Caravan Weights - Advice Needed Please

[ProfJohnL]

Hello Rooster your last post contains some factual errors:

quote
"As has already been pointed out, there are weight related No-No's that must be adhered to. They are never go over 100% of known (not published) Kerbweight)."

This has been one of the myths the caravan industry has managed to impress on much of the caravanning population. It has no legal basis. A tow vehicle can tow any trailer that does not exceed the car manufactures stated towing limit.- Though its not always wise to do so. The drivers licence categories may limit them to no more than 100%

Whilst I may argue the facts above, I always recommend the trailer should be as small as possible.

quote
"Never exceed the maximum weight on the ball,the hitch-head weight may be higher,that does not count."

The posting is incomplete and thus wrong; It is possible to have a tow ball rated to a higher limit than the caravan (e.g. some 4x4s). In that case the 'hitch head weight' does count and must not be exceeded.

Quote
"Observe the maximum axle weights,ALL of THEM. Don't forget that the weight on your tow-ball is actually counted as in the car payload but it does not come off the vans."

You are absolutely rights about observing each axle load limit, but the part about accounting for the nose load does not conform to the latest thinking.

This is a topic that has been discussed extensively on the forum, and the latest document to come to light confirms the trailers nose load is counted as part of the towing vehicles payload, but not part of the trailers weight. This is where the detail of definitions becomes important, because the current thinking is that the Gross Trailer Weight (what the whole trailer weighs) is made up of two elements, the axle(s) load plus the nose load. the axle load is the 'trailer weight', as differentiated from the Gross Trailer Weight. The Gross Trailer Weight must not exceed the MTPLM.

This implies that the nose load is taken away from the MTPLM leaving a smaller loading margin for the main axle.

As Lutz has pointed out on several occasions, it would be illogical and wrong to count the nose load as part of both the car and the trailer, as it would be counted twice.

Sorry for the length of this reply.

 

[Adria542Chick]

Hi Rooster
Thanks for taking the time to reply.
We tow with a 2 litre tdci 136 Mondeo.
We never ever exceed the caravans MTPLM, and always make sure that our noseweight is correct.
We have been caravanning for the past 2 years, and have always towed sensibly and legally. This has always been our main priority as we have 3 children.
Unfortunately, having the luxury of the motor mover fitted has now bought this problem to light.
However, today we have weighed and cleared out non-essential items from the caravan (including the hoover 5kg

) and we are left with 25kg which will be used as clothing hung in the wardrobe. Everything else has now been accounted for, and food items and other clothing items will be carried in the boot)
I made a spreadsheet when we first started caravanning with all the items in the caravan a) in case it ever got stolen and b) beside each item is the weight. I now feel 100% satisfied that we are towing legally and correctly and that the caravan will not be overweight.
Took me a while but I can now sit back and think .............. phew !!!!! and look forward to our Easter holiday.
****** Thank You to all of you that commented *******

 

[TheTravellingRooster]

Hello Rooster your last post contains some factual errors:

quote
"As has already been pointed out, there are weight related No-No's that must be adhered to. They are never go over 100% of known (not published) Kerbweight)."

This has been one of the myths the caravan industry has managed to impress on much of the caravanning population. It has no legal basis. A tow vehicle can tow any trailer that does not exceed the car manufactures stated towing limit.- Though its not always wise to do so. The drivers licence categories may limit them to no more than 100%

Whilst I may argue the facts above, I always recommend the trailer should be as small as possible.

quote
"Never exceed the maximum weight on the ball,the hitch-head weight may be higher,that does not count."

The posting is incomplete and thus wrong; It is possible to have a tow ball rated to a higher limit than the caravan (e.g. some 4x4s). In that case the 'hitch head weight' does count and must not be exceeded.

Quote
"Observe the maximum axle weights,ALL of THEM. Don't forget that the weight on your tow-ball is actually counted as in the car payload but it does not come off the vans."

You are absolutely rights about observing each axle load limit, but the part about accounting for the nose load does not conform to the latest thinking.

