If the public is told a car can tow a maximum weight of 3500kg by the car manufacturer why should they question this? Most people have never seen anything other than pig trailers. I've only ever seen one picture of a car towing anything but a pig trailer (Leyland Brothers photo). So everyone will assume that their maximum tow weight is with a caravan. I think that no one would then start looking into the maximum rear axle weight, because the car manufacturer has said that they can tow up to maximum tow weight.
It would (will?) be an interesting legal case if a manufacturer is sued for failing to give clarity to tow weights.
My experience in product development is that if we're a bit iffy (technical term :) ) about something with the product the rule is "Put it in the manual" and then when the customer says it won't do X we say "It says so in the manual".
Flame Man said
10:43 AM Nov 4, 2021
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
montie said
11:00 AM Nov 4, 2021
Flame Man wrote:
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
I'm a tad confused here.....
Axle Group Rating is set by the chassis and suspension manufacturers and is an engineered rating. This rating is not always stamped on the compliance plate and is sometimes stamped separately.
GTM on the other hand is simply a product of subtracting the measured empty ball weight from the plated ATM and stamping the figure on the compliance plate. No science or engineering involved.
It is illegal for the measured weight to exceed the AGR or the ATM.
Flame Man said
02:25 PM Nov 4, 2021
montie wrote:
Flame Man wrote:
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
I'm a tad confused here.....
Axle Group Rating is set by the chassis and suspension manufacturers and is an engineered rating. This rating is not always stamped on the compliance plate and is sometimes stamped separately.
GTM on the other hand is simply a product of subtracting the measured empty ball weight from the plated ATM and stamping the figure on the compliance plate. No science or engineering involved.
It is illegal for the measured weight to exceed the AGR or the ATM.
No engineered ratings have been exceeded and everything is legal with no modifications necessary.
The G&S chassis which has a compliance plate stating GTM 3200Kg (this is also called axle group rating)
The caravan VIN plate had a GTM of 2900Kg (ATM 3200 - 300TBW)
The new compliance certificate has a GTM 3200Kg (G&S rating) and an ATM 3500Kg
It is common practice for some caravan manufacturers to derate the GTM below the chassis manufacturer engineered rating.
montie said
03:19 PM Nov 4, 2021
Flame Man wrote:
montie wrote:
Flame Man wrote:
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
I'm a tad confused here.....
Axle Group Rating is set by the chassis and suspension manufacturers and is an engineered rating. This rating is not always stamped on the compliance plate and is sometimes stamped separately.
GTM on the other hand is simply a product of subtracting the measured empty ball weight from the plated ATM and stamping the figure on the compliance plate. No science or engineering involved.
It is illegal for the measured weight to exceed the AGR or the ATM.
No engineered ratings have been exceeded and everything is legal with no modifications necessary.
The G&S chassis which has a compliance plate stating GTM 3200Kg (this is also called axle group rating)
The caravan VIN plate had a GTM of 2900Kg (ATM 3200 - 300TBW)
The new compliance certificate has a GTM 3200Kg (G&S rating) and an ATM 3500Kg
It is common practice for some caravan manufacturers to derate the GTM below the chassis manufacturer engineered rating.
So the Axle Group Rating is 3200kg and the ATM is 3500kg.
You will need a minimum of 300kg ball weight in order not to exceed the AGR and even then your sailing close to the wind.
If I've got your figures correct you are relying on ball weight to stay legal.
-- Edited by montie on Thursday 4th of November 2021 03:21:20 PM
Flame Man said
07:08 PM Nov 4, 2021
montie wrote:
Flame Man wrote:
montie wrote:
Flame Man wrote:
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
I'm a tad confused here.....
Axle Group Rating is set by the chassis and suspension manufacturers and is an engineered rating. This rating is not always stamped on the compliance plate and is sometimes stamped separately.
GTM on the other hand is simply a product of subtracting the measured empty ball weight from the plated ATM and stamping the figure on the compliance plate. No science or engineering involved.
It is illegal for the measured weight to exceed the AGR or the ATM.
No engineered ratings have been exceeded and everything is legal with no modifications necessary.
The G&S chassis which has a compliance plate stating GTM 3200Kg (this is also called axle group rating)
The caravan VIN plate had a GTM of 2900Kg (ATM 3200 - 300TBW)
The new compliance certificate has a GTM 3200Kg (G&S rating) and an ATM 3500Kg
It is common practice for some caravan manufacturers to derate the GTM below the chassis manufacturer engineered rating.
So the Axle Group Rating is 3200kg and the ATM is 3500kg.
