Tyres

[PaulT]

I previously posted regarding a tyre puncture. Another point has arisen and I'd be grateful for some advice.

The tyres on the caravan are less than a year old and are Michelin Energy Saver's as fitted by Bailey. The specification is 185/65 R14 86 T. They appear to be normal car tyres. I seem to remember when I bought tyres for my previous single axle 'van that they had to be (I think) 92 or 94T. Was this because the SA tyres ran at much higher pressure?

It appears that Michelin no longer make the Energy Saver but have replaced it with the Energy Saver Plus which has a different tread pattern. Is it ok to fit this tyre with the 3 other, original, tyres? The spare tyre, which I had to use, was a completely different make and tread design.

Thanks for any advice received.

 

[Damian]

Tread pattern has no significance.
It is the makeup of the tyre that is important.
They must be of the same construction on the same axle, ie Radial or Crossply.
As long as the loading rating is right for the van then the tyre is OK to go.

 

[Abinitio1989]

They appear to be normal car tyres. I seem to remember when I bought tyres for my previous single axle 'van that they had to be (I think) 92 or 94T. Was this because the SA tyres ran at much higher pressure?

TA generally require a lower load rating as they are not carrying as much load-per-tyre as in a SA.

A SA at 1500kg is about ~700kg per tyre (some on the hitch) whereas a TA at 2100kg is only about ~500kg per tyre

 

[Anseo]

TA generally require a lower load rating as they are not carrying as much load-per-tyre as in a SA.

A SA at 1500kg is about ~700kg per tyre (some on the hitch) whereas a TA at 2100kg is only about ~500kg per tyre

What is an SA and a TA please,

 

[WoodlandsCamper]

SA = Single Axle
TA = Twin Axle

 

[Anseo]

SA = Single Axle
TA = Twin Axle

Thank you.

 

[ProfJohnL]

They appear to be normal car tyres. I seem to remember when I bought tyres for my previous single axle 'van that they had to be (I think) 92 or 94T. Was this because the SA tyres ran at much higher pressure?

TA generally require a lower load rating as they are not carrying as much load-per-tyre as in a SA.

A SA at 1500kg is about ~700kg per tyre (some on the hitch) whereas a TA at 2100kg is only about ~500kg per tyre

Hello Abinitio1989,

Your assumption about the load rating for the tyres on a TA is I am afraid flawed. If everything was perfectly level (which it never is) then yes the load on each tyre would be the towed load divided by four. Sadly outfits are never level for long, and in fact during various towing maneuvers the loads on the tyres can change dramatically, and because the the twin axles do not load share there is often a considerable differential between front and rear axle loads.

Be guided by the caravan manufacturers specifications for tyres, as they should have done all the necessary calculations and testing to validate the size and rating of tyres to be used.

 

[WoodlandsCamper]

SA = Single Axle
TA = Twin Axle

Thank you.

Just a tip - you can click on the [THANK YOU] button rather than have to type the word.

 

[Abinitio1989]

Prof

I was merely demonstrating why SA caravans GENERALLY (!) require higher load ratings on the tyres in answer to the OP's post

You're welcome ;-)

 

[ProfJohnL]

Prof

I was merely demonstrating why SA caravans GENERALLY (!) require higher load ratings on the tyres in answer to the OP's post

You're welcome ;-)

Hello Abinitio1989

The load rating for the tyres has far more to do with the weight of the caravan rather than the number of axles. Generally (but not always) TA caravan tend to be bigger and heavier than SA caravan, so are likely to have a higher load rating.

Unlike trailers with a load sharing bogie arrangement (like many HGV axle systems) the axles and in fact each wheel on a TA caravan is independently sprung, and the load applied is not automatically shared between the axles. Consequently and as I suggested above there are plenty of occasions when the load on a TA caravan transfers between axles, so at time any one axle may be carrying the bulk of the load, so tyre ratings have to be chosen to include these unbalanced loadings.

If this were not so, then when a TA does suffer a tyre deflation the load on the same side will be passed entirely to the remaining inflated tyre. If the tyre only rated to carry 25% of the towed load, when the additional load is imposed the tyre would be grossly overload and itself very susceptible to catastrophic failure.

Each of TA's tyres should be rated to carry at least 50% of the caravans towed weight..