This is a topic that has been discussed extensively on the forum, and the latest document to come to light confirms the trailers nose load is counted as part of the towing vehicles payload, but not part of the trailers weight. This is where the detail of definitions becomes important, because the current thinking is that the Gross Trailer Weight (what the whole trailer weighs) is made up of two elements, the axle(s) load plus the nose load. the axle load is the 'trailer weight', as differentiated from the Gross Trailer Weight. The Gross Trailer Weight must not exceed the MTPLM.

This implies that the nose load is taken away from the MTPLM leaving a smaller loading margin for the main axle.

As Lutz has pointed out on several occasions, it would be illogical and wrong to count the nose load as part of both the car and the trailer, as it would be counted twice.

Sorry for the length of this reply.

Hi Prof John L. Thank you for your remarks /observations and 'slightly off key scrutiny'. Whilst I accept that I should have elaborated and expanded on the never exceed hitch-head weight. I know that the never go over 100% of known(not published) kerb-weight is not a legal requirement,it was said more on the basis of common sense and not having a "tail wagging the dog" situation.
The point regarding the trailer/caravan weight is (in my opinion) a rather subjective one;insomuch as,if the trailer was put onto a weighplate and a nose weight gauge put into the hitch-head cup and set on pads to simulate a ride height of the chosen tow-vehicle when hitch up to ball and with the nose weight adjusted and the jockey wheel raised this would give a true total weight of that trailer as a stand-alone entity.
If the trailer/caravan was then put onto the tow-vehicle ball and past onto that same weigh-plate it is assumed that the trailer would then weigh less by that tow-ball figure and therefore the car would weigh heavier by that same amount than before it was weighed as a stand-alone entity.
If that was the case then there would be two definitions (the subjective bit), the theoretical & the actual of the trailer/caravan.
Being that for the purpose of law enforcement the unit is weighed as a whole (train weight). The axle weights of the individual units of the outfit are evaluated and in the the case of a caravan that the MTPLM is not exceeded, as is the gross weight of the tow-vehicle also not exceeded. This is where the Dynamic Axle Weigh-Bridges as used by VOSA come into play. The problem with the caravan being weighed as a stand alone on a VOSA system is that it would have to be rolled (manually) across the weigh cells on the jockey wheel and then the road wheels to establish an as near as weight being that the jockey wheel is set back from the hitch-head.
If the MiRO was established from a bridge and the caravan was loaded to the MTPLM by accounted for weighing of everything and the unit was hitched up then that should ensure that the caravan is on the "safe side" of the MTPLM. I say this because it is possible that an over enthusiastic (possibly Spanish or French Plod) might decide to put the weight as of the MTPLM to the test on a stand-alone basis. By being under by the ball or hitch-head weight (which ever is the lower) would ensure a "Belt & Braces Job" and avoid retribution.

 

[ProfJohnL]

Hello Rooster,

The theoretical 100% ceiling on towing ratios and the concept of a trailer wagging the tow vehicle really does not stand up to scientific scrutiny for two major reasons.

Firstly what we call towing ratio, is rarely a true representation of the actual mass distribution. Remember that the ratio is calculated on the caravans MTPLM divided by the cars Kerbweight. Whilst it is conceivable that a caravan may well be loaded up to its MTPLM, I have yet to see a towing car that is as low as it’s kerbweight, so the real ratio will automatically be lower than the traditional calculated value. In most cases I would estimate that a towing car is at least 200Kg or more heavier than its kerbweight when towing which suggests that an 100% outfit of 1500kg (kerbweight) with a 1500kg caravan will actually be 1700 and 1500Kg giving a real 88% ratio. In practice the car may be even heavier leading to even lower real percentages.

Secondly, When you review the numbers of instability related incidents, (whether or not they result in a total loss of control), the chances that all these outfits are at or above 100% (by the traditional method) is highly unlikely. This leads to the logical deduction that instability can affect outfits at values below 100%, But how much below? A lot of caravanners report they stick religiously to the oft recommended 85% UK ratio, and yet there are still instability incidents. Factor in the first reason I gave and the traditional 85% figure actually falls to a real 71% or less.

