# Tyre pressures - I know - I know



## acctutor (Oct 3, 2009)

Hi,

I have tried to sort this out myself, e-mailed Michellin (no response), and searched the tyres posts - no joy.

New Frankia - 2020kg on the front, 2480kg on the rear. Michellin tyres 225 x 75 x 16. Four wheels.

The tyre safe booklet does not cover my tyre size.

Any suggestions (sensible!!!), on tyre pressures, I am running 70psi on the front and 75psi on the back (cold values) at the moment, BUT when running long distance, the tyrepal system tells me that the two rear tyres rise to 84psi when warm, and the two front to 81psi.

Is this acceptable?

At Berchtesgaden at the moment - raining since 04:00 - but clearing now.

All the best

Bill & Patsy


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## VanFlair (Nov 21, 2009)

Hi Bill and Patsy

I have just taken our motorhome away to park it otherwise I would have a tyre pressure vs axle load chart for your tyre size.

I also know that the Michelin recommended pressures would not be that high on the front axle I am running slightly over the chart value and run 62 psi cold in the front (rises to about 72 driving) and weight is 2100 kg, I also know that a lot of people have been told by the tyre manufacturers to run 80 psi in the rears.

Your maximum allowed pressure will be 80 or 85 psi cold so the simple answer is yes you are OK, tyrepal will scare you at first as the pressure rises with driving, I found the rise to be no different from winter in the UK to summer in UK spring in Spain.



Martin


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## mentaliss (Oct 23, 2012)

VanFlair said:


> Hi Bill and Patsy
> 
> I have just taken our motorhome away to park it otherwise I would have a tyre pressure vs axle load chart for your tyre size.
> 
> ...


 Sorry Martin, but from many posts/ topic's that I have read from this site and Motorhome Fun Michelin tell everybody to inflate their tyres to 80psi........


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## Pudsey_Bear (Sep 25, 2008)

Mine is 70 F 65 R, But a rule of thumb is to just look at them and if they look low then they probably are, the bottom of the tyre should stick out about 15-25mm ish


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## bigtwin (May 24, 2008)

mentaliss said:


> Sorry Martin, but from many posts/ topic's that I have read from this site and Motorhome Fun Michelin tell everybody to inflate their tyres to 80psi........


Not so, Michelin gave me 54 psi front and 72psi rear (sorry but I don't have the axle weights to hand to provide a reference for the OP).

Ian


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## BritStops (Mar 3, 2011)

So could a chart be made up that shows psi per axle depending on weight and tyre size?

Or is it also dependent on tyre make and type (camper vs truck, etc.)?

Steve


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## pippin (Nov 15, 2007)

When I emailed Michelin I received an almost immediate response with pressure recommendations.


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## cabby (May 14, 2005)

perhaps both of you should check that the question is going to the same address that pippin's email went to then.

cabby


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## VanFlair (Nov 21, 2009)

Hi mentaliss

I agree with what you say regarding rear pressures (and did point this out), in the last few years tyre companies have started quoting 80 for motorhomes as we can't be trusted not to overload, most people who ask the tyre company are given a realistic figure for the front axle as over pressure on the steering axle is not good for steering.

With our Iveco base chassis book we have the table that I mentioned, OK 8 years old but works for me.

Our tyres are nowhere near fully loaded so why run full pressure, load index 121 is 2900kg and we run at 2100kg, full pressure rattles my fillings.

Martin


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## acctutor (Oct 3, 2009)

*Tyres*

Hi,

Thanks for all your replies - I think I can begin to see the light at the end of the tunnel (unless some bu**ger switches the torch off!!).

I re-read the tyre safe book, and found I had missed the tyre load index table, I also found a thread with an entry from grizzlyJ (I think), who mentioned the formula.

Divide the max load by max pressure to give kgs per psi.

Multiply this by the actual load to give the working pressure (and add 5psi for long runs.

With my tyres (Load index 116), this gives the front pressures at 63psi and the rear at 77psi. I am bit concerned that the rear tyres are working so close to the maximum, 2480kg actual, 2500kgs max, but hopefully full air suspension may assist with this.

Don't you love it when the salesman says "I doubt you will even get within 50kgs of the 4500 limit" - yeh right?????.

This the salesman who agreed to weigh the van on arrival to give an accurate base figure - and then conveniently forgot. I feel a strong letter coming on.

All the best, and thanks for all the help. From a rainy Berchtesgaden

Bill & Patsy.


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## mentaliss (Oct 23, 2012)

VanFlair said:


> Hi mentaliss
> 
> I agree with what you say regarding rear pressures (and did point this out), in the last few years tyre companies have started quoting 80 for motorhomes as we can't be trusted not to overload, most people who ask the tyre company are given a realistic figure for the front axle as over pressure on the steering axle is not good for steering.
> 
> ...


 ________________________________________
Interesting isn't it on 3 different occasions Mich' with the axle weights given still said their tyre pressures should be 80psi, all tyres (Agilis camper tyres)......... Continental (Vanco) gave me the precise tyre pressure when given the axle weights...Mich' should, but won't give the same service I wonder why??


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## BillCreer (Jan 23, