Hi All, Im doubting the science!! I have calculated the supposed correct tyre pressure for my 21' twin axle van and it comes out at 36PSI. Then Isee one caravan sales mob say things like if y our trvelling at approx 100 kph on avarage then your pressure should be 50 psi (or half of whatever speed you travel at ie dirt road and sand). I thought that was a bit hit and miss. I then read whre others are running at between 40 and 50 psi. My max psi on the side of the tyre is 50 psi. IAt 36 psi my tyres look "flat". Hence ...doubting the sciense.
Peter_n_Margaret said
02:10 PM Oct 21, 2024
Depends entirely on the tyre and the actual weight it supports.
Without that information it is a guess.
Get the correct information first.
Cheers,
Peter
FootlooseFizzers said
03:59 PM Oct 21, 2024
235/75R 15 109T AGM 2900 does that help?
Peter_n_Margaret said
05:13 PM Oct 21, 2024
For an ACTUAL load on a load sharing suspension of 2,900kg = 725kg per tyre.
The maximum load for a 109 rated tyre is 1,030kg at 50psi.
725 divided by 1030 x 50 = 36psi.
This is IF all your info is correct, the ACTUAL weight is 2,900kg and the suspension IS load sharing. If not, all bets are off.
Cheers,
Peter
Radar said
06:36 PM Oct 21, 2024
FootlooseFizzers wrote:
235/75R 15 109T AGM 2900 does that help?
We have been running 36 psi in 2 set of tyres for the last 6 years, approx 100 000 kilometers.
Ps, for all the science you can find, I will still run similar tyre pressure as I have in the rear of my car.
Big Mal said
07:43 AM Oct 22, 2024
For over 50 years I have always set my tyres so that the HOT pressure is 4-6 psi above the cold pressure, it has proven to be a reliable method in that tyre wear and control have never been an issue.......... I have tried a calculator based on weight, size etc but found they seem to be too low.
I love that salesmans guide, first Ive heard of it........ Its total BS as a P rated tyre may only have a max pressure of 45psi (cold) while a 265 75R16 LT like on my rig has 80psi max, an XL may have just 50 psi so I think you will find the old analoge 4-6 psi above cold pressure works very well. Above 6 psi pressure is low and tyre will heat up, and wear on the edges plus too hot can cause premature failure, under 4 psi then its too high and in time you will find the centre will show more wear .... Use a TPMS or stop an hour down the road and check them .....................
The tyre construction, size, brand along with suspension all mean that there no hard fast psi setting for a vehicle ........... Tyre placards should be used as a starting point not a mandatory setting ..
Peter_n_Margaret said
08:32 AM Oct 22, 2024
Big Mal wrote:
For over 50 years I have always set my tyres so that the HOT pressure is 4-6 psi above the cold pressure,
My advice is do NOT use this method.
The best option is to use the manufacturer's pressure charts. The second best is to use the proportional method from the info printed on the tyre that I used here.
Cheers,
Peter
dogbox said
09:07 AM Oct 22, 2024
Big Mal wrote:
For over 50 years I have always set my tyres so that the HOT pressure is 4-6 psi above the cold pressure, it has proven to be a reliable method in that tyre wear and control have never been an issue.......... I have tried a calculator based on weight, size etc but found they seem to be too low.
I love that salesmans guide, first Ive heard of it........ Its total BS as a P rated tyre may only have a max pressure of 45psi (cold) while a 265 75R16 LT like on my rig has 80psi max, an XL may have just 50 psi so I think you will find the old analoge 4-6 psi above cold pressure works very well. Above 6 psi pressure is low and tyre will heat up, and wear on the edges plus too hot can cause premature failure, under 4 psi then its too high and in time you will find the centre will show more wear .... Use a TPMS or stop an hour down the road and check them .....................
The tyre construction, size, brand along with suspension all mean that there no hard fast psi setting for a vehicle ........... Tyre placards should be used as a starting point not a mandatory setting ..
old school thinking that still works. if the pressure goes up more than 5 psi from cold to hot put a bit more air in to reduce the flexing of tires, if pressure does not change reduce tire pressure till it does, SIMPLES
FootlooseFizzers said
01:49 PM Oct 22, 2024
Thanks all. I do have a TPMS installed on the van so I will run them at 36 PSI as per the calculation and monitor it with the TPMS
Peter_n_Margaret said
02:44 PM Oct 22, 2024
dogbox wrote:
old school thinking that still works. if the pressure goes up more than 5 psi from cold to hot put a bit more air in to reduce the flexing of tires, if pressure does not change reduce tire pressure till it does, SIMPLES
Totally unreliable. Depending on which side is in shade and which is in the sun can cause 5psi difference.