I strongly suspect that there are more instability incidents with outfit at much lower ratios which in turn strongly suggests that 100% towing (either traditional method or real life values) is not the glass ceiling towing limit it has been made out to be. This suggests there is no particular sudden change in towing characteristics when the ratio reaches 100%

As a supporting view, I draw your attention to the car manufacturers. If towing at more than 100% were proven to be as unadvisable as the UK caravan industry tries to tell us, then the car manufactures who are highly sensitive to safety related issues would cap their vehicles towing limits to no more than 100%, They don’t because they test their vehicles to asses their towing abilities so they bring factual test results into consideration rather than hypothetical limits. So they do not see it as being the big “no no” Similarly in Europe and elsewhere abroad, towing above 100% is quite commonplace.

Now please don’t think that I am advocating everyone should load up as high as possible. No! There is a vast body of evidence that demonstrate that as the towed weight increases the outfit does tend towards instability of various kinds. I also advocate that the driver considers not just the towing ratio but all the other factors that affect the safety of the outfit. I continue to recommend that you should keep the trailer as light as possible but I wont give any artificial limits to the statement.

There is nothing wrong in setting a personal preference of 85 or 100% but the traditional reasons for sticking to these specific limits do not have credibility.

With regards the issue of assessing an outfits weights: I think I understand your description but can I clarify a few points with you?

Quote
“The point regarding the trailer/caravan weight is (in my opinion) a rather subjective one;insomuch as,if the trailer was put onto a weighplate and a nose weight gauge put into the hitch-head cup and set on pads to simulate a ride height of the chosen tow-vehicle when hitch up to ball and with the nose weight adjusted and the jockey wheel raised this would give a true total weight of that trailer as a stand-alone entity.

If the trailer/caravan was then put onto the tow-vehicle ball and past onto that same weigh-plate it is assumed that the trailer would then weigh less by that tow-ball figure and therefore the car would weigh heavier by that same amount than before it was weighed as a stand-alone entity.

If that was the case then there would be two definitions (the subjective bit), the theoretical & the actual of the trailer/caravan.”
Unquote

Do you mean you add the nose load to the axle load? – if so then yes; that gives the Gross trailer weight, and that must not exceed the MTPLM limit. I do not see any subjective element to it.

The axle load of the trailer is the trailed weight and not its gross weight. VOSA should understand that important difference.

As for assessing the gross weight of a caravan using the dynamic system, there is actually no problem with rolling the jockey wheel and then the main wheels over the pad, and adding their values together. Although the jockey wheel is set back from the tow hitch and thus will record a different value compared to the hitch load, for good scientific reasons the weight of any object (i.e. the caravan) can be measured by adding together all the vertical loads imposed an ALL the supporting structures. If you had a system of sufficient accuracy you would find that the difference between the load on the hitch and the load supported by the jockey wheel would be exactly offset by a change on the main axle load.

The only two cautionary notes would be; Firstly the jockey wheel load and the axle load must be taken with the jockey wheel set at the same height and the wheel facing in the same relative direction to the caravan body,

And secondly this method by its self does not establish the nose load, however if you also have the trailer axle load when attached to the car, then the trailer Gross weigh minus the towed axle load gives the nose load, which takes us back to 4 paragraphs above.

I hope this helps

 

[Lutz]

The easiest method of checking an outfit whether everything is legal, including whether the caravan is still within its MTPLM, is to measure the gross vehicle weight of the car first (drive the outfit on to the weighbridge but only until the car is standing on it) then the whole outfit (car and caravan), to get the gross train weight. Finally, unhitch the car and weigh it alone. The second reading minus the third will be the gross weight of the caravan.
No need to put the jockey wheel on the weighbridge if you measure like that.
The difference between the first and the third reading will be the noseweight.

 

[ProfJohnL]

Good evening Lutz,

Trust you to spot a simpler method. well done.