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Topic: Tyre pressure - confused!!!!!!

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Onewheelonmywagon

14/5/2015 at 12:19pm
Location: W Midlands
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Using the 29 psi figure in the manual assumes that exactly the same spec tyres are fitted to the caravan as when it rolled out of the factory.

Opensauce

14/5/2015 at 12:34pm
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It does not matter about the spec of the tyres, the pressure you run at remains the same. If for example you fit 8ply commercial van tyres that allow much higher pressure than equivalent car tyres you only run them at that higher pressure if vehicle requires it.

All you need is tyres of sufficient load rating for your caravan. If those tyres exceed that load rating by a large margin it does not mean you have to run them at the sort of pressure required if they were fitted to a vehicle that requires tyres of that load rating.

Its often incorrectly posted on here that caravans should be fitted with commercial van tyres when car tyres are ok provided they are of sufficient load rating.

Your tyre size is available in car tyre at 86 load rating, 530kg load per tyre so more than adequate for any twin axle tourer, it matters not if you over spec tyre load rating as you have done, the pressure recommended for caravan remains the same.

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trog100

14/5/2015 at 6:43pm
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i have a boat trailer.. with the boat on the tyres get inflated to 80 psi.. without the boat 15 psi is better..

at 80 psi the trailer without the boat on literally does bounce.. like two feet in the air when it hits a pot hole.. he he

but the common idea that over inflating tyres is a good thing will not be shifted.. many caravans are running around with tyres at the max pressure simply because that is what it says on the side of the tyre..

on the other hand we have people saying the vans are not built well enough.. he he

they are not which is why i dont make my tyres (and suspension) any harder than it has to be.. :)

trog

jadatis

14/5/2015 at 7:22pm
Location: Holland
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Quote: Originally posted by acampingwewillgo on 13/5/2015
Thanks Jadatis. I'll go for 35 psi.

And then after this post of yours , still some say its to much and bumping can occur.

In the beginning of my tyrepressure story I already determined the load%.
This is the percentage what the real load on a tire is of the load the pressure is calculated for.
Best is to explain with an example.

Say you calculate pressure for 1000 ( lbs, kg or whatever)

Then if realy on the tyre 850 = Load% 85% then People and Animal ( for instance your horses) begin to experiënce discomfort by bouncing.

If in real 800 = L% 80% then caravans go that much bumping , that screws can come loose from woodwork , and real glasses can break.

In real it has to do with the deflection, and those L% must be seen for the pressure calculated for 160km/99m/h.
I also use this idea in my spreadsheet.

Over 1000 would give damage to tyre if you drive 160km/99m/h
Rule of tumb is for every 10km/6.5m/h lower then 99m, damage begins when 3% higher load on tire.
For N speedrated ( up to 140km/h) 2x 3%=6% so at 1060 damage begins when realy riding 140km/87m/h.

And that is what I used with that lowering higher speed and highening for lower speed I did .

So with 35 filled you can even haver more then a combined overload and unequal load the my mentioned 179kg before bumping begins.

Planning to review my calculator spreadsheet again.
Then you also have to give speedcode of tyre and the maximum speed you sertainly wont go over for a minute.
Then the spreadsheet calculates automatically for 160km/99m/h though no person drives that fast with caravan behind, and going 11% higher so L% 90% makes it possible to have some weightdifference and underload before bumping begins.
At lower pressure as the 35 psi for your 120km/h even more weightdifference and underload possible before bumping.

so if you want to have advice for your horsetrailer give the needed data ( you know now wich) and I will give advice.
Weight of horses is pretty accurately estimated I think and empty weight of trailer given somewhere.

-------------
signed in for tyre-pressure

CaerYnArfon

14/5/2015 at 7:33pm
Location: St Jory De Chalais
Outfit: XC90 Fleetwood 560 4

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Go with the manual...

navver

14/5/2015 at 10:27pm
Location: West country
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Actually I think the manual is spot on. Deduct the nose weight from the load also a smidge for not being 100% of MTPLM and 29 is about right.

glittering-prize

14/5/2015 at 10:57pm
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my twin axle is a 2004 lunar and that is 36 psi on all 4 tyres.If you need new tyres just get transit van tyres.I got all 4 fitted on storage site for 160 quid which i think is great.

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Onewheelonmywagon

15/5/2015 at 11:56am
Location: W Midlands
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Quote: Originally posted by Opensauce on 14/5/2015It does not matter about the spec of the tyres, the pressure you run at remains the same......

I wonder then how the caravan manufacturer arrived at a figure of 29 psi?
I don't think it was by using a random number generator. I suspect they used the MTPLM of the van and CALCULATED the required pressure based on the spec of the tyres they were fitting as standard at the time.

Here's a method from a bloke that's been a caravan engineer for 25 years. You might recognise the formula.