Different tyre construction types can cause large differences too.
Every tyre has manufacturer's pressure/load data available. Why not use it and be sure?
Cheers,
Peter
dogbox said
05:16 PM Oct 22, 2024
Peter_n_Margaret wrote:
dogbox wrote:
old school thinking that still works. if the pressure goes up more than 5 psi from cold to hot put a bit more air in to reduce the flexing of tires, if pressure does not change reduce tire pressure till it does, SIMPLES
Totally unreliable. Depending on which side is in shade and which is in the sun can cause 5psi difference.
Different tyre construction types can cause large differences too.
Every tyre has manufacturer's pressure/load data available. Why not use it and be sure?
Cheers,
Peter
i don't know about the "be sure" point still a few variables to be considered example as to how much weight is actually on each tire. just check once in a while should not take long to work out what works for your set up
watsea said
07:05 PM Oct 22, 2024
My van has independent suspension. I have read allow an extra 25% for your tyre loading if independent suspension.
At my vans weight equal to its ATM, and assuming 10% of the weight is on the tow ball.
So taking the tyres sidewall loading and doing the maths as per P & M for the weight on the tyres plus 25% gives a tyre pressure result that matches the tyre pressure given on the vans compliance plate. (Probably my apostrophes will not show in the post. Grrr.)
dogbox said
09:31 AM Oct 23, 2024
watsea wrote:
My van has independent suspension. I have read allow an extra 25% for your tyre loading if independent suspension.
At my vans weight equal to its ATM, and assuming 10% of the weight is on the tow ball.
So taking the tyres sidewall loading and doing the maths as per P & M for the weight on the tyres plus 25% gives a tyre pressure result that matches the tyre pressure given on the vans compliance plate. (Probably my apostrophes will not show in the post. Grrr.)
with independent suspension the load is not shared and may not be the same on each tire. so you may have to adjust accordingly
watsea said
10:43 AM Oct 23, 2024
dogbox wrote:
watsea wrote:
My van has independent suspension. I have read allow an extra 25% for your tyre loading if independent suspension.
At my vans weight equal to its ATM, and assuming 10% of the weight is on the tow ball.
So taking the tyres sidewall loading and doing the maths as per P & M for the weight on the tyres plus 25% gives a tyre pressure result that matches the tyre pressure given on the vans compliance plate. (Probably my apostrophes will not show in the post. Grrr.)
with independent suspension the load is not shared and may not be the same on each tire. so you may have to adjust accordingly
Yep. I think that is why there was the suggestion to allow a 25% increase in load to the wheels.
Even with load sharing suspension, is there any guarantee that the load on each side of a van is equal anyway? Some vans even have the respective side springs unbalanced because of different weights on each side.
Also, I think that the average caravan owner does not carry wheel scales so I would expect some tolerances can be accommodated by tyres near their correct pressure. Caravan load can vary during a trip by consumption of its food and water contents, or vice versa.
Radar said
01:47 PM Oct 23, 2024
watsea wrote:
dogbox wrote:
watsea wrote:
My van has independent suspension. I have read allow an extra 25% for your tyre loading if independent suspension.
with independent suspension the load is not shared and may not be the same on each tire. so you may have to adjust accordingly
Yep. I think that is why there was the suggestion to allow a 25% increase in load to the wheels.
Even with load sharing suspension, is there any guarantee that the load on each side of a van is equal anyway? Some vans even have the respective side springs unbalanced because of different weights on each side.
Also, I think that the average caravan owner does not carry wheel scales so I would expect some tolerances can be accommodated by tyres near their correct pressure. Caravan load can vary during a trip by consumption of its food and water contents, or vice versa.
Now that would be getting to pedantic.
As for carrying wheel scales, I do for when people start to ,,,,,me off enough with their talk, I reach into my air compressor bag and pull the scales out and say to them, "we have time let's weigh your caravan and remove the access weight" generally that quietens them down.
The difference between our 4 tyre's on our caravan was 30 kgs, 593 and 623 kgs
-- Edited by Radar on Wednesday 23rd of October 2024 01:55:19 PM
Big Mal said
04:14 PM Oct 23, 2024
Peter_n_Margaret wrote:
Big Mal wrote:
For over 50 years I have always set my tyres so that the HOT pressure is 4-6 psi above the cold pressure,
My advice is do NOT use this method.