You will need a minimum of 300kg ball weight in order not to exceed the AGR and even then your sailing close to the wind.
If I've got your figures correct you are relying on ball weight to stay legal.
-- Edited by montie on Thursday 4th of November 2021 03:21:20 PM
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.
I found the caravan tows much better with full water tanks and ball weight of 340Kg.
montie said
07:50 AM Nov 5, 2021
Just a note...van measured ball weight is variable and can change quickly as the load changes. Often water tanks have a large effect depending on whether they are full or empty or in between.
In a situation where you are relying on measured ball weight to stay legal it would be advisable to monitor it on a regular basis.
Most engineers, not all, will not approve an ATM upgrade that exceeds the Axle Group Rating.
Derek Barnes said
08:15 AM Nov 5, 2021
montie wrote:
Just a note...van measured ball weight is variable and can change quickly as the load changes. Often water tanks have a large effect depending on whether they are full or empty or in between. In a situation where you are relying on measured ball weight to stay legal it would be advisable to monitor it on a regular basis.
Most engineers, not all, will not approve an ATM upgrade that exceeds the Axle Group Rating.
It is interesting that the whole dynamic of the caravan would change to a certain extent depending on where your fresh water and grey water tanks were located and whether you stayed in a caravan park (water is discharged from the caravan) or stayed in a National Park (water is just transferred from the fresh water to grey water tanks).
PeterD said
10:27 AM Nov 5, 2021
Derek Barnes wrote: It is interesting that the whole dynamic of the caravan would change to a certain extent depending on where your fresh water and grey water tanks were located and whether you stayed in a caravan park (water is discharged from the caravan) or stayed in a National Park (water is just transferred from the fresh water to grey water tanks).
It does alter the ball weight a bit but generally not drastically. Back when I was around 80 kg weight we had a 15' pop-top van with a front kitchen. I walked the length of the van whilst a friend observed the ball weight. It was only 30 kg heavier when I was in the front of the van than when I was standing in the rear.
yobarr said
11:01 AM Nov 5, 2021
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
yobarr said
11:06 AM Nov 5, 2021
PeterD wrote:
Derek Barnes wrote: It is interesting that the whole dynamic of the caravan would change to a certain extent depending on where your fresh water and grey water tanks were located and whether you stayed in a caravan park (water is discharged from the caravan) or stayed in a National Park (water is just transferred from the fresh water to grey water tanks).
It does alter the ball weight a bit but generally not drastically. Back when I was around 80 kg weight we had a 15' pop-top van with a front kitchen. I walked the length of the van whilst a friend observed the ball weight. It was only 30 kg heavier when I was in the front of the van than when I was standing in the rear.
What you say,Peter,is quite right,but a lot depends on the location of tanks.Under my van there are 5 water tanks,and a grey water tank,each plumbed individually.In order to maintain ball weight somewhere near right,I use these selectively as 420kg of water can make a huge difference to ball weight.Your comnent about the slight change in your ball weight is simple physics,and would depend on whether your were a "slim Jim" or a man of more ample proportions! Cheers
-- Edited by yobarr on Friday 5th of November 2021 10:40:45 PM
montie said
11:47 AM Nov 5, 2021
KevinJ wrote:
I was under the impression that a vehicle with a Maximum Tow Mass (Braked) of say 3000kg could tow a caravan with an ATM of say 3500kg PROVIDED the mass of the caravan while sitting alone on a weighbridge with both the rear axles and the jockey wheel on the weighbridge did not weigh more than the 3000kg the vehicle is allowed to tow (I'm assuming all other ratings are not exceeded in this example). Illogical if you want to load it with gear and go travelling but what if a friend asks you to take his van from Site A and deliver it to Site B and the van is empty?
The measured weight of the unhitched van must not legally exceed the plated ATM.
Flame Man said
12:47 PM Nov 7, 2021
yobarr wrote:
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
yobarr said
07:02 PM Nov 7, 2021
Flame Man wrote:
yobarr wrote:
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.any
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
Interesting.I would be most interested to learn what the axle weight on your caravan (GTM) is with WDH tensioned,and what the weight on your car's front axle is when the WDH is tensioned.Something doesn't add up here,but since I do not have access to my records,and my memory is not what it once was,I do not know the JGC's specs,but I would be astounded if the weight on the wheels of your car is 10% greater than the weight on your van's axle group,which is a prerequisite if safety is of any concern,when the trailer being towed is a PIG trailer.(most caravans). This is why there is a law that says that if a towing vehicle has a GVM above 4500kg,any PIG trailer it tows must always weigh less than the towing vehicle. For this reason,many earthmoving contractors have to load a couple of buckets of dirt onto the back of their truck when carrying their excavators etc on their PIG trailer.The sooner a similar law is enacted for all vehicles,the safer we all will be.Think "Tail wagging the Dog". Cheers
P.S How can your water tanks be "full or empty"? Do you fill them before you travel,or do you drain them completely? Strange.
