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Tyre Pressure ?



 
 
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  #1 (permalink)  
Old July 10th 04, 11:08 PM posted to uk.rec.caravanning
Iain Mortimer
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Posts: 1
Default Tyre Pressure ?

Hello was hoping someone could help after the correct tyre pressures for a
88 Avondale Mayfair Derwent 5 Berth.

Iain


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  #2 (permalink)  
Old July 11th 04, 08:26 AM posted to uk.rec.caravanning
Hitch Lock
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Posts: 692
Default Tyre Pressure ?

In an earlier contribution to this discussion,
Iain Mortimer tina.iain wrote:

Hello was hoping someone could help after the correct tyre pressures
for a 88 Avondale Mayfair Derwent 5 Berth.

Iain


I don't know the recommendation for this particular van, but you can
probably work it out by the following method:

Look at the tyres to see whether there is a marking on them along the lines
of: "Max load XXX Kg at YY psi"

If there is, look on the caravan plate for the maximum permitted weight. [It
may be in cwt, which you'll have to convert to Kg]. Divide this by 2
(assuming it's a single axle van) to get the maximum load per wheel. Compare
this with the tyre marking. Assuming that the caravan load per wheel is
*less* than permitted for the tyre (if it's *more*, you're in trouble!)
scale the pressure in the ratio of the loads.

For example, if the tyre can carry 600 Kg at 50 psi but the caravan weighs
1000 Kg (500 Kg per wheel) use a tyre pressure of 500/600 * 50 = 42 psi
(approx).

HTH.
--
Cheers,
Hitch Lock
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  #3 (permalink)  
Old July 11th 04, 09:55 AM posted to uk.rec.caravanning
Andrew Kay
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Posts: 86
Default Tyre Pressure ?

"Hitch Lock" wrote in message
...

I don't know the recommendation for this particular van, but you can
probably work it out by the following method:


Look at the tyres to see whether there is a marking on them along the

lines
of: "Max load XXX Kg at YY psi"


For example, if the tyre can carry 600 Kg at 50 psi but the caravan weighs
1000 Kg (500 Kg per wheel) use a tyre pressure of 500/600 * 50 = 42 psi
(approx).


The area of the footprint that the tyre makes on the road is approximately
equal to the downforce on that wheel (in pounds) divided by the tyre
inflation pressure (the tyre wall has some effect, but not a significant
one). For a given tyre size, the footprint should be more or less the same
irrespective of the model of the tyre to make sure that the tread sits flat
on the road across its width and that there is no uneven tyre wear. It
does not make sense, therefore, to change the inflation pressure as you
suggest (for the same tyre size) depending upon the maximum allowable
inflation pressure - as you will be changing the size of the tyre footprint
on the road.

I have a Lunar Saturn which has 165 R13 tyres. When new, it was fitted with
Contintental tyres which had a maximum weight rating fractionally *less
than* half the maximum permitted weight of the caravan. Lunar's
recommendation was that they were run at the maximum rated tyre pressure -
36psi. I have recently fitted some Michelin tyres to replace them which
have pretty much the same load rating (slightly higher, its a little over
half the caravan max weight) but a much higher maximum tyre pressure of
50psi. By your calculation, I should be running them at nearly 50psi - much
higher than the original tyres. If I did, the tyre footprint would be much
smaller and I'd wear the tyres much more in the middle than at the edges.
Sorry, your method just is not correct.

As an aside, the maximum tyre load strictly applies only at the maximum
rated speed of the tyre. Manufacturers derive the figures by testing to
ensure that the maximum allowable tyre temperature is not execeeded. This
explains why Lunar could fit tyres to my Saturn that appear, at first
glance, to be inadequate for the purpose. Caravans are limited to 60mph in
the UK and the tyres run far cooler than they would at the maximum rated
speed.

I'd suggest that owners stick to the caravan manufacturers recommended tyre
pressures, irresepective of the make of tyre fitted - providing it is of the
same size as the orignal fitment.