The best option is to use the manufacturer's pressure charts. The second best is to use the proportional method from the info printed on the tyre that I used here.
Cheers,
Peter
The thing you overlooked is that the cold pressure originally is what is recommended either by some calculator or makers chart, not some whim and when they get hot the pressure will rise and no chart or calculation is available to say what that may be as depending on road surface, ambient temperature, speed and loading the variables are immense and thats why the basic rule of thumb used by many truckies, tyre people and manufacturers is 4-6 psi above cold, it gives you an indication of the correct pressure for those conditions which usually can mean an adjustment of 1-2 psi cold to get that sweet spot, if not checked then easier to have a blow hot with too hot a tyre .....
I run 265 60R18 with123LR on tug and 265 75R16 again 123LR on van, just changed the rubber on tug and today is day one of a tour and new tyres at 45rear same as previous which would go to 50 hot on a 30 deg day, these new ones are running at 51 may need to p the cold to 56 to get the sweet 5 psi, see how they go tomorrow....... and while 10c ambient only adds/subtracts 1 psi to the cold pressure, the road temp can vary from 40 to 70 deg or more ... If I start a run on the coast I may adjust the pressure as I get further west, hotter roads means hotter tyres ..............
A look at many sites that explain the topic in detail confirms my description is not some old drivers myth, lots of outback and local travel from trucks to cars and blows outs were never from incorrect pressure ..
-- Edited by Big Mal on Wednesday 23rd of October 2024 04:17:54 PM
-- Edited by Big Mal on Thursday 24th of October 2024 06:40:13 AM
Peter_n_Margaret said
08:40 AM Oct 24, 2024
The 2 pressure calculation links that you quoted are copied from each other and are identical to the one I used earlier in this thread which uses the tyre manufacturer's data. The temperature increase method does not come from any reputable tyre manufacturer's data because it is totally unreliable. I repeat my advice to use the tyre manufacturer's data for the tyre you use (from that moulded into the tyre, or the charts) along with ACTUAL weight data (with an allowance if you have non-load sharing suspension) and ignore other advise. Cheers, Peter
-- Edited by Peter_n_Margaret on Thursday 24th of October 2024 10:27:19 AM
Big Mal said
03:20 PM Oct 24, 2024
The links were added to just be an easier explanation, but it seems you know more and my old Instructors were totally wrong and as someone fully capable of tyre maintenance and repair in a workshop or out in the field I'll bow out of the discussion and leave you with your own ideas ...............
Peter_n_Margaret said
06:38 PM Oct 24, 2024
If they know more about tyres than Michelin, they must be pretty special.
Cheers,
Peter
Ian G said
06:56 PM Oct 24, 2024
While on the subject, what is a safe pressure to reduce to when moving from the tar to corrugated dirt. In my case I have a single axle van with AGM of 2000 kg. On the tar I run mine at 64 psi, tyres are 116, rated max 80 psi. So what would be a safe minimum PSI I could reduce it to on the dirt ? Ian
Peter_n_Margaret said
07:41 PM Oct 24, 2024
Ian G wrote:
While on the subject, what is a safe pressure to reduce to when moving from the tar to corrugated dirt. In my case I have a single axle van with AGM of 2000 kg. On the tar I run mine at 64 psi, tyres are 116, rated max 80 psi. So what would be a safe minimum PSI I could reduce it to on the dirt ? Ian
Whatever pressure provides a reasonable ride Ian.
The important part of the decision is that the speed you drive at is closely related to the pressure and load on the tyres. Drive a little too fast for a bit too long and the tyres will overheat internally and will fail. This may not be apparent for days or months later when the damage causes the tyre to fail.
Low pressure and driving much slower will always give the best result.
I prepared this chart for our use for this very question. It was derived directly from Michelin published data for the specific tyre we use, but generally this type of info is unfortunately difficult to find from a credible source.
Sorry it is on its side.
This is what the tyres look like.
We recently drove the southern half of the Canning Stock Route. We typically drove the 1050km at between 20 and a maximum of 30kmph, just above the yellow line. We were with friends who had exactly the same tyres but were lighter than us. Neither of us had any tyre problems.
I saved a couple of 1 minute video clips from the dash cam to demonstrate speed differences over corrugations.
Thanks Peter, so you are dropping approx 10 psi, in my case that would be around 50 psi which is higher than most drive on the tar.
Peter_n_Margaret said
09:48 AM Oct 25, 2024
Ian G wrote:
Thanks Peter, so you are dropping approx 10 psi, in my case that would be around 50 psi which is higher than most drive on the tar.
