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Tyre Pressures

T

Trev864

I recently bought a 1993 Craftsman Miracle 154 caravan. I can't see anywhere in the handbook where it tells you what the correct tyre pressures should be.

Does anyone know where or how I could find out please?
 
Trevor, if you have the service handbook, in addition to the user handbook, the pressures will be in the Service book.
 
Failing any other information, the maximum permitted pressure is usually shown on the tyre sidewall in very small letters. Somewhere at or just under this fighure is what I would use in the absence of anything more detailed.
 
Last year I needed to find out what pressure to use in a certain caravan tyre loaded at 60% maximum. I thought there would be readily available a formulae to calculate this; but I could'nt find it anywhere on the internet. I asked one of the major manufacturers, but they said it was "restricted information"!

The pressure to use at maximum loading for any tyre is easilly found, either from the data stamped on the tyre itself, or from various websites.

Eventually, I obtained the booklet "Tyre Tips for Caravan & Trailer Tents" from the BTMA*. This has tables for some tyre sizes showing pressures to use at loads below maximum.

So I plotted out a graph for each size, using % of maximum pressure as one axis, and % of maximum load as the other.

I found that all tyre sizes produced points on the same line (within about 2%), so I therefore had the formulae (down to about 40% load).

So, to replicate this, get a piece of linear graph paper with the axes designated 0 to 100 (%).

Label one % max. load, the other % max. pressure.

Put two points, one at 40% max. load, 30% max. pressure; and the other, of course, 100% both.

Draw a straight line between the two.

You can then relate this to your particular van.

Eg a 195/70R14 has a load index of 91 (615Kg per wheel), (at 36psi).

If on a van weighing 800Kg., load per wheel = 400Kg which is 65% loading. The graph shows 59% pressure = 21psi.

I submit this in good faith but without guarantee as to its "correctness".

If you have "solid" information that contradicts this graph, please post it.

Note the booklet pressure tables show AXLE loads, whereas the Load Index Table shows per wheel load.

*BTMA 020 7457 5040 (they also do a booklet for motorhomes), don't think you can download or order them online.
 
I'm sorry Chris but I have to disagree with your interpretation. If you interpolate your graph there would be a point where you would run the tyre with no pressure. Think of your car from running with driver to fully loaded, the chances are that you change the pressure by only 10%. 21psi would be grossly under inflated for any tyre increasing the risk of blowout, instability, tyre wear, etc.

If an owner does not have the maufacturers hand book with the correct pressures in them then a tyre dealer or caravan dealer should be able to advise the pressure, which is likely to the pressure for that tyre and accurate to within 10% for the load of the van to which it is fitted. If for the 195/70R14 example you give the pressure for that tyre is going to be 36psi - no variation, (you do not have to inflate tyres on a new van once you own it because you have loaded it).
 
Thanks AlanW for your comments.

Perhaps I should have mentioned it specifically, but I consider the graph only usable down to about 40% load.

The BRMA table for the 195/70R14 only goes down to 73% load (450Kg), but other sizes go down to 40%. As all the table figures for this size are "on the line", it seemed reasonable to me to consider that at lower loadings the points would remain on the line. Hence my calculation of 21psi.

Get the booklet & do the calculations & you will see what I mean.

With a modernish Van, it's easy to get the pressure from the manufacturer; in my case the firm was no longer trading.

I tried going round to several tyre dealers for advice, but they really had no idea; I got answers like "we usually put 30 psi in" nothing logical whatsoever, and none of them had tables like the BRMA booklet!

So I had to draw out the graph, and was surprised to find that all the figures from the booklet were "on the line" (down to 40%) I therefore concluded that I had found the formulae.
 
I'm sorry Chris but I have to disagree with your interpretation. If you interpolate your graph there would be a point where you would run the tyre with no pressure. Think of your car from running with driver to fully loaded, the chances are that you change the pressure by only 10%. 21psi would be grossly under inflated for any tyre increasing the risk of blowout, instability, tyre wear, etc.

If an owner does not have the maufacturers hand book with the correct pressures in them then a tyre dealer or caravan dealer should be able to advise the pressure, which is likely to the pressure for that tyre and accurate to within 10% for the load of the van to which it is fitted. If for the 195/70R14 example you give the pressure for that tyre is going to be 36psi - no variation, (you do not have to inflate tyres on a new van once you own it because you have loaded it).
Click to expand...
Alan, I agree with what you are saying, I would have thought that if your payload is 250kg, then the pressure would be the same if the caravan is empty or fully laden.