-- Edited by yobarr on Sunday 7th of November 2021 07:18:27 PM
Flame Man said
02:30 PM Nov 8, 2021
yobarr wrote:
Flame Man wrote:
yobarr wrote:
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.any
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
Interesting.I would be most interested to learn what the axle weight on your caravan (GTM) is with WDH tensioned,and what the weight on your car's front axle is when the WDH is tensioned.Something doesn't add up here,but since I do not have access to my records,and my memory is not what it once was,I do not know the JGC's specs,but I would be astounded if the weight on the wheels of your car is 10% greater than the weight on your van's axle group,which is a prerequisite if safety is of any concern,when the trailer being towed is a PIG trailer.(most caravans). This is why there is a law that says that if a towing vehicle has a GVM above 4500kg,any PIG trailer it tows must always weigh less than the towing vehicle. For this reason,many earthmoving contractors have to load a couple of buckets of dirt onto the back of their truck when carrying their excavators etc on their PIG trailer.The sooner a similar law is enacted for all vehicles,the safer we all will be.Think "Tail wagging the Dog". Cheers
P.S How can your water tanks be "full or empty"? Do you fill them before you travel,or do you drain them completely? Strange.
-- Edited by yobarr on Sunday 7th of November 2021 07:18:27 PM
As I stay in Caravan parks I have no need to carry water (empty case) I do now (after ATM upgrade) tow with full water tanks as I believe the caravan is less susceptible to cross winds and truck air turbulence, the fact that both tanks are full and located either side of the axle group means I see no increase in TBW with full tanks
After many trips to a weighbridge to check loading and adjust WDH, the rig is legal and within specification with a buffer for additional payload if that occurs on long trips.
WDH tensioned.
Caravan GTM 2940
Jeep Front Axle 1240
Jeep Rear Axle 1660kg
yobarr said
08:31 PM Nov 8, 2021
Flame Man wrote:
yobarr wrote:
Flame Man wrote:
yobarr wrote:
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.any
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
Interesting.I would be most interested to learn what the axle weight on your caravan (GTM) is with WDH tensioned,and what the weight on your car's front axle is when the WDH is tensioned.Something doesn't add up here,but since I do not have access to my records,and my memory is not what it once was,I do not know the JGC's specs,but I would be astounded if the weight on the wheels of your car is 10% greater than the weight on your van's axle group,which is a prerequisite if safety is of any concern,when the trailer being towed is a PIG trailer.(most caravans). This is why there is a law that says that if a towing vehicle has a GVM above 4500kg,any PIG trailer it tows must always weigh less than the towing vehicle. For this reason,many earthmoving contractors have to load a couple of buckets of dirt onto the back of their truck when carrying their excavators etc on their PIG trailer.The sooner a similar law is enacted for all vehicles,the safer we all will be.Think "Tail wagging the Dog". Cheers
P.S How can your water tanks be "full or empty"? Do you fill them before you travel,or do you drain them completely? Strange.
As I stay in Caravan parks I have no need to carry water (empty case) I do now (after ATM upgrade) tow with full water tanks as I believe the caravan is less susceptible to cross winds and truck air turbulence, the fact that both tanks are full and located either side of the axle group means I see no increase in TBW with full tanks
After many trips to a weighbridge to check loading and adjust WDH, the rig is legal and within specification with a buffer for additional payload if that occurs on long trips.WDH tensioned.
Caravan GTM 2940
Jeep Front Axle 1240
Jeep Rear Axle 1660kg
Seems you've done well with your weights,William.Congratulations.We can now only hope that others are equally responsible,but of major concern is that the weight on your car's wheels is less than the weight on your van's wheels. Always, if safety is of any concern,the weight on your car's wheels should be at least 10% greater than the weight on your van's wheels,but at least you're taking things seriously.Well done.Cheers
Flame Man said
09:20 PM Nov 8, 2021
yobarr wrote:
Flame Man wrote:
yobarr wrote:
Flame Man wrote:
yobarr wrote:
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.any
I found the caravan tows much better with full water tanks and ball weight of 340kg
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
Interesting.I would be most interested to learn what the axle weight on your caravan (GTM) is with WDH tensioned,and what the weight on your car's front axle is when the WDH is tensioned.Something doesn't add up here,but since I do not have access to my records,and my memory is not what it once was,I do not know the JGC's specs,but I would be astounded if the weight on the wheels of your car is 10% greater than the weight on your van's axle group,which is a prerequisite if safety is of any concern,when the trailer being towed is a PIG trailer.(most caravans). This is why there is a law that says that if a towing vehicle has a GVM above 4500kg,any PIG trailer it tows must always weigh less than the towing vehicle. For this reason,many earthmoving contractors have to load a couple of buckets of dirt onto the back of their truck when carrying their excavators etc on their PIG trailer.The sooner a similar law is enacted for all vehicles,the safer we all will be.Think "Tail wagging the Dog". Cheers
P.S How can your water tanks be "full or empty"? Do you fill them before you travel,or do you drain them completely? Strange.