 

[Dustydog]

Prof
We have been here before. Remember the axle loading on my TA supplied by Al-ko pics are on here.
Strangely enough the tyres on heavy TA and SA are similarly rated but the pressures are light years apart eg TA 29psi. SA 65psi .

 

[saint-spoon]

SA = Single Axle
TA = Twin Axle

Thank you.

Just a tip - you can click on the [THANK YOU] button rather than have to type the word.

Thanks for the tip WC

 

[ProfJohnL]

Prof
We have been here before. Remember the axle loading on my TA supplied by Al-ko pics are on here.
Strangely enough the tyres on heavy TA and SA are similarly rated but the pressures are light years apart eg TA 29psi. SA 65psi .

Yes Dusty,
But not everyone on this thread has read the thread you refer too, but thanks for the input. The pressures can be lower, but the rating must be correct for the load.

 

[chrisn7]

Last line - but they aren't. Typically the load index on a TA van is 25% of the MTPLM plus 15-20% of that figure. For example, mine have a load index equivalent to 575 Kg for a 1900 kg van.

Using Tyrepal, I don't believe there is a significant variation in axle loading when running in a stable situation such as a motorway. All 4 tyres are set at equal pressures with a digital gauge, and then, once up to running temp, all 4 maintain equal and similar pressures within 1psi.
This is a van running 14" wheels. Similar parameters with my old van, which had 13" wheels, towed by the same car with self levelling, showed a clear distinction on the Tyrepal, with higher temps and pressures on the rear axle. This would appear to have been due to the caravan being fractionally lower, causing the rear tyres to be more heavily loaded.

 

[Hutch]

I have just looked up Coachmans 2016 tech specs and the single axle Vip runs on 195/70 14 104 @ 65psi
The laser twin axle runs on 185/70 14 88 @35 psi. .
Just for info.

 

[ProfJohnL]

Last line - but they aren't. Typically the load index on a TA van is 25% of the MTPLM plus 15-20% of that figure. For example, mine have a load index equivalent to 575 Kg for a 1900 kg van.

Using Tyrepal, I don't believe there is a significant variation in axle loading when running in a stable situation such as a motorway. All 4 tyres are set at equal pressures with a digital gauge, and then, once up to running temp, all 4 maintain equal and similar pressures within 1psi.
This is a van running 14" wheels. Similar parameters with my old van, which had 13" wheels, towed by the same car with self levelling, showed a clear distinction on the Tyrepal, with higher temps and pressures on the rear axle. This would appear to have been due to the caravan being fractionally lower, causing the rear tyres to be more heavily loaded.

Hello Chrisn,

I wouldn't expect to significantly different pressures between the axles, as tyres attempt to be a constant pressure device. If you significantly increase the load on a tyre, the tyre changes shape and puts more surface in contact with the ground. with out increasing the internal pressure by the same degree. It also depends on the size of the load change and the speed at which it occurs and is it repetitive or single strike loading.

Even at town speeds, the changes in loads on tyres are quite dramatic, consider cornering, or going over a pothole or speed calming hump. If you have cycled on a bike without suspension, can you recall how much vibration is transmitted to your arms, All the forces involved have had to come up through the tyres, so there simply isn't a steady state condition except when you are stationary.

Its highly unlikely that your tyre monitoring system will be giving instantaneous peak pressure and temperature readings as the changes would be occurring so quickly as to be unreadable on a digital numeric. I guess the system will be producing a rolling average which is resolved over several seconds.

 

[JonnyG]

Prof
We have been here before. Remember the axle loading on my TA supplied by Al-ko pics are on here.
Strangely enough the tyres on heavy TA and SA are similarly rated but the pressures are light years apart eg TA 29psi. SA 65psi .

Yes Dusty,
But not everyone on this thread has read the thread you refer too, but thanks for the input. The pressures can be lower, but the rating must be correct for the load.

