I know what it stands for, but how do manufacturers arrive at this figure. Is it the capacity of the over run brake; or the chassis, or something else? Just wondered?
Mel
Mel
MPTLM: how is it calculated?
- Thread starter Mel
- Start date
Hello Mel,
That's a good question. I do not know the accurate answer, but I will speculate.
On the basis that many manufactures will if you ask increase the MTPLM of some caravans, suggests they deliberately choose to set a lower MTPLM, There is no mechanical reason to set lower MTPLMS, so there must be some incentive.
With many caravanners trying to follow the industry guidelines for towing ratios of 85% or less by setting a lower MTPLM, it may make the caravan fall into the caravanner's weight range.
Similarly drivers with only CAT B licences will be looking for vans with lower MTPLM'S so here again the lower values makes the caravan more accessible to a wider customer base.
Seems its likely to be about maximising sales.
hmy:
That's a good question. I do not know the accurate answer, but I will speculate.
On the basis that many manufactures will if you ask increase the MTPLM of some caravans, suggests they deliberately choose to set a lower MTPLM, There is no mechanical reason to set lower MTPLMS, so there must be some incentive.
With many caravanners trying to follow the industry guidelines for towing ratios of 85% or less by setting a lower MTPLM, it may make the caravan fall into the caravanner's weight range.
Similarly drivers with only CAT B licences will be looking for vans with lower MTPLM'S so here again the lower values makes the caravan more accessible to a wider customer base.
Seems its likely to be about maximising sales.
hmy:
There will also be limiting factors such as wheel bearings, tyre loadings and suspension dampers strength of chasis etc. These items can be made stronger, but normally weight and cost will also go up, so somewhere along the way, the designers come up against the accountants.
Hutch.
Hutch.
Then of course there's the extra £60 or so for the weight upgrade, just a new sticker but in our case nearly 200kg increase in payload.
EH52ARH said:
Sorry Hutch but I think you have missed the point.
Whilst the MTPLM can never exceed the mechanical specifications of the parts used. However the MTPLM can be set lower by the manufacturer for the reasons I described above.
All EU manufactured caravans have to have a CE type approval before being sold within the EU Each approval costs a considerable sum, so manufacturers are not likely to offer chassis or axle upgrades which would require rebuilding parts of the caravan also have to CE approved. - very expensive for the very small numbers of caravanner's who would consider it.
In most cases where a manufacturer allows an increase in the MTPLM, there are no mechanical upgrades so its just a paper excersise and in a few cases they recommend tyre pressure changes or possible different tyres.
Morning Prof, what I was suggesting as being the limiting factors, for the MTPLM, are the design limits of such things as bearings etc, manufactures do not fit the top of the design items, at a higher cost, where a suitabe lower cost item is , possible to be fitted, to keep costs down, but to be able to get their type approval for their product.
Hutch.
Hutch.
I think in most cases the limit is set lower than the physical capability of the caravan as a sales tactic. Keeping the figure low lets more people believe it is within the towing capability of their car where by setting it higher the figure might exceed the car manufacturers towing limit or as said earlier the 85% or some other figure the purchaser is looking at.
The figure should be taken seriously though as unless you get the caravan replated it is the legal maximum weight and exceeding this could lead to prosecution although a defence that the manufacturer can replate it without modification might just work but not worth risking.
You should also be aware that it might be the maximum in real terms. When we bought our Lunar I did ask if it could have the MTPLM increased as the car had the capacity, but the salesman checked with Lunar and was told no as it was the maximum the chassis would take.
The figure should be taken seriously though as unless you get the caravan replated it is the legal maximum weight and exceeding this could lead to prosecution although a defence that the manufacturer can replate it without modification might just work but not worth risking.
You should also be aware that it might be the maximum in real terms. When we bought our Lunar I did ask if it could have the MTPLM increased as the car had the capacity, but the salesman checked with Lunar and was told no as it was the maximum the chassis would take.
The limiting things, are the combination of the weakest link, weither it be the last two bolts between the tow hitch and the chasie or the valve in the wheel which hold the pressure into the tyre, its all worked out by design engineer s to be the best spec and then spoilt by the accountants who want the best return for their money, changing a screw that is say 10 mm long for one that is 8 mm, it still holds but maybe with not quite the same bite, but at a saving of pence and a few grams in weight.
I understand Hutches point, and such savings are more likely to occur in a mass productions environment, and there are some well known major manufacturers who go to great lengths to save fractions of a penny on components, but none of the UK caravan manufacturers can be classed as mass producers, they tend to batch produce in tens of a particular model, and will produce a wide range of models from the same factory.