Cheers
Andrew




  #4 (permalink)  
Old July 11th 04, 12:05 PM posted to uk.rec.caravanning
Hitch Lock
external usenet poster
 
Posts: 692
Default Tyre Pressure ?

In an earlier contribution to this discussion,
Andrew Kay wrote:


The area of the footprint that the tyre makes on the road is
approximately equal to the downforce on that wheel (in pounds)
divided by the tyre inflation pressure (the tyre wall has some
effect, but not a significant one).


It's precisely for this reason that you *should* change the tyre pressure if
the imposed load is substantially less than the maximum which the tyre can
carry. This keeps the footprint the correct size. If you run a lightly
loaded tyre at its maximum pressure, the footprint will be too small!

Why do you think your car tyre pressures are different front to rear,
although the tyres are exactly the same?
--
Cheers,
Hitch Lock
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  #5 (permalink)  
Old July 11th 04, 12:19 PM posted to uk.rec.caravanning
Andrew Kay
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Posts: 86
Default Tyre Pressure ?

"Hitch Lock" wrote in message
...
In an earlier contribution to this discussion,
Andrew Kay wrote:


The area of the footprint that the tyre makes on the road is
approximately equal to the downforce on that wheel (in pounds)
divided by the tyre inflation pressure (the tyre wall has some
effect, but not a significant one).


It's precisely for this reason that you *should* change the tyre pressure

if
the imposed load is substantially less than the maximum which the tyre can
carry. This keeps the footprint the correct size. If you run a lightly
loaded tyre at its maximum pressure, the footprint will be too small!


This isn't worth arguiing about. You are just wrong - and anyone following
yor advice could put themselves at risk. Like I said - stick to the
manufacturers recommendations.

Pressure = Force/Area. If the downforce is the same (which is it, the
caravan doesn't change!!!), then to keep the same size footprint the tyre
pressure must be the same. Simple sums, really.

Why do you think your car tyre pressures are different front to rear,
although the tyres are exactly the same?


The downforce on front & rear axles are not the same.

Cheers
Andrew Kay


  #6 (permalink)  
Old July 11th 04, 01:53 PM posted to uk.rec.caravanning
Hitch Lock
external usenet poster
 
Posts: 692
Default Tyre Pressure ?

In an earlier contribution to this discussion,
Andrew Kay wrote:

"Hitch Lock" wrote in message
...
In an earlier contribution to this discussion,
Andrew Kay wrote:


The area of the footprint that the tyre makes on the road is
approximately equal to the downforce on that wheel (in pounds)
divided by the tyre inflation pressure (the tyre wall has some
effect, but not a significant one).


It's precisely for this reason that you *should* change the tyre
pressure if the imposed load is substantially less than the maximum
which the tyre can carry. This keeps the footprint the correct size.
If you run a lightly loaded tyre at its maximum pressure, the
footprint will be too small!


This isn't worth arguiing about. You are just wrong - and anyone
following yor advice could put themselves at risk. Like I said -
stick to the manufacturers recommendations.

Pressure = Force/Area. If the downforce is the same (which is it, the
caravan doesn't change!!!), then to keep the same size footprint the
tyre pressure must be the same. Simple sums, really.

Why do you think your car tyre pressures are different front to rear,
although the tyres are exactly the same?


The downforce on front & rear axles are not the same.

Cheers
Andrew Kay



I've got no problem with sticking with the manufacturer's recommendation.
The problem was that the OP didn't know what the caravan manufacturer *had*
recommended, so I presented a method which would enable him to reach a
reasonable conclusion.

You seem to be confusing the maximum load rating (and related pressure) as
marked on a tyre, with a vehicle or caravan manufacturer's recommendation on
what pressure to use in a particular context.

You are actually contradicting yourself in alternate utterances. On the one
hand, you are saying (correctly) that load = pressure x area and that you
need to adjust the pressure according to the load in order to keep the
footprint area correct.

You even accept that the front and rear tyre pressures on a car need to be
different because the loads are different.

