supported by both the tractor and plane however the tractor to has to two the weight of both the airplane and tow bar
(PS HGV error above, HGV are usually 44 tons with a 30 ton max trailer which has to be towed no matter where it is supported, apparently some of these tractors and trailers have movable axles to distribute the weight but it is still towing 30 tones and that is what they base their calculations on, not what is supported where )
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Navigateur is correct. The noseweight is part of the total weight of the car which its engine is providing the power to move forward. Now if the car were towing the full weight of the trailer, including the noseweight, it would be pulling the noseweight twice, once as part of its own total weight and again as part of the total weight of the trailer. That doesn't make sense.
That is also why the towed load, by definition, is the axle load of the trailer, not its total weight.
Here is the actual wording, taken out of the regulations:
" ‘technically permissible maximum towable mass’ (TM) means the maximum mass of one or more trailers that may be towed by a towing vehicle which corresponds to the total load transmitted to the ground by the wheels of an axle or a group of axles on any trailer coupled to the towing vehicle;"
Note that only the axle load of the trailer is referred to, not its total mass.
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The above respondents seem to have overlooked that a car which is not towing is lighter than a car that is.
If the trailer were not attached, the difference would be the noseweight which is no longer there, and the engine would have to develop correspondly less power to propel the vehicle on its own.
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All very interesting, I love watching this type of forum debate
The fact is at the end of the day if it didn't leave the factory with it fitted and was added by a 3rd party then it's extra weight and will reduce the user margin for all the rubbish we stuff inside.
When I was an apprentice we had to prepare new lorries to go to the test station to be plated (the blue certificate that goes in the cab) that figure then stayed with it and the difference between that and it's manufactures gross weight was what it could be carried.
I won't tell you about some of things that went on though, like removing spare wheels and carriers for the test then refitting them later
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I'm pretty sure these engineering lecturers have not overlooked the fact that a trailer need a towing vehicle.
You are still not getting the fact that weights which are downward forces have nothing to do with horizontal driving forces. The car being slightly heavier will mean that it's frictional forces on the tyres are greater and therefore it has to do more work, the car also has to overcome the frictional force of the caravan, and this is in effect what towing (or pushing or pulling anything) is. It is making sure you have enough power to overcome the initial frictional limiting forces to start movement and then to balance the driving force with them to keep moving at your chosen velocity.
The weight of your trailer wherever it is only affects friction.
‘technically permissible maximum towable mass’ (TM) means the maximum mass of one or more trailers that may be towed by a towing vehicle which corresponds to the total load transmitted to the ground by the wheels of an axle or a group of axles on any trailer coupled to the towing vehicle;"
I think the problem lies with that fact that you (one) has to understand the difference between mass and weight. I'm talking mass, you're talking weight.
Weight is a force, mass is not. Most of the time you can use one for the other but not here.
Weight is used to calculate the frictional force. What the above is saying is that you use weight downwards - the total load transmitted to the ground by the wheels to calculate the friction and you use mass horizontally to calculate the driving forces. No matter where the weight of the car + trailer is supported you still have to tow the mass of your trailer. The mass of your trailer cannot change. In other words what you wrote cannot occur :
However, the car is not towing the MTPLM but only the MTPLM minus the noseweight.
and indeed cannot make sense. Mass in measures in Kg, weight (a force) and here is measured in Newtons (mass x 9.81) You cannot take mass away from weight, they are two totally different things. Yes the weights are distributed across the car and van but not the mass.
The regulations and caravan industry talk about mass of your trailer not weight precisely because weight acts downwards and mass affects horizontal forces, ie what force you need to tow, push, pull a trailer of a certain mass
I suggest as well as finding some experts you also look at some videos like this:https://www.youtube.com/watch?v=IPaEcygK-M8
Notice that weight isn’t mentioned at all - it’s all mass. In more advanced questions you would calculate the friction using the weight pressing down. So weight no matter where it is supported as you say only affect friction. Weight plays no part horizontally. Mass does
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sorry pressed reply before I meant to.
To covert the mass of your trailer to calculate weight - the force pressing down on the road multiply eq 1500kg x 9.81 m/s2 (acceleration due to gravity) To calculate friction then calculate the reaction of the road to this (not always the same as the) weight), perhaps find the weight on each individual tyre, and multiply it by the coefficient of friction for rubber and tarmac (the stickiness factor) and you have worked out the frictional force on that tyre.
bonus question what was the imperial unit of mass?
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Weight is nothing more than the force of gravity acting on a mass. Therefore, unless the acceleration due to gravity acting on the car is different to that acting on the trailer (which it won't be) then it is irrelevant whether you talk about mass or weight, so long as you are consistent. The relationships between car and trailer remain the same.
Fact is that you are not towing the full weight of the trailer. You are carrying that part which is the noseweight, but not towing it. The noseweight increases the rear axle load of the car (and reduces the front axle load slightly), but the sum of all axle loads of car and trailer remain the same regardless of whether the trailer is hitched or not (on condition that you treat the jockey wheel as an axle) and it's that sum that the engine has to provide the power to get the whole outfit over a hill, for example.
The car's mass includes the mass of its payload and the noseweight is part of the payload.
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All this might be begining to confuse me if I was not able to think of my favourite vehicle - the ballast tractor. You will probably have seen them moving ridiculously heavy loads, often working in pairs or threes.
The load sits on its own wheels - dozens of them - and the tractor pulls it all along with a drawbar. Only weight applied to the tractor is part of the weight of that drawbar. That is why the tractor has to carry ballast, otherwise it would just spin wheels. The force needed to move the load is all on the drawbar, often called the drawbar pull. When two tractors in use, then first pulls the second, second pulls the load.
Going uphill that pull increases as the load is being moved to a higher place. Going down the opposite, and the pull can become a push. That is why a third tractor is often employed at the rear to reduce the amount of push on the lead tractor(s).
None of the tractors carry any weight except themselves and their ballast.
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No offense intended but a comment like your first one shows a very schoolboy understanding of physics. There is a huge huge difference between mass and weight. Google it. I certainly would have failed my degree, actually A levels as well with comments like that. I am amazed you can say that and there is no point discussing these things if you can't understand the fundamental difference between mass and weight. Like I said no offense. You are towing a mass not a weight. Huge difference.
Also the engine has to overcome friction, that is all.
If there is no difference between weight and mass when does it talk about mass in official documents that have legal standing so much rather than weight? maximum allowable mass, then kerbweight? why not kerb mass? Maybe because horizontal calculations are not affect by weight? only mass?
Anyway nice to try and discuss these thing with you
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well using the mover may have meant you didn't get as sweaty so less changes of clothes?
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