No. My pressures on the highway are around 45/65psi and for the Canning we were around 20/30psi, so less than half of normal pressures. We weigh around 6.5T.
What pressure you choose is dependent on the speed you are prepared to drive at and the load carried.
I trust and use the Michelin data implicitly and have done so for 20 years. I do not trust any other source, despite every 4WDer or truckie having a better idea.
Cheers,
Peter
Big Mal said
08:15 AM Oct 28, 2024
I don't know what your arguing about really, I agree with your chart as it gives the same cold pressures that I base things using the formula on a calculator, what Im saying is when the tyres get HOT the pressure shown will show if its under or over inflated for the terrain your travelling , conditions will make changes neccessary but only minor changes are usually needed , like Snowys in winter have a different pressure to Winton in summer and the 4psi bit gives a good indicator ............... Cooper Tyres explain the why www.coopertires.com.au/tyres-explained/tyre-pressure-guide/
dogbox said
02:50 PM Oct 28, 2024
Big Mal wrote:
I don't know what your arguing about really, I agree with your chart as it gives the same cold pressures that I base things using the formula on a calculator, what Im saying is when the tyres get HOT the pressure shown will show if its under or over inflated for the terrain your travelling , conditions will make changes neccessary but only minor changes are usually needed , like Snowys in winter have a different pressure to Winton in summer and the 4psi bit gives a good indicator ............... Cooper Tyres explain the why www.coopertires.com.au/tyres-explained/tyre-pressure-guide/
x2
jegog said
12:57 AM Nov 16, 2024
As a confidence boost I have an remote thermometer from Bunnings and check tyre, wheel hub and brake shoe/disk when stopped.
Hi All, Im doubting the science!! I have calculated the supposed correct tyre pressure for my 21' twin axle van and it comes out at 36PSI. Then Isee one caravan sales mob say things like if y our trvelling at approx 100 kph on avarage then your pressure should be 50 psi (or half of whatever speed you travel at ie dirt road and sand). I thought that was a bit hit and miss. I then read whre others are running at between 40 and 50 psi. My max psi on the side of the tyre is 50 psi. IAt 36 psi my tyres look "flat". Hence ...doubting the sciense.
Without that information it is a guess.
Get the correct information first.
Cheers,
Peter
The maximum load for a 109 rated tyre is 1,030kg at 50psi.
725 divided by 1030 x 50 = 36psi.
This is IF all your info is correct, the ACTUAL weight is 2,900kg and the suspension IS load sharing. If not, all bets are off.
Cheers,
Peter
We have been running 36 psi in 2 set of tyres for the last 6 years, approx 100 000 kilometers.
Ps, for all the science you can find, I will still run similar tyre pressure as I have in the rear of my car.
I love that salesmans guide, first Ive heard of it........ Its total BS as a P rated tyre may only have a max pressure of 45psi (cold) while a 265 75R16 LT like on my rig has 80psi max, an XL may have just 50 psi so I think you will find the old analoge 4-6 psi above cold pressure works very well. Above 6 psi pressure is low and tyre will heat up, and wear on the edges plus too hot can cause premature failure, under 4 psi then its too high and in time you will find the centre will show more wear .... Use a TPMS or stop an hour down the road and check them .....................
The tyre construction, size, brand along with suspension all mean that there no hard fast psi setting for a vehicle ........... Tyre placards should be used as a starting point not a mandatory setting ..
My advice is do NOT use this method.
The best option is to use the manufacturer's pressure charts. The second best is to use the proportional method from the info printed on the tyre that I used here.
Cheers,
Peter
old school thinking that still works. if the pressure goes up more than 5 psi from cold to hot put a bit more air in to reduce the flexing of tires, if pressure does not change reduce tire pressure till it does, SIMPLES
Totally unreliable. Depending on which side is in shade and which is in the sun can cause 5psi difference.
Different tyre construction types can cause large differences too.
Every tyre has manufacturer's pressure/load data available. Why not use it and be sure?
Cheers,
Peter
i don't know about the "be sure" point still a few variables to be considered example as to how much weight is actually on each tire. just check once in a while should not take long to work out what works for your set up
At my vans weight equal to its ATM, and assuming 10% of the weight is on the tow ball.
So taking the tyres sidewall loading and doing the maths as per P & M for the weight on the tyres plus 25% gives a tyre pressure result that matches the tyre pressure given on the vans compliance plate. (Probably my apostrophes will not show in the post. Grrr.)
with independent suspension the load is not shared and may not be the same on each tire. so you may have to adjust accordingly
Yep. I think that is why there was the suggestion to allow a 25% increase in load to the wheels.
