Tyres play a critical role in determining the Handlingand Stability characteristics of all caravans on the road.
On a few frightening occasions, the tyres selected by a 'van manufacturer have been dangerously "under-engineered", in that their Maximum Load Rating was less than the Maximum Load that the tyres could legally (as per the GTM Rating) be subjected to!
In far too many cases, the tyres selected by the 'van manufacturer have been hideously "over-engineered", in that their Maximum Load Rating was much higher than the Maximum Loadthat the tyres would ever be subjected to!
While there may be some other opinions on how much a tyre should be loaded to, for caravan tyres it is reasonable to have the maximum Load applied to a tyre, at around 90 - 95% of the Rating. Certainly it should not be around 50 - 60%.
"BIGGER ain't always BETTER"... certainly applies to caravan wheels and tyres.
If you are not happy with the way your 'van handles on the road, or if you have any other concerns about your 'van's Tyre Size or Inflation Pressures, you should discuss the issues with the 'van Manufacturer - and also seek the advice of a professional Tyre specialist - to best-ensure that the fitted tyres are, or are not, of the optimum specification.
Manufacturers often state the "Recommended Inflation Pressure" - for the Full-Loaded Condition - as the "Maximum Permitted Pressure" that is embossed on the tyre sidewall... even though the tyres are only loaded to a fraction of their Rating!
No wonder the 'van then skips and bounces around on the road, the 'van vibrates horribly, and the central portion of the tyres' tread wears out very quickly.
Sometimes manufacturers state the "Recommended Inflation Pressure" - for the Full-Loaded Condition - as the "Stated Pressure" for that particular tyre size, when subjected to the actual maximum load... as prescribed in the Tyre and Rim Association of Australia's "Standards Manual".
No wonder the 'van then sways and wallows around on the road, and the outer portions of the tyres' tread wear out very quickly.
Neithersituation is satisfactory with respect to safe and predictable handling and road-holding.
If only one inflation inflation pressure is stated, you need to seriously question the 'van manufacturer's knowledge of suspensions and tyres.
The "Tyre Placard" - which is usually incorporated in the "Trailer Plate" - is a vital part of a caravan.
The intentof affixing a Tyre Placard to a caravan is to provide an Assurance from the Manufacturer - to a potential-buyer that the selected tyre Size and Load Rating - along with the recommended Tyre Inflation Pressures - have been professionally determined, so as to provide the optimum caravan handlingand stability performance, at both the empty and the fully-loadedconditions.
Sadly, for many caravans, this is clearly not the case. Incorrect information has been the prime cause of manycomplaints of unacceptable - and often dangerous - caravan handling and stability problems... and all too often, serious accidents.
The Tyre Placard must show at least the following information:
Caravan manufacturer's recommended (complete) tyre Size and Description... Diameter, Width, Construction, Aspect Ratio
Tyre Load Rating
Tyre Speed Rating
Cold Inflation Pressures
The Tyre Load Rating mustbe stated in kg.
The Tyre Inflation Pressures must be stated in kPa.
For load-sharing suspension systems, the maximum legal individual Tyre Load equals the GTM Rating divided by the Number of tyres fitted to the 'van.
The individual Tyre Load - when the 'van is empty - equals the (Tare Mass minus the Empty Ball-Loading) divided by the Number of tyres fitted to the 'van.
If - as is often the case - your 'van's "Empty Ball-Loading just "happens" to equal the ATM Rating minus the GTM Rating" or the "GTM Rating just "happens" to equal the ATM Rating minus the Empty Ball-Loading", it is vital that you seriously question the 'van manufacturer, as to how the GTM Rating and the Empty Ball-Loading were determined.
You simply cannotcompare (fixed, allocated Ratings... which are maximum limits applicable only for the fully-loaded condition) with Actual Masses, such as the Ball-Loading and the Axle-Loading, that vary, depending on how the 'van is loaded!!!
For non-load-sharing suspension systems (on tandem-axle 'vans), the front and rear tyres will have different loadings, if the 'van is notlevel(in a side view).