Adrian Tyldesley

When trying to work out pressures ignore the spec of the tyres at your peril.

trog100

16/5/2015 at 8:35am
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the max pressure is marked on the tyre.. the max loading is marked on the tyre..

if the actual loading on the tyre is only half the max loading the running pressure would be half the max pressure.. simple enough to work out..

also if the tyres are being run too soft the increase in running temperature will be higher..

also the ideal the starting pressure will vary depending on the ambient temperature.. on a freezing cold day the initial tyre pressure will read lower than on a scorching hot day..

personally i think the entire caravan tyre blow out thing is a myth.. they dont blow out any more than a car tyre does..

what happens is simple.. a normal puncture occurs.. the caravan tyres loses pressure.. unlike with a car the driver is totally unaware of this..

after xxxx amount of time the caravan tyre gets way too hot and the tyre de-laminates and flies apart causing what appears to be a tyre blow out..

caravan tyres do not blow out they simple get run too flat for too long and eventually de-laminate and fly apart..

people might think they would be aware of a van tyre going down.. in reality they are not.. or not until the inevitable de-lamination occurs.. they are then.. he he..

if i am correct all the pictures of exploding caravan tyres are false.. they are actually pictures of what happens when a tyre is run flat for too long.. pictures of a de-laminated tyre..

no matter how new or good your tyres are a simple puncture will cause this to happen.. simply because the driver is totally unaware of what is going on with the tyres on a caravan until its too late..

i dont check my tyre pressure whilst on the move.. i do put my hand on them when i stop and feel how hot they are.. warm is about right.. and they should all feel the same..

trog

Post last edited on 16/05/2015 08:44:17

VangoMan02

16/5/2015 at 9:13am
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Does anyone else feel we are over complicating a simple thing like tyre pressure?

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clbewi

16/5/2015 at 9:44am
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Quote: Originally posted by VangoMan02 on 16/5/2015
Does anyone else feel we are over complicating a simple thing like tyre pressure?

Couldn't agree more!
I don't know what's happening to this forum, a simple question gets asked and it turns into an argument. There people on here often go off on a tangent and it ends up in a massive argument.. It was just a question about tyre pressures.

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Claire and Colin

arc systems

16/5/2015 at 10:21am
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185/65R14C and 93 load index, from that tyres single axle caravan pressure chart the max load is 650kg at 54psi, that makes it a 6ply commercial tyre. The lowest pressure I've got from the chart is 36psi and thats for 940kg, per pair of tyres. ie single axle.

So using this lower figure as a starting point and calculating more accurately from there, 32psi is correct for 739kg single axle or 1678kg on a twin axle.

On a technical note, pressure does not support a load, it is a volume of air. Required pressure then is the result of getting the required larger volume of air into the available space inside any given tyre.
For a given load then, it is now obvious that volume of air is the constant but pressure will vary depending on tyre size.
It is not then safe to assume that handbook pressures are correct, unless, the current tyres are the same size and TYPE as original.

Onewheelonmywagon

16/5/2015 at 11:03am
Location: W Midlands
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If you believe tyre pressures are simple, and don't want to see a debate about it, then there are plenty of other topics on the forum to follow.

VangoMan02

16/5/2015 at 11:21am
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Quote: Originally posted by Onewheelonmywagon on 16/5/2015If you believe tyre pressures are simple, and don't want to see a debate about it, then there are plenty of other topics on the forum to follow.

Having been caravanning for 40 years, I do think tyre pressures are one of the easiest things to work out and as a backup the handbook tells you.

As you say it is a debate so I am therefore entitled ..as you are to give an opinion, there are other opinions to view if you don't like mine.

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trog100

16/5/2015 at 2:37pm
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just to add some more.. he he he

its all related to the cross section of the tyre.. the greater the cross section the less pressure is required to support any given weight..

the greater the cross section the more square inches are supporting the tyre.. if the tyre is run too hard it makes the pretty basic caravan suspension even harder than it needs to be.. this isnt good for the flimsy caravan body structure.. it also puts less rubber on the road which is not good for grip and the tyre will wear out in the middle and not evenly over its tread width..

if the tyre is run too soft it will flex too much and generate more heat.. it will also run concave and wear out on the outside edges and again not evenly over the tread width..

there is no absolutely correct tyre pressure.. for slower speed bumpy roads a bit softer is better.. for high speed smooth roads a bit harder is better..

the recommended figure is just an average not an absolute.. me i go for the softer side and make sure my tyres are not running too warm.. i also wish my caravan had some real suspension instead of the basic (cheap) rubber system it has.. :)

trog

acampingwewillgo

16/5/2015 at 9:58pm
Location: Chiltern Hills
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Okay so I change my tyre pressures by a few PSI - my Draper gauge gave me one figure, my foot pump gave me a different figure (5 PSI) out and tomorrow I shall try my 12v plug in pump - what's the bet that I get a different reading from that too!!!!!!

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