Tyre pressure discrepancy ... check yours!

Minxy

Minxy

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Carthago Compactline
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Since 1996, had Elddis/Swift/Rapido/Rimor/Chausson MHs. Autocruise/Globecar PVCs/Compactline i-138
Yesterday hubby was checking the tyre pressure on our Globecar Campscout on the Maxi heavy chassis (4250kg MTPLM), he checked the pressure recommendations on the label on the door pillar which show 79.5 psi all round ... however ... when he got down on his hands and knees he noticed that the max pressure for the tyres is 69 psi! :confused:

This is a bit concerning - if he'd put in 79 psi it would be 10 psi over the tyres 'capability' ... thus it got me thinking ... when putting air in tyres does anyone actually check what the tyres themselves state or just go by the label???? o_O

DSCF0703.JPG
psi.JPG
tyre size.JPG
 
Continental’s technical support have stated ( elsewhere on this forum) that the 69 psi only applies to the US market and can be ignored everywhere else.

Your sign also says “ at full load” ie 4250. You should check axle weight at a weighbridge then inflate to the recommended tyre pressures on each axle for that weight.
 
I always look at the max load/pressure recommendations on the tyre and work out the proportional pressure according to the actual load on the basis that tyre manufacturers know more about tyres than converters.
 
@Mr Chrysalis
Thanks for the info and link to the other thread, what makes American so 'special'? The bar and psi are the same for us and them ... so why is it only 69 for Northern America?

https://www.sensorsone.com/tyre-pressure-conversion-table/

American vs Imperial psi, kPa and bar
I’m using a tyre pressure gauge which was purchased in America, it is scaled in a choice of units: kPa, bar and psi. Is there any difference between the America and Imperial conversion?

The pressure unit bar is a European metric unit which has the same value in every country. Kilopascals (kPa)are derived form the SI unit for pressure which is the pascal (Pa), which is also commonly used in Europe. Pounds per square inch (psi or pfsi) are American and British Imperial units and the value is the same in both measurement systems.
  • bar: Metric, 100,000 Pa
  • kPa: SI, Metric, 1,000 Pa
  • psi: American, British Imperial, 6,894.76 Pa

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Minxy Girl said:
@Mr Chrysalis
Thanks for the info and link to the other thread, what makes American so 'special'? The bar and psi are the same for us and them ... so why is it only 69 for Northern America?

https://www.sensorsone.com/tyre-pressure-conversion-table/

American vs Imperial psi, kPa and bar
I’m using a tyre pressure gauge which was purchased in America, it is scaled in a choice of units: kPa, bar and psi. Is there any difference between the America and Imperial conversion?

The pressure unit bar is a European metric unit which has the same value in every country. Kilopascals (kPa)are derived form the SI unit for pressure which is the pascal (Pa), which is also commonly used in Europe. Pounds per square inch (psi or pfsi) are American and British Imperial units and the value is the same in both measurement systems.
  • bar: Metric, 100,000 Pa
  • kPa: SI, Metric, 1,000 Pa
  • psi: American, British Imperial, 6,894.76 Pa
Click to expand...

Does your tyre not have marks relating max pressure to max load for single and dual use, every tyre that I have had has these marks but I have never had CP tyres.
 
Minxy Girl said:
@Mr Chrysalis
Thanks for the info and link to the other thread, what makes American so 'special'? The bar and psi are the same for us and them ... so why is it only 69 for Northern America?

https://www.sensorsone.com/tyre-pressure-conversion-table/

American vs Imperial psi, kPa and bar
I’m using a tyre pressure gauge which was purchased in America, it is scaled in a choice of units: kPa, bar and psi. Is there any difference between the America and Imperial conversion?

The pressure unit bar is a European metric unit which has the same value in every country. Kilopascals (kPa)are derived form the SI unit for pressure which is the pascal (Pa), which is also commonly used in Europe. Pounds per square inch (psi or pfsi) are American and British Imperial units and the value is the same in both measurement systems.
  • bar: Metric, 100,000 Pa
  • kPa: SI, Metric, 1,000 Pa
  • psi: American, British Imperial, 6,894.76 Pa
Click to expand...
I don’t know why the US market is different, but it probably has something to do with the litigious culture. Maybe someone got hurt when a tyre exploded at 69.5 psi or something
 
matamoros said:
Does your tyre not have marks relating max pressure to max load for single and dual use, every tyre that I have had has these marks but I have never had CP tyres.
Click to expand...
Yup, 69 psi ...

DSCF0706.JPG
 
Mr Chrysalis said:
I don’t know why the US market is different, but it probably has something to do with the litigious culture. Maybe someone got hurt when a tyre exploded at 69.5 psi or something
Click to expand...
Maybe ... or possibly due to the 'size' of some Americans they can't cope with a harsher ride ... although you'd think with all the extra padding ... :imoutahere:
 
matamoros said:
I always look at the max load/pressure recommendations on the tyre and work out the proportional pressure according to the actual load on the basis that tyre manufacturers know more about tyres than converters.
Click to expand...

Genuine question; is the relationship between pressure and load linear?

Hopefully so, otherwise your approach could result in pressures that are way over/below the correct pressures.

Ian
 
Have you got the tyre that the converter
Intended????
 
bigtwin said:
Genuine question; is the relationship between pressure and load linear?

Hopefully so, otherwise your approach could result in pressures that are way over/below the correct pressures.