..

hi .prof.i think you've got this wrong.....what dusty has stated actually debunks what you stated earlier and indeed are repeating here.. the load index is the amount a tyre car support at near as dam it the tyres MAX psi. if the psi is half its max psi then the tyre will only be able to support ROUGHLY half of its index load rating.... .. so if a tyre can support 600kg and its max psi is 60 psi.... then if you lower the pressure to 30 psi it would only be safe to 300kg.. ..as you can see from dusty's example same index loaded tyres and approx same weight caravan the T/A has its tyres at half the psi of a single axle caravan of the same weight. because the tyres share the weight 4 tyres as opposed to 2..therefore for whatever reason and assuming dusty example is correct and i see no reason for it not to be, a T/A that has a puncture in dusty's example could not safely support a fully loaded T/A on 3 tyres..mind i do find that strange .. ps .sorry this info applies to commercial / heavy duty tyres that inflate above 50psi .... ... Normal car tyres are different max load index is rated at depending on which system is used 35/42psi..

 

[ProfJohnL]

Hello Jonny,

I don't think it debunks what I stated, The point is that if a TA looses a tyre then it does put extra stress onto the three remaining tyres and especially the one tyre on the same side as the failure.If manufacturer wants the outfit to remain manageable, then they should specify tyres that can handle this scenario, other wise it will be a chain reaction that if you loose one tyre you must consider the remaining tyre on the same side to be damaged because of over loading and thus at high risk failure in quick succession.

However tyre manufacturers must know that if a tyre is rated to say 600kg, then in its service life it will experience shock loads (from normal travel across roads and their defects) well in excess of the static rated load, which is why you don't tend to see large numbers of cascade tyre failures.

 

[JonnyG]

hi prof this is how i understand it and many moons ago it did apply. a normal car tyre can carry its max index load at around 36 psi /42psi . there are several so called standards and i'm too old to start trying to work which is which given how the internet is full of mis information.. but i believe the euro one is 36psi.. under this psi figure the load index decreases as a percentage of the psi drop. ie ten percentage drop in psi equals 10 percent drop in load carrying ability.. given dusty's example i assume the tyre in question are commercial given their high psi rating of 65psi ... and my data on commercial tyres is max psi equals max load index... but there,s more,when i was gagged, ok banned a year or two back on a similar topic[ my fault,] something of use did come out of it,what came to light was some car tyres actually state load index or max Ibs at a psi rating normally max psi .i believe this is for the US market never the less it shows a correlation between weight carrying ability and psi.. obviously one could say, but were is it ? If we again use dusty's example and assume the tyres are commercial and therefore the correlation is max load index equals max psi , say max psi is 65psi.. those same tyres when pumped to only 29psi would have lost a atleast 50% of there load carrying ability... on the other hand if they somehow where euro rated tyres or whatever the tech term is.. and max load index was at 36psi then 29psi would only be a loss of about 20% of the max rated load index big difference right..now i do not know for sure which applies, if either does, but it does show what a minefield load index and tyre psi is..worse having looked at many caravan forums that give charts that are supposed to be of help on how one works out your caravan tyres. Psi. versus load index where it seems they use and assume max load [ load index] is achieved at MAX psi as the starting point for there equations without ever explaining where this fact comes from if indeed it isn't just an assumption made from what little info on this topic is available on a tyre...As these topics and the advise given do pop up on a regular basis it would be surely useful if somebody could shed some proper authority on the correlation between what load a tyre can carry at a given psi and what psi is needed to achieve the tyre load index.. it once was 36psi but today i have no idea what is what..

 

[Hutch]

I found this bit of information on another website. It might help us

The other bit of information we need to know from the tyre is the Maximum Tyre Inflation Pressure and Load. This is usually located on the side wall below the tyre data in smaller text. It will say something like “MAXIMUM LOAD 720 Kg’s MAXIMUM PRESSURE 52 PSI” it might have the pressure given in Bar.

Once you have the MTPLM of your caravan and the maximum pressure for your tyre we can now work out the correct tyre pressure.

Take the figure for the maximum pressure for the tyre and divide it by the maximum weight from the table above. Now multiply this figure by the actual load you are going to put on the tyre – so half the MTPLM for a single axle caravan or a quarter of the MTPLM for a twin axle caravan. The resulting figure should be the correct inflation pressure for each tyre.