But then, in the next breath, you appear to say (totally incorrectly) that
if a tyre is marked as having a maximum load of 600 Kg at 50 psi, you should
inflate it to 50 psi under all circumstances - even if you're only imposing
a 500 Kg load on it. If you're *not* saying that, please explain exactly
what you *are* saying.

I agree with you that the maximum plated weight of a caravan isn't suddenly
going to change. However, the caravan may well not still have its original
tyres - and the tyres fitted may have a higher load rating (as indeed mine
do). The method I outlined *will* yield a sensible pressure for the actual
tyres fitted and the actual imposed load, and is *not* plainly wrong as you
suggest.
--
Cheers,
Hitch Lock
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  #7 (permalink)  
Old July 11th 04, 11:13 PM posted to uk.rec.caravanning
Andrew Kay
external usenet poster
 
Posts: 86
Default Tyre Pressure ?

----- Original Message -----
From: "Hitch Lock"

But then, in the next breath, you appear to say (totally incorrectly)

that
if a tyre is marked as having a maximum load of 600 Kg at 50 psi, you

should
inflate it to 50 psi under all circumstances - even if you're only

imposing
a 500 Kg load on it.


No, I did not say that. If you re-read what I wrote in my previous posts,
you will see what I wrote.

I agree with you that the maximum plated weight of a caravan isn't

suddenly
going to change. However, the caravan may well not still have its

original
tyres - and the tyres fitted may have a higher load rating (as indeed mine
do). The method I outlined *will* yield a sensible pressure for the actual
tyres fitted and the actual imposed load, and is *not* plainly wrong as

you
suggest.


If you replace a tyre on a caravan with another tyre of the same size then
you should inflate it to the pressure recommended by the van manufacturer
for the original tyre. The *maximum* permitted inflation pressure of the
new tyre is irrelevant.

As I mentioned previously, I have recently replaced the tyres on my Lunar
Saturn (1030kg max weight). The original Contintental tyres had a max
inflation pressure of 36psi and a Lunar recommended inflation pressure of
36psi. I replaced them with Michelins which have a max load rating of 550kg
and max inflation pressure of 50psi. If I used your method of estimating
the required inflation pressure for the Michelins, I would have to pump them
up to 47psi - which is far from being a sensible result.

I think you might be coming unstuck by assuming that the max load and max
inflation pressure of a tyre are somehow related and that you can scale one
against the other - e.g. that my Michelin tyres have a max load of 550kg at
50psi, but it would fall to 275kg at 25psi. It ain't so. The maximum
load and inflation pressure ratings are independent of each other.

I have spent half an hour of so searching the web to try to find some
justification for the methodology you suggest. I can find none at all. All
the tyre manufacturers simply say that the correct inflation pressure is the
one recommended by the vehicle manufacturer. If you can point towards any
reasonably definitive source (tyre or vehicle manufacturer) to justify what
you say then I'll have another look - but until then, I will continue to
regard it as misguided & potentially dangerous.

Regards
Andrew Kay


  #8 (permalink)  
Old July 12th 04, 10:25 PM posted to uk.rec.caravanning
Hitch Lock
external usenet poster
 
Posts: 692
Default Tyre Pressure ?

In an earlier contribution to this discussion,
Andrew Kay wrote:

All the tyre manufacturers simply say that the correct
inflation pressure is the one recommended by the vehicle
manufacturer.



I don't have a problem with that.

However, what are you supposed to do if - for any reason - you can't find
out what the vehicle/caravan manufacturer *did* recommend? That, as I
understand it, was the position of the OP.

You say that you have not found anything on the web to support my
methodology. You have presumably not found anything to support yours
either - or you would have told us about it!

You have said that:
* the correct footprint should be maintained for a given tyre
* footprint area is determined by dividing the load by the pressure (with
the sidewall having little effect)

It therefore follows that if a tyre is designed to support a particular load
at a particular pressure, if you ask it to support only x% of the design
load , you also inflate it to x% of the design pressure - to a first
approximation. This will achieve the same footprint as it would have when
supporting the design load at the design pressure.