For maximum traction - and maximum tread life - the loading needs to be spread evenly across the full width of the tread.
This can only be achieved if the correct inflation pressure is used so that the tread is at the same temperature across its full width.
Under-inflation:
If a tyre is not inflated up to the Pressure that is required to match the actual tyre Loading, the tyre tread will not have full contact with the road surface, causing the outer portions of the tread to be subjected to increased loading, with subsequent increased temperatures and increased wear.
The traction between the tyre and the road surface will be decreased, and the sidewalls of the tyre will significantly flex, causing the 'van to wallowand sway.
Over-inflation:
If a tyre is inflated above the Pressure that is required to match the actual tyre Loading, again the tyre tread will not have full contact with the road surface, this time causing the centre portion of the tread to be subjected to increased loading, with subsequent increased temperatures and increased wear.
Again, the traction between the tyre and the road surface will be decreased, causing the tyre to bulge, and the 'van to bounce and skip.
In both cases, the handling and stability of the 'van will be impaired, and the tyre life appreciably reduced. While most of the tread will have plenty of kilometres of travel remaining, the outer peripheries of an under-inflated tyre will be worn down to the Tread Wear Indicators, thus rendering the tyre unroadworthy, while for an over-inflated tyre, the inner periphery will quickly be worn down to a dangerous and illegal level.
Peter_n_Margaret said
02:38 PM Dec 14, 2021
Colin1947 wrote:
While there may be some other opinions on how much a tyre should be loaded to, for caravan tyres it is reasonable to have the maximum Load applied to a tyre, at around 90 - 95% of the Rating. Certainly it should not be around 50 - 60%.
You need to explain - "why not?"
Cheers,
Peter
yobarr said
03:14 PM Dec 14, 2021
Peter_n_Margaret wrote:
Colin1947 wrote:
While there may be some other opinions on how much a tyre should be loaded to, for caravan tyres it is reasonable to have the maximum Load applied to a tyre, at around 90 - 95% of the Rating. Certainly it should not be around 50 - 60%.
You need to explain - "why not?"
Cheers,
Peter
Peter,I am in agreement with you on this one.My GTM is 3150kg,but my tyres have carrying capacity of 1550kg each,or 6200kg,so about 50%. I wouldn't have it any other way.Similar to these little twin-cabs with their highly stressed little engines screaming their guts out to get anywhere, while my little understressed V8 just chugs along all day without me needing to change gears. Rear axle on car runs at 2300kg,but that is only 74% of the tyres' carrying capacity.Like you,I look forward to a logical explanation.Cheers
-- Edited by yobarr on Tuesday 14th of December 2021 04:15:41 PM
Jaahn said
01:29 PM Dec 15, 2021
Hi
My on this long statement from Colin. It cannot be argued that the carrying capacity of the tire size and pressure recommended for any vehicle must exceed the load rating for that vehicle. Indeed the fitting of an information plate that does not do that would be in breech of several standards IMHO.
I would then say that recommended pressures for tires should be within reasonable limits close to recommended by the manufacturer for that size and load. It is not recommended to run lower pressures unless you have special conditions and you are reducing the speed proportionally also. But is commonly recommended to run "a bit" higher depending on the conditions. I see no problems there if the starting point is the recommended pressure for the load.
I think my problem is with Colins' statement on the wear problems with different pressures. Most of the statements of shoulder wear or center wear is very old information from cross ply times and is not really relevant to belted radial tires IMHO. I would have to say from my long experience the tire industry trots out that story regularly to explain poor tire wear when infact the wheel alignment is the real cause IMHO and the alignment you paid for was so poorly done as to be ineffective.
As for not fitting bigger tires and running them at a bit less pressure to suit the load, this is regularly done on 4WDs and light trucks and causes no problem. I might do this and be happy. Indeed there is a huge industry flogging this to the punters What was the perceived problem seen in the statement ??