Ian
Click to expand...
I have a complicated formula based on the load rating, but every time I have used it with loads relatively close to the maximum it has been the linear relationship.

I have used this formula for a number of years on caravans, on the 5'er and pickup, the pickup covered around 100000 miles without uneven wear or tyre problems.

I will root out the original formula and post later.
 
Our MH came with similar setup (need's 80 psi in the rear) so I changed them, as I've run into pedantic officers of the law when bike tyre's have not for Highway use on them, which only applies in the USA, just couldn't be assed with the grief.

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Yes but you may have not been upgraded from standard van tyre 69lb to tyre recomended or expected by converter hence 79lb got to be looked into
 
B063F253-2EBC-4130-B7ED-EF2D495CCF29.jpeg
Our van has original Vancouver tyres on stamped at 65 psi max. but look at the small print further round the tyre
 
@Minxy Girl this is an old and contentious issue that will go on for ever.

Most PVC converters simply quote Fiats max ‘builders and delivery van’ pressures to cover their own @rse! The pressures all but knock yer fillings out on a less than perfectly smooth road.

I used to run our Globescout at 55 front and 60 back and the ride was nice and smoooooth :)

At the time I referred to Continental and they actually recommended a slightly lower pressure!

Interestingly our new Hymercar has pressures that are also much lower than Fiat and Hymer state that their pressures MUST be used regardless of what Fiat or the tyre maker say:eek:

(I’m not near the van today but Hymer state something like 3.5 bar front and 4 bar rear, FAR removed from Fiats 5.5 bar all round!

After nearly 40000 miles on the old van I am much happier with the lower pressures :D2
 
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bigtwin said:
Genuine question; is the relationship between pressure and load linear?

Hopefully so, otherwise your approach could result in pressures that are way over/below the correct pressures.

Ian
Click to expand...

The original formula is simply a more complicated version of direct proportionality as in my example.

Common sense would tell me that tyres will have a minimum working safe pressure but vehicle manufacturers will fit tyres suitable for the loads likely to be imposed and the variance will not be large, especially for motorhomes that are normally loaded close to maximum.
 
You are almost certainly running them at far too high a pressure. :) Get the axle loads measured on a weighbridge and then use Continental's guide for the correct pressure. We have similar tyres and vehicle and I've run them at 3.25 bar (47 psi) for about 30K miles without problems.

This only applies to Continental tyres! Other manufacturers differ widely, some refusing to give any advice.

The latest guide can be downloaded from here:

https://blobs.continental-tires.com...998ef8b3/download-technical-databook-data.pdf

@Minxy Girl's tyres are on page 86 and 87 I think. It is best to print these two pages off as you need to read across from the left hand page to the right hand one.
 
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matamoros said:
The original formula is simply a more complicated version of direct proportionality as in my example.

Common sense would tell me that tyres will have a minimum working safe pressure but vehicle manufacturers will fit tyres suitable for the loads likely to be imposed and the variance will not be large, especially for motorhomes that are normally loaded close to maximum.
Click to expand...

I agree with the ‘bolded’ Statement but that simply changes the gradient of the assumed linear relationship (which, of course, would change the interpolated pressures).

I thought that there may be other influences that may have meant that the relationship wasn’t linear (e.g. the amount of flex in the tyre walls and perhaps changes in the internal volume).

However, it’s unlikely(?) that any non linearity would make a huge difference.

Ian

PS - just did a quick google search and found this (neither read nor digested!)
 
@Minxy Girl it looks like you have the same tyres, weight and chassis as me.... here is the relevant part of an email direct from continental tyres... hope this helps.....

I run at 70psi rear and 65psi front....(y)

DB314B55-323A-4F89-AA5F-9C85DD39B184.jpeg
 
Another vote for the easy and user friendly service provided by continental.
If every tyre manufacturer did that how much easier would things be.

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bigtwin said:
I agree with the ‘bolded’ Statement but that simply changes the gradient of the assumed linear relationship (which, of course, would change the interpolated pressures).

I thought that there may be other influences that may have meant that the relationship wasn’t linear (e.g. the amount of flex in the tyre walls and perhaps changes in the internal volume).

However, it’s unlikely(?) that any non linearity would make a huge difference.

Ian

PS - just did a quick google search and found this (neither read nor digested!)
Click to expand...
Well I started reading and then it got a bit complicated so I cut to the Conclusions and what do you know(y) suits me as this is what I use,

  1. The simple ratio method given by Equation 9 provides a slightly more conservative, but computationally simpler, method of developing a safe operating pressure for tires operating at less than maximum loads.

    Pressure/max pressure = load/max load

    Martin
 
funflair said:
Well I started reading and then it got a bit complicated so I cut to the Conclusions and what do you know(y) suits me as this is what I use,

  1. The simple ratio method given by Equation 9 provides a slightly more conservative, but computationally simpler, method of developing a safe operating pressure for tires operating at less than maximum loads.

    Pressure/max pressure = load/max load

    Martin
Click to expand...

That was the conclusion that I came to after a similar scan through a complicated scientific study.
 
DBK said:
@Minxy Girl's tyres are on page 86 and 87 I think. It is best to print these two pages off as you need to read across from the left hand page to the right hand one.
Click to expand...
Ah, the above makes sense now! So I should use the FA S and FA R figures. (y)
 
RAS = Rear axle single wheel.
RAT = Rear axle twin wheel.
Use the line 8 116 FA S and RA S which relate to the CP tyres you have.

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