Incidentally, when I used this approach on my own caravan to work out what
pressure to use with new tyres of the same size but higher load index, I
ended up very close to the caravan manufacturer's recommendation for the
original tyres. I don't understand why this didn't work for you.
--
Cheers,
Hitch Lock
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  #9 (permalink)  
Old July 12th 04, 11:06 PM posted to uk.rec.caravanning
Andrew Kay
external usenet poster
 
Posts: 86
Default Tyre Pressure ?

"Hitch Lock" wrote in message
...

Incidentally, when I used this approach on my own caravan to work out what
pressure to use with new tyres of the same size but higher load index, I
ended up very close to the caravan manufacturer's recommendation for the
original tyres. I don't understand why this didn't work for you.


Let's see if it works for my car. The tyres are BF Goodrich 245/70 R16.
The tyre rated maximum load is 1150kg and maximum pressure is 65psi.

The maximum axle loading at the front is 1200kg & at the rear is 1500kg.
Correct me if you think I am wrong, but using your methdology, I should
inflate the tyres to:

Front: 65psi * 600kg /1150kg = 34psi
Rear 65psi * 750kg/1150kg. = 42psi

Nope - it doesn't work for my car either. I think I'll stick to Isuzu's
recommendation of 30psi and 35psi, if you don't mind.

The reason reason why it didn't work for me is because the methodology is
wrong. It is entirely fortuitous that you got answers that were close to
the manufacturers estimate.

You mentioned "my method". I don't have a "method". I said that I would
stick to the caravan manufacturers recommendation for a tyre of the same
size. This is what all tyre suppliers will tell you. It is what all tyre
manufacturers do tell you. There are plenty of references on the web, if
you care to look them up - try keywords such as "correct, inflation
pressure", (tire OR tyre)

The reason why the tyre pressure should be the same for any replacement road
tyre *of the same size* is very simple. If the tyre pressure is the same
then the footprint size will be pretty much the same as the vehicle
manufacturer intended. I think I'm pretty happy with that. I am not,
however, happy to *calculate* tyre pressures based upon some half-baked
theory that has no theoretical basis - and just doesn't work anyway.

Cheers
Andrew Kay


  #10 (permalink)  
Old July 13th 04, 09:40 AM posted to uk.rec.caravanning
Hitch Lock
external usenet poster
 
Posts: 692
Default Tyre Pressure ?

In an earlier contribution to this discussion,
Andrew Kay wrote:


You mentioned "my method". I don't have a "method". I said that I
would stick to the caravan manufacturers recommendation for a tyre of
the same size. This is what all tyre suppliers will tell you. It is
what all tyre manufacturers do tell you. There are plenty of
references on the web, if you care to look them up - try keywords
such as "correct, inflation pressure", (tire OR tyre)

If the information is readily available, you could have pre-empted this
discussion by looking it up and responding to the OP. Why didn't you?

I accept that my method probably produces pressures which are slightly
higher than recommended by vehicle/caravan manufacturers. In the absence of
recommended figures it is better to err on the high side. This will give
better cornering stabilty at the possible expense of a firmer ride and a
slight tendancy to wear the centre of the tread. On the other hand, running
them at too *low* a pressure will result in excessive cornering slip angles
and, more importantly, overheating due to excessive sidewall flexing -
greatly increasing the risk of blowouts.

How do you think that manufacturers arrive at their recommendations? Having
worked for a vehicle manufacturer, I am very well aware that there are no
magic right answers - and that the whole thing is empirical - taking account
of stability, ride quality, noise, fuel consumption, tyre wear, to name but
a few. But you have to start somewhere. No great harm will come to anyone
who starts from the guidelines which I outlined in my original post.

We are probably not that far apart! You keep saying "stick to the
manufacturer's recommendations". I agree. But you still haven't said what
you should do if there aren't any! For example, just suppose you were to
build a one-off d-i-y trailer . . .

--
Cheers,
Hitch Lock
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