As I stay in Caravan parks I have no need to carry water (empty case) I do now (after ATM upgrade) tow with full water tanks as I believe the caravan is less susceptible to cross winds and truck air turbulence, the fact that both tanks are full and located either side of the axle group means I see no increase in TBW with full tanks
After many trips to a weighbridge to check loading and adjust WDH, the rig is legal and within specification with a buffer for additional payload if that occurs on long trips.WDH tensioned.
Caravan GTM 2940
Jeep Front Axle 1240
Jeep Rear Axle 1660kg
Seems you've done well with your weights,William.Congratulations.We can now only hope that others are equally responsible,but of major concern is that the weight on your car's wheels is less than the weight on your van's wheels. Always, if safety is of any concern,the weight on your car's wheels should be at least 10% greater than the weight on your van's wheels,but at least you're taking things seriously.Well done.Cheers
Thanks for your kind words, my first concern was to make sure the rig was legal and within manufacturers specification.
It is possible to tow up to a 3.5 ton caravan with a number of vehicles with close attention to loading of vehicle/van and a properly adjusted WDH (in my case)
I take on board your comments about the vehicle ideally being heavier than the van, the only answer I have to mitigate this is I did fit a Dexter stability control system to the van which should stop the tail from wagging the dog under aggressive braking.
It would (will?) be an interesting legal case if a manufacturer is sued for failing to give clarity to tow weights.
My experience in product development is that if we're a bit iffy (technical term :) ) about something with the product the rule is "Put it in the manual" and then when the customer says it won't do X we say "It says so in the manual".
The GTM is the one on the chassis manufacturer compliance plate attached to the chassis, in my case this was 3200kg.
On the caravan manufacturer compliance plate this is called the axle group load capacity (3200) the GTM was derated by 300kg which was the ball loading at tare, which effectively means the chassis GTM becomes the ATM in the eyes of the caravan manufacturer, this seems to be common practice by some caravan manufacture, but it also means that lots of caravans can have their payload increased by approx 50% if the chassis GTM is used on the caravan compliance (VIN) plate. The reason no modifications are required is because the chassis manufacturer has designed/certified the chassis to take 3200kg, all that is required is a new compliance certificate from a licensed certifier (cost $660)
I'm a tad confused here.....
Axle Group Rating is set by the chassis and suspension manufacturers and is an engineered rating. This rating is not always stamped on the compliance plate and is sometimes stamped separately.
GTM on the other hand is simply a product of subtracting the measured empty ball weight from the plated ATM and stamping the figure on the compliance plate. No science or engineering involved.
It is illegal for the measured weight to exceed the AGR or the ATM.
No engineered ratings have been exceeded and everything is legal with no modifications necessary.
The G&S chassis which has a compliance plate stating GTM 3200Kg (this is also called axle group rating)
The caravan VIN plate had a GTM of 2900Kg (ATM 3200 - 300TBW)
The new compliance certificate has a GTM 3200Kg (G&S rating) and an ATM 3500Kg
It is common practice for some caravan manufacturers to derate the GTM below the chassis manufacturer engineered rating.
So the Axle Group Rating is 3200kg and the ATM is 3500kg.
You will need a minimum of 300kg ball weight in order not to exceed the AGR and even then your sailing close to the wind.
If I've got your figures correct you are relying on ball weight to stay legal.
-- Edited by montie on Thursday 4th of November 2021 03:21:20 PM
Just to be clear, if fully loaded with an ATM 3500KG ball weight 350Kg (10% rule) then AGR 3150Kg
which is still within spec and legal, in reality with full water tanks, clothes, food etc the weights are
GTM 2940, TBW 340, ATM 3280Kg this still gives me an additional 220Kg of payload if required and a handy buffer to have for the lap around Oz.