Jaahn
-- Edited by Jaahn on Wednesday 15th of December 2021 01:37:09 PM
-- Edited by Jaahn on Wednesday 15th of December 2021 03:24:30 PM
watsea said
02:23 PM Dec 15, 2021
There are probably a number of people running "over engineered" tyres. They are probably more than satisfied.
Though I wonder, do they ever consider how much extra weight, extra friction power, wear and tear, and hence extra running costs that they end up having to endure?
Whenarewethere said
02:33 PM Dec 15, 2021
What road surface, ie corrugated unsealed or worse, has one reduced tyre pressure accordingly & reduced speed to appropriate conditions for the tyre to cope.
KM575 said
06:23 PM Dec 15, 2021
My vans compliance plate indicates to use cold pressure of 50psi. The tyres that came with the van from new also have a max cold pressure of 50psi. However if I use one of the tyre pressure calculators online eg the G&S tyre pressure calculator, based on number of wheels, tyre load rating, max tyre pressure as above and actual load on the vans wheels of van (I use my GTM assuming I a fully loaded, but usually travel a couple of hundred kg below), the calculator says to use 39.18psi. This is way less than 50psi on the compliance plate I have experimented a bit and currently use 42psi as using this cold pressure it rises between 4 to 6 psi when the tyres are warmed up to travelling temp. I have no abnormal tyre wear and van seems to ride well. Do van manufacturers just use the max indicated on the tyre or do they have some other calculator to fill in the compliance plate ?
-- Edited by KM575 on Wednesday 15th of December 2021 06:29:45 PM
-- Edited by KM575 on Wednesday 15th of December 2021 09:00:05 PM
Jaahn said
09:20 AM Dec 16, 2021
KM575 wrote:
My vans compliance plate indicates to use cold pressure of 50psi. The tyres that came with the van from new also have a max cold pressure of 50psi. However if I use one of the tyre pressure calculators online eg the G&S tyre pressure calculator, based on number of wheels, tyre load rating, max tyre pressure as above and actual load on the vans wheels of van (I use my GTM assuming I a fully loaded, but usually travel a couple of hundred kg below), the calculator says to use 39.18psi. This is way less than 50psi on the compliance plate I have experimented a bit and currently use 42psi as using this cold pressure it rises between 4 to 6 psi when the tyres are warmed up to travelling temp. I have no abnormal tyre wear and van seems to ride well. Do van manufacturers just use the max indicated on the tyre or do they have some other calculator to fill in the compliance plate ?
Hi
You have taken a sensible approach to find a good pressure IMHO. Good travelling.
I do not believe the van manufacturers have any magic calculator for the pressure but just use the tires max as a fail safe pressure.
Jaahn
-- Edited by KM575 on Wednesday 15th of December 2021 06:29:45 PM
-- Edited by KM575 on Wednesday 15th of December 2021 09:00:05 PM
Mamil said
08:03 PM Dec 16, 2021
I was told a formula long ago that supposedly allows you to calculate the right pressure for any tyre and any load.
It assumes that the ratio between pressure and load at the tyre's maximum rating continues in a linear fashion all through it's usage range.
So, if you read the max. load and max pressure from your tyre wall, and weigh your rig to find it's GTM, you can calculate the required pressure as follows:
Cold inflation pressure in psi = (Max. tyre pressure in psi / max. tyre load in KG) X (GTM / number of wheels)
I'd be interested to hear what the tyre experts think of that?
-- Edited by Mamil on Thursday 16th of December 2021 08:04:57 PM
yobarr said
08:37 PM Dec 16, 2021
Mamil wrote:
I was told a formula long ago that supposedly allows you to calculate the right pressure for any tyre and any load.
It assumes that the ratio between pressure and load at the tyre's maximum rating continues in a linear fashion all through it's usage range.
So, if you read the max. load and max pressure from your tyre wall, and weigh your rig to find it's GTM, you can calculate the required pressure as follows:
Cold inflation pressure in psi = (Max. tyre pressure in psi / max. tyre load in KG) X (GTM / number of wheels)
I'd be interested to hear what the tyre experts think of that?