Lets look at an example for a single axle caravan:

Tyre Details: 205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1200 Kg’s

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1200 / 2) = 53.7 PSI (inflation factor times half the weight of the caravan)

So the correct tyre pressure is 54 PSI

Working out for a twin axle caravan:

Tyre Details: 205 / 65 R 15 94 H – Maximum Inflation Pressure 60 PSI

Weight of caravan: 1800 Kg’s

60 / 670 = 0.08955 (Max Pressure for tyre divided by load rating)

0.08955 x (1800 / 4) = 40.29 PSI (inflation factor times one-quarter of the weight of the caravan)

So the correct tyre pressure is 41 PSI

 

[JonnyG]

hi EH52ARH, Yep i have come across that and others before which is where the confusion comes from , If there is no such thing as a proper standard ie all tyres need to comply to a fixed X psi that can carry your index rating then you have tyres that carrying there max load at a huge differing psi, ranging from 40 psi to 60 plus psi being needed to meet those tyres max load index.. it cannot be so. think about it you standard car Tyre state 35 psi in normal use and the max tyre pressure is say 45psi and you change to another brand which just so happens to have a with higher max psi for its tyres of say 65psi you still use the cars handbook tyre pressure recommendations on those new tyres given the assumption used in your copied post ie max load index is equal to max tyre pressure you would have under inflated tyres on your car.. no there has to be a standard there always is and that makes the info from that and many more websites iffy, if not then all car hadbooks tyre pressures can onluy apply to a tyre with the same index loading and the same max tyre psi...

 

[Hutch]

Johnny G, understand where your coming from,
Just been out to the car and the max load on each tyre is 875 kg @ 51 psi, working through the calculations with max weight of the car at 2500 kg. Workw out at 36.5 psi. Hyundai state run at 33 psi, which give an increase of 3 psi for running temp increase this , would be correct.
But it states on the tyres thes codes are for Australia, canada and USA. Only.
So as you say must be something some where else.

 

[ProfJohnL]

Hi Jonny and Hutch,

I agree with both of your posts, the situation is confusing regards the methods of establishing normal running pressures, but I was looking at the excess load caused by a tyre failure on a TA, so is not 'normal' running.

In the context of pressure changes caused by load changes in tyres, then remember that pressure in a closed vessel is determined by the mass of gas and its temperature. If you ignore temperature changes for a moment, if you compress the volume to half its original size, then the pressure will double.

But when you look at a tyre on any vehicle, the bulk of the tyre's circumference is unaffected by the contact patch, and if you were to double the load on the tyre, the contact patch will grow but it will still be much less than the total circumference, so the change in volume (and thus pressure) is minimal and thus not proportional to the change in load.

If fact pressure change in is far smaller in bigger diameter tyres than smaller tyres.

 

[Hutch]

We must remeber.....that when a tyre on a twin axle, deflates, the load is not totally removed from the axle.
The tyre deflates but the load goes onto the deflated wheel. Through the deflated tyre and rim. . Same load but on a deflated wheel. . Or am I agaain missing something.

 

[Dustydog]

Hi Jonny and Hutch,

I agree with both of your posts, the situation is confusing regards the methods of establishing normal running pressures, but I was looking at the excess load caused by a tyre failure on a TA, so is not 'normal' running.

In the context of pressure changes caused by load changes in tyres, then remember that pressure in a closed vessel is determined by the mass of gas and its temperature. If you ignore temperature changes for a moment, if you compress the volume to half its original size, then the pressure will double.
Agreed . Boyles Law. P1V1 =P2V2

But when you look at a tyre on any vehicle, the bulk of the tyre's circumference is unaffected by the contact patch, and if you were to double the load on the tyre, the contact patch will grow but it will still be much less than the total circumference, so the change in volume (and thus pressure) is minimal and thus not proportional to the change in load .
Not sure ? The tyre is a pressure vessel of the same volume , allowing for marginal expansion ,why would Boyles Law not apply
If fact pressure change in is far smaller in bigger diameter tyres than smaller tyres.

Doesn't that totally contradict Boyles Law. I assume like you Prof the temperature is a constant.

It is worth mentioning that the Al-ko axle loading on my 1700ks TA is 1000kgs per axle. Is this enough if one axle has to carry the entire load. I mention this for the interest of those who have not read the earlier thread on this subject. Prof, The other factor worth discussion the that the footprint of the tyre does not remain constant. Low pressure will give a flatter or even concave print. High pressure a convex more rounded print. Neither extremities is ideal. At the correct manufacturers pressure there will be maximum surface contact. ][/color]