I found the caravan tows much better with full water tanks and ball weight of 340Kg.
In a situation where you are relying on measured ball weight to stay legal it would be advisable to monitor it on a regular basis.
Most engineers, not all, will not approve an ATM upgrade that exceeds the Axle Group Rating.
It is interesting that the whole dynamic of the caravan would change to a certain extent depending on where your fresh water and grey water tanks were located and whether you stayed in a caravan park (water is discharged from the caravan) or stayed in a National Park (water is just transferred from the fresh water to grey water tanks).
It does alter the ball weight a bit but generally not drastically. Back when I was around 80 kg weight we had a 15' pop-top van with a front kitchen. I walked the length of the van whilst a friend observed the ball weight. It was only 30 kg heavier when I was in the front of the van than when I was standing in the rear.
It would be interesting to hear what car you propose to apply a 340kg towball weight to,as there is almost none that can handle that,despite all the claimed figures.A 340kg towball weight applies between 480kg and 520kg to the car's rear axle,and there are very few cars that can handle this under "normal" holiday usage.None of the more popular twin-cab utes can safely tow more than about 3100kg ATM,and most of the bigger cars also fail.Weights are a minefield if you don't understand them,and I have,until now,simply been watching this thread,with interest.I look forward to your next post.Cheers
What you say,Peter,is quite right,but a lot depends on the location of tanks.Under my van there are 5 water tanks,and a grey water tank,each plumbed individually.In order to maintain ball weight somewhere near right,I use these selectively as 420kg of water can make a huge difference to ball weight.Your comnent about the slight change in your ball weight is simple physics,and would depend on whether your were a "slim Jim" or a man of more ample proportions! Cheers
-- Edited by yobarr on Friday 5th of November 2021 10:40:45 PM
The measured weight of the unhitched van must not legally exceed the plated ATM.
The vehicle is a Jeep Grand Cherokee and again within the factory design specification and legal, the overhang wheelbase ratio is 0.42 which gives 482kg on the rear axle, after fitting a WDH this goes down to 412kg. Fully loaded with 2 passengers full fuel tanks the rear axle is 1660kg (design 1678kg)
The 2 water tanks on my caravan ( either side of axles) make no difference to TBW as I either have them full or empty (nothing in between) and stay in caravan parks.
Interesting.I would be most interested to learn what the axle weight on your caravan (GTM) is with WDH tensioned,and what the weight on your car's front axle is when the WDH is tensioned.Something doesn't add up here,but since I do not have access to my records,and my memory is not what it once was,I do not know the JGC's specs,but I would be astounded if the weight on the wheels of your car is 10% greater than the weight on your van's axle group,which is a prerequisite if safety is of any concern,when the trailer being towed is a PIG trailer.(most caravans). This is why there is a law that says that if a towing vehicle has a GVM above 4500kg,any PIG trailer it tows must always weigh less than the towing vehicle. For this reason,many earthmoving contractors have to load a couple of buckets of dirt onto the back of their truck when carrying their excavators etc on their PIG trailer.The sooner a similar law is enacted for all vehicles,the safer we all will be.Think "Tail wagging the Dog". Cheers
P.S How can your water tanks be "full or empty"? Do you fill them before you travel,or do you drain them completely? Strange.
-- Edited by yobarr on Sunday 7th of November 2021 07:18:27 PM
As I stay in Caravan parks I have no need to carry water (empty case) I do now (after ATM upgrade) tow with full water tanks as I believe the caravan is less susceptible to cross winds and truck air turbulence, the fact that both tanks are full and located either side of the axle group means I see no increase in TBW with full tanks
After many trips to a weighbridge to check loading and adjust WDH, the rig is legal and within specification with a buffer for additional payload if that occurs on long trips.
WDH tensioned.
Caravan GTM 2940
Jeep Front Axle 1240
Jeep Rear Axle 1660kg
Seems you've done well with your weights,William.Congratulations.We can now only hope that others are equally responsible,but of major concern is that the weight on your car's wheels is less than the weight on your van's wheels. Always, if safety is of any concern,the weight on your car's wheels should be at least 10% greater than the weight on your van's wheels,but at least you're taking things seriously.Well done.Cheers
Thanks for your kind words, my first concern was to make sure the rig was legal and within manufacturers specification.
It is possible to tow up to a 3.5 ton caravan with a number of vehicles with close attention to loading of vehicle/van and a properly adjusted WDH (in my case)
I take on board your comments about the vehicle ideally being heavier than the van, the only answer I have to mitigate this is I did fit a Dexter stability control system to the van which should stop the tail from wagging the dog under aggressive braking.