That's the formula I use,Andy,and it seems to work OK.When I put the formula to a mate,who reckons he knows about this stuff,he studied it at length,and gave it the thumbs up.Cheers
Peter_n_Margaret said
11:26 PM Dec 16, 2021
The linear relationship is not 100% but it is close if you don't have the manufacturer's data. I have complete data for the tyres I run on the OKA and from that I produced the following chart.
Could you not use tyre temps ? To pressure ? Start from
Higher side work down till temp starts to rise . Then add more pressure till temp stabilises ??
Peter_n_Margaret said
09:02 PM Dec 18, 2021
Aus-Kiwi wrote:
Could you not use tyre temps ? To pressure ? Start from Higher side work down till temp starts to rise . Then add more pressure till temp stabilises ??
No.
The temperature that is relevent is in the core of the tyre construction. I know no way of measuring that.
On the other hand, I find the pressure Vs Load Vs speed data supplied by the tyre manufacturer to be consistently reliable.
Cheers,
Peter
Aus-Kiwi said
10:10 PM Dec 18, 2021
So motor racing etc dont use tyre temps ?
Jaahn said
11:42 AM Dec 19, 2021
Aus-Kiwi wrote:
So motor racing etc dont use tyre temps ?
Of course they do Kiwi, but what relevance is that to a caravan or MH flogging along a mostly straight-ish road at 40deg in the back blocks or some horrible dirt roads. Generally racing is about corners and getting the suspension setup for even tread temps for max grip.
Why 'reinvent the wheel' again when the tire manufacturers have done all the work years ago and you only need to ask or look on line. Then write it in texta near the wheels if you have trouble remembering it.
Jaahn
Aus-Kiwi said
05:09 PM Dec 19, 2021
Thats the point the tyre delaminates off tyre gets too hot . There was a post on here way back about checking tyre pressures and double checking with temp gauge !!
Tyres - Size, Ratings & Inflation Pressures
Tyres play a critical role in determining the Handling and Stability characteristics of all caravans on the road.
On a few frightening occasions, the tyres selected by a 'van manufacturer have been dangerously "under-engineered", in that their Maximum Load Rating was less than the Maximum Load that the tyres could legally (as per the GTM Rating) be subjected to!
In far too many cases, the tyres selected by the 'van manufacturer have been hideously "over-engineered", in that their Maximum Load Rating was much higher than the Maximum Load that the tyres would ever be subjected to!
While there may be some other opinions on how much a tyre should be loaded to, for caravan tyres it is reasonable to have the maximum Load applied to a tyre, at around 90 - 95% of the Rating. Certainly it should not be around 50 - 60%.
"BIGGER ain't always BETTER"... certainly applies to caravan wheels and tyres.
If you are not happy with the way your 'van handles on the road, or if you have any other concerns about your 'van's Tyre Size or Inflation Pressures, you should discuss the issues with the 'van Manufacturer - and also seek the advice of a professional Tyre specialist - to best-ensure that the fitted tyres are, or are not, of the optimum specification.
Manufacturers often state the "Recommended Inflation Pressure" - for the Full-Loaded Condition - as the "Maximum Permitted Pressure" that is embossed on the tyre sidewall... even though the tyres are only loaded to a fraction of their Rating!
No wonder the 'van then skips and bounces around on the road, the 'van vibrates horribly, and the central portion of the tyres' tread wears out very quickly.
Sometimes manufacturers state the "Recommended Inflation Pressure" - for the Full-Loaded Condition - as the "Stated Pressure" for that particular tyre size, when subjected to the actual maximum load... as prescribed in the Tyre and Rim Association of Australia's "Standards Manual".
No wonder the 'van then sways and wallows around on the road, and the outer portions of the tyres' tread wear out very quickly.
Neither situation is satisfactory with respect to safe and predictable handling and road-holding.
If only one inflation inflation pressure is stated, you need to seriously question the 'van manufacturer's knowledge of suspensions and tyres.