The reason the pressures on a car differs, is because of the added weight of the passengers and luggage,

best regards, Martin
 
I felt your argument was so sufficiently flawed that I discussed it with my tyre dealer today. His belief was that at 21psi a 195/70R14 would be dangerously underinflated and probably illegal. His recommendation was a minimum pressure of 36psi (on a caravan).

Even if I got the book (as you suggest) it does not mean that I would agree with extrapolating the graph as far as you have done. As a Maths teacher we have to teach pupils to ONLY read a graph WITHIN the known limits. If you have taken the line outside those limits the graph has no value.
 
If you find you have the need to extrapolate the load/tyre pressure table or chart, you've probably got the wrong size tyres in the first place. The normal working range of reinforced tyres is about 29 to 42 psi. If you need to go much higher than that, the axle load is higher than recommended for that size of tyre and if the pressure needs to be much under 29 psi, the tyre is overdimensioned.

Here's a sample table of tyre pressure versus load for a tyre with a load index of 94 (=670kg)

Tyre Pressure Load

(bar) (psi) (kg)

2.0 29.0 500

2.1 30.5 520

2.2 31.9 535

2.3 33.4 555

2.4 34.8 575

2.5 36.3 595

2.6 37.7 615

2.7 39.2 635

2.8 40.6 650

2.9 42.1 670
 
Good Morning Lutz,

Not wanting to become involved with the very techie nature of this thread, our van is 1854 kg MTPLM. Tyre pressure is 50 psi on the data plate, thats more than 20 % above you 42 psi max Have I got something wrong ???

Regards

Val & Frank
 
Thanks for your further comments.

AlanW;

Where did your tyre dealer get his 36psi from?, if he looked at the BRMA booklet he would see that 25psi is listed for a 450kg load (195/70R14), is his manufacturers association wrong? I suggest he just guessed it. So anyone taking his advise for a 900Kg van would be 44% overinflated!, would you run at that figure?

This pressure/load is the lowest table figure for that particular size, I accept I have extrapolated my 21psi from the graph, but based on the fact that the line is straight, and other sizes give figures down to their 40% loads. So I consider the extrapolation reasonable, but only down to 40%, which I have already stated.

If you phone the BRMA they will post the booklet foc, please plot out the graph and I think you will follow my reasoning; perhaps a good exercise for your students?

Lutz;

Thanks for the table, it matches the one in the booklet, and the points are "on the line" of the graph.

Are you able to disclose where it came from, as, apart from the BRMA booklet, I could'nt find such tables last year?
 
No, you haven't got anything wrong. What has happened is that the caravan manufacturer may have tried to reduce costs by fitting a tyre which is smaller than ideally suitable. As a consequence, the pressure has had to be increased accordingly.

So long as the pressure is still within the maximum permitted by the tyre manufacturer, there is nothing wrong with this practice other than it increases tyre wear. Perhaps this is not so much of an issue with caravans where tyres are often replaced due to old age, well before they are completely worn.
 
Thanks for your further comments.

AlanW;

Where did your tyre dealer get his 36psi from?, if he looked at the BRMA booklet he would see that 25psi is listed for a 450kg load (195/70R14), is his manufacturers association wrong? I suggest he just guessed it. So anyone taking his advise for a 900Kg van would be 44% overinflated!, would you run at that figure?

This pressure/load is the lowest table figure for that particular size, I accept I have extrapolated my 21psi from the graph, but based on the fact that the line is straight, and other sizes give figures down to their 40% loads. So I consider the extrapolation reasonable, but only down to 40%, which I have already stated.

If you phone the BRMA they will post the booklet foc, please plot out the graph and I think you will follow my reasoning; perhaps a good exercise for your students?

Lutz;

Thanks for the table, it matches the one in the booklet, and the points are "on the line" of the graph.

Are you able to disclose where it came from, as, apart from the BRMA booklet, I could'nt find such tables last year?
Click to expand...
The data was taken from the Continental tyres website.
 
Reading the information here I am now wondering how manufacturers come to their conclusion regarding tyre pressures.

My van has an MTPLM of 1481kg and runs on 63psi.
 
Like I said, there is nothing wrong with such extremely high tyre pressures so long as they are still within the absolute maximum allowed by the tyre manufacturer. Such practice only increases tyre wear but it's not dangerous.

At any rate it's better than the other extreme, underinflating an overdimensioned tyre. That is plain unsafe.
 
Hi All, I go back a few months when I posted some advice on the forum from the Caravan Club Techies. I had a couple of punctures in quick sucession on the van. I was concerned that the 185 tyres rated at 775kg were recomended by Avondale to run at 65psi - the maximum pressure and the maximum load on the tyre.

Caravan Club advice was to change to 195 and reduce the pressure to 54psi. I did this also uprating the tyre to 825kg
 
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