Even with load sharing suspension, is there any guarantee that the load on each side of a van is equal anyway? Some vans even have the respective side springs unbalanced because of different weights on each side.
Also, I think that the average caravan owner does not carry wheel scales so I would expect some tolerances can be accommodated by tyres near their correct pressure. Caravan load can vary during a trip by consumption of its food and water contents, or vice versa.
Now that would be getting to pedantic.
As for carrying wheel scales, I do for when people start to ,,,,,me off enough with their talk, I reach into my air compressor bag and pull the scales out and say to them, "we have time let's weigh your caravan and remove the access weight" generally that quietens them down.
The difference between our 4 tyre's on our caravan was 30 kgs, 593 and 623 kgs
-- Edited by Radar on Wednesday 23rd of October 2024 01:55:19 PM
The thing you overlooked is that the cold pressure originally is what is recommended either by some calculator or makers chart, not some whim and when they get hot the pressure will rise and no chart or calculation is available to say what that may be as depending on road surface, ambient temperature, speed and loading the variables are immense and thats why the basic rule of thumb used by many truckies, tyre people and manufacturers is 4-6 psi above cold, it gives you an indication of the correct pressure for those conditions which usually can mean an adjustment of 1-2 psi cold to get that sweet spot, if not checked then easier to have a blow hot with too hot a tyre .....
I run 265 60R18 with123LR on tug and 265 75R16 again 123LR on van, just changed the rubber on tug and today is day one of a tour and new tyres at 45rear same as previous which would go to 50 hot on a 30 deg day, these new ones are running at 51 may need to p the cold to 56 to get the sweet 5 psi, see how they go tomorrow....... and while 10c ambient only adds/subtracts 1 psi to the cold pressure, the road temp can vary from 40 to 70 deg or more ... If I start a run on the coast I may adjust the pressure as I get further west, hotter roads means hotter tyres ..............
A look at many sites that explain the topic in detail confirms my description is not some old drivers myth, lots of outback and local travel from trucks to cars and blows outs were never from incorrect pressure ..
https://rvdaily.com.au/how-to-calculate-the-correct-tyre-pressure-for-your-caravan/
https://www.windsorrvs.com.au/blog/complete-caravan-tyre-pressurechart-maintenance-guide
-- Edited by Big Mal on Wednesday 23rd of October 2024 04:17:54 PM
-- Edited by Big Mal on Thursday 24th of October 2024 06:40:13 AM
The 2 pressure calculation links that you quoted are copied from each other and are identical to the one I used earlier in this thread which uses the tyre manufacturer's data.
The temperature increase method does not come from any reputable tyre manufacturer's data because it is totally unreliable.
I repeat my advice to use the tyre manufacturer's data for the tyre you use (from that moulded into the tyre, or the charts) along with ACTUAL weight data (with an allowance if you have non-load sharing suspension) and ignore other advise.
Cheers,
Peter
-- Edited by Peter_n_Margaret on Thursday 24th of October 2024 10:27:19 AM
Cheers,
Peter
Whatever pressure provides a reasonable ride Ian.
The important part of the decision is that the speed you drive at is closely related to the pressure and load on the tyres. Drive a little too fast for a bit too long and the tyres will overheat internally and will fail. This may not be apparent for days or months later when the damage causes the tyre to fail.
Low pressure and driving much slower will always give the best result.
I prepared this chart for our use for this very question. It was derived directly from Michelin published data for the specific tyre we use, but generally this type of info is unfortunately difficult to find from a credible source.
Sorry it is on its side.
This is what the tyres look like.
We recently drove the southern half of the Canning Stock Route. We typically drove the 1050km at between 20 and a maximum of 30kmph, just above the yellow line. We were with friends who had exactly the same tyres but were lighter than us. Neither of us had any tyre problems.
I saved a couple of 1 minute video clips from the dash cam to demonstrate speed differences over corrugations.
https://youtu.be/m7ud9EdMh20
https://youtu.be/jgmRkbM4BJ4
Cheers,
Peter
No. My pressures on the highway are around 45/65psi and for the Canning we were around 20/30psi, so less than half of normal pressures. We weigh around 6.5T.
What pressure you choose is dependent on the speed you are prepared to drive at and the load carried.
I trust and use the Michelin data implicitly and have done so for 20 years. I do not trust any other source, despite every 4WDer or truckie having a better idea.
Cheers,
Peter
x2