A few years ago I saw quite a few manufacturers production facilities, and having seen their shop floors with the numbers of staples, nails and screws, and other components strewn across the floor I don't think they seriously look for these sub penny savings.
In terms of design process, caravan manufacturers will start a new model design with a very clear perception of the target customer group and the range of tow vehicles they are likely to have. This will largely dictate the MTPLM they need to aim for to make the model attractive to the widest possible range of customers.
In addition each manufacturer will have a clear idea of how much payload they need to allow for each model based on the number of berths in the caravan.
Armed with these (and other manufacture defined weights, loads and tolerances) they will design the caravan to bring it in at, or below the target MTPLM.
Part of the process involves choosing the size and load capacity of the chassis components. All the 90% of European caravan manufacturers use chassis parts from just two companies. These chassis manufacturers don't make bespoke chassis for UK caravan makers, instead they sell sets of standard parts. The caravan manufacturers will select the parts. The standard components chosen may end up having a slightly larger load capacity than the caravans designed MTPLM. This gives the caravan manufacture some scope to increase the MTPLM if the customer wants it.
A few years ago I saw quite a few manufacturers production facilities, and having seen their shop floors with the numbers of staples, nails and screws, and other components strewn across the floor I don't think they seriously look for these sub penny savings.
In terms of design process, caravan manufacturers will start a new model design with a very clear perception of the target customer group and the range of tow vehicles they are likely to have. This will largely dictate the MTPLM they need to aim for to make the model attractive to the widest possible range of customers.
In addition each manufacturer will have a clear idea of how much payload they need to allow for each model based on the number of berths in the caravan.
Armed with these (and other manufacture defined weights, loads and tolerances) they will design the caravan to bring it in at, or below the target MTPLM.
Part of the process involves choosing the size and load capacity of the chassis components. All the 90% of European caravan manufacturers use chassis parts from just two companies. These chassis manufacturers don't make bespoke chassis for UK caravan makers, instead they sell sets of standard parts. The caravan manufacturers will select the parts. The standard components chosen may end up having a slightly larger load capacity than the caravans designed MTPLM. This gives the caravan manufacture some scope to increase the MTPLM if the customer wants it.
Makes sense Prof.
We needed the extra MTPLM because of all the extra kit involved with my disability. Luckily we already had the tow car to cope with this. The relatively low MTPLM for a twin axle 6 berth at the start, would mean that a family could have a larger range of tow vehicles that would be less expensive to run on a day to day basis. An important consideration for many families these days, also a better choice of company cars to choose from as well.
We needed the extra MTPLM because of all the extra kit involved with my disability. Luckily we already had the tow car to cope with this. The relatively low MTPLM for a twin axle 6 berth at the start, would mean that a family could have a larger range of tow vehicles that would be less expensive to run on a day to day basis. An important consideration for many families these days, also a better choice of company cars to choose from as well.
Thank you all. There was I in my purist innocence assuming it would be based on a clever equation in which the chassis load would be divided by the drag factor and multiplied by the square root of the tyre pressure plus the coefficient of friction applied by the brake and subtracted from the universal constant, C. Or something like that.
And it is all down to marketing.... Should have known.
Mel
And it is all down to marketing.... Should have known.
Mel
so what about cars then? they have a chassis, wheels, suspension. ect ect .. how does the thinking on that go?
JonnyG said:
Very few cars have a chassis these days, everything is bolted to the bodywork. As cars are mass produced in large quantities by robots then design engineering played a major part initially. All the calculations will have been done to enable the vehicle to travel at a much higher speed than a 'van, whereas vans are only produced in smaller quantities and the profit isn't there to do computer simulations, so guesswork based on minimal calculations is prevalent. Caravan design/manufacture is still a cottage industry compared to cars, so you are not comparing like for like.
JonnyG said:
Hello Jonny,
I'm not sure I understand the context of your point.
The title of the thread is " MTPLM how is it calculated". MTPLM only relates to trailers, not cars.
But its almost certain that car manufacturers will go through a similar design exercise of having a notional idea of what MAM they want for their new design, and then as they develop the design choose the necessary specification of wheels and tyres and all other components to meet the companies criteria.
With the sophisticated CAD CAM systems the auto manufacturers have these days, they can build a digital model of the project and know with a fair degree of certainty how much all the parts will weigh before they even cut metal.
I would bet that the car manufacturers can predict to better than 1% what the final car would weigh. I don't think caravan manufacturers have the luxury of that degree of certainty.
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