The "Tyre Placard" - which is usually incorporated in the "Trailer Plate" - is a vital part of a caravan.
The intent of affixing a Tyre Placard to a caravan is to provide an Assurance from the Manufacturer - to a potential-buyer that the selected tyre Size and Load Rating - along with the recommended Tyre Inflation Pressures - have been professionally determined, so as to provide the optimum caravan handling and stability performance, at both the empty and the fully-loaded conditions.
Sadly, for many caravans, this is clearly not the case. Incorrect information has been the prime cause of many complaints of unacceptable - and often dangerous - caravan handling and stability problems... and all too often, serious accidents.
The Tyre Placard must show at least the following information:
The Tyre Load Rating must be stated in kg.
The Tyre Inflation Pressures must be stated in kPa.
For load-sharing suspension systems, the maximum legal individual Tyre Load equals the GTM Rating divided by the Number of tyres fitted to the 'van.
The individual Tyre Load - when the 'van is empty - equals the (Tare Mass minus the Empty Ball-Loading) divided by the Number of tyres fitted to the 'van.
If - as is often the case - your 'van's "Empty Ball-Loading just "happens" to equal the ATM Rating minus the GTM Rating" or the "GTM Rating just "happens" to equal the ATM Rating minus the Empty Ball-Loading", it is vital that you seriously question the 'van manufacturer, as to how the GTM Rating and the Empty Ball-Loading were determined.
You simply cannot compare (fixed, allocated Ratings... which are maximum limits applicable only for the fully-loaded condition) with Actual Masses, such as the Ball-Loading and the Axle-Loading, that vary, depending on how the 'van is loaded!!!
For non-load-sharing suspension systems (on tandem-axle 'vans), the front and rear tyres will have different loadings, if the 'van is not level (in a side view).
For maximum traction - and maximum tread life - the loading needs to be spread evenly across the full width of the tread.
This can only be achieved if the correct inflation pressure is used so that the tread is at the same temperature across its full width.
Under-inflation:
If a tyre is not inflated up to the Pressure that is required to match the actual tyre Loading, the tyre tread will not have full contact with the road surface, causing the outer portions of the tread to be subjected to increased loading, with subsequent increased temperatures and increased wear.
The traction between the tyre and the road surface will be decreased, and the sidewalls of the tyre will significantly flex, causing the 'van to wallow and sway.
Over-inflation:
If a tyre is inflated above the Pressure that is required to match the actual tyre Loading, again the tyre tread will not have full contact with the road surface, this time causing the centre portion of the tread to be subjected to increased loading, with subsequent increased temperatures and increased wear.
Again, the traction between the tyre and the road surface will be decreased, causing the tyre to bulge, and the 'van to bounce and skip.
In both cases, the handling and stability of the 'van will be impaired, and the tyre life appreciably reduced. While most of the tread will have plenty of kilometres of travel remaining, the outer peripheries of an under-inflated tyre will be worn down to the Tread Wear Indicators, thus rendering the tyre unroadworthy, while for an over-inflated tyre, the inner periphery will quickly be worn down to a dangerous and illegal level.
You need to explain - "why not?"
Cheers,
Peter
Peter,I am in agreement with you on this one.My GTM is 3150kg,but my tyres have carrying capacity of 1550kg each,or 6200kg,so about 50%. I wouldn't have it any other way.Similar to these little twin-cabs with their highly stressed little engines screaming their guts out to get anywhere, while my little understressed V8 just chugs along all day without me needing to change gears. Rear axle on car runs at 2300kg,but that is only 74% of the tyres' carrying capacity.Like you,I look forward to a logical explanation.Cheers
-- Edited by yobarr on Tuesday 14th of December 2021 04:15:41 PM
Hi
My
on this long statement from Colin. It cannot be argued that the carrying capacity of the tire size and pressure recommended for any vehicle must exceed the load rating for that vehicle. Indeed the fitting of an information plate that does not do that would be in breech of several standards IMHO.
I would then say that recommended pressures for tires should be within reasonable limits close to recommended by the manufacturer for that size and load. It is not recommended to run lower pressures unless you have special conditions and you are reducing the speed proportionally also. But is commonly recommended to run "a bit" higher depending on the conditions. I see no problems there if the starting point is the recommended pressure for the load.
I think my problem is with Colins' statement on the wear problems with different pressures. Most of the statements of shoulder wear or center wear is very old information from cross ply times and is not really relevant to belted radial tires IMHO. I would have to say from my long experience the tire industry trots out that story regularly to explain poor tire wear when infact the wheel alignment is the real cause IMHO and the alignment you paid for was so poorly done as to be ineffective.
As for not fitting bigger tires and running them at a bit less pressure to suit the load, this is regularly done on 4WDs and light trucks and causes no problem. I might do this and be happy. Indeed there is a huge industry flogging this to the punters
What was the perceived problem seen in the statement ??
Jaahn
-- Edited by Jaahn on Wednesday 15th of December 2021 01:37:09 PM
-- Edited by Jaahn on Wednesday 15th of December 2021 03:24:30 PM
Though I wonder, do they ever consider how much extra weight, extra friction power, wear and tear, and hence extra running costs that they end up having to endure?
What road surface, ie corrugated unsealed or worse, has one reduced tyre pressure accordingly & reduced speed to appropriate conditions for the tyre to cope.
My vans compliance plate indicates to use cold pressure of 50psi.
The tyres that came with the van from new also have a max cold pressure of 50psi.
However if I use one of the tyre pressure calculators online eg the G&S tyre pressure calculator, based on number of wheels, tyre load rating, max tyre pressure as above and actual load on the vans wheels of van (I use my GTM assuming I a fully loaded, but usually travel a couple of hundred kg below), the calculator says to use 39.18psi. This is way less than 50psi on the compliance plate
I have experimented a bit and currently use 42psi as using this cold pressure it rises between 4 to 6 psi when the tyres are warmed up to travelling temp.
I have no abnormal tyre wear and van seems to ride well.
Do van manufacturers just use the max indicated on the tyre or do they have some other calculator to fill in the compliance plate ?
-- Edited by KM575 on Wednesday 15th of December 2021 06:29:45 PM
-- Edited by KM575 on Wednesday 15th of December 2021 09:00:05 PM
I was told a formula long ago that supposedly allows you to calculate the right pressure for any tyre and any load.
It assumes that the ratio between pressure and load at the tyre's maximum rating continues in a linear fashion all through it's usage range.
So, if you read the max. load and max pressure from your tyre wall, and weigh your rig to find it's GTM, you can calculate the required pressure as follows:
Cold inflation pressure in psi = (Max. tyre pressure in psi / max. tyre load in KG) X (GTM / number of wheels)
I'd be interested to hear what the tyre experts think of that?
-- Edited by Mamil on Thursday 16th of December 2021 08:04:57 PM
That's the formula I use,Andy,and it seems to work OK.When I put the formula to a mate,who reckons he knows about this stuff,he studied it at length,and gave it the thumbs up.Cheers
The linear relationship is not 100% but it is close if you don't have the manufacturer's data.
I have complete data for the tyres I run on the OKA and from that I produced the following chart.
Sorry it is sideways......
Cheers,
Peter
Higher side work down till temp starts to rise . Then add more pressure till temp stabilises ??
No.
The temperature that is relevent is in the core of the tyre construction. I know no way of measuring that.
On the other hand, I find the pressure Vs Load Vs speed data supplied by the tyre manufacturer to be consistently reliable.
Cheers,
Peter
Of course they do Kiwi, but what relevance is that to a caravan or MH flogging along a mostly straight-ish road at 40deg in the back blocks or some horrible dirt roads. Generally racing is about corners and getting the suspension setup for even tread temps for max grip.
Why 'reinvent the wheel' again when the tire manufacturers have done all the work years ago and you only need to ask or look on line. Then write it in texta near the wheels if you have trouble remembering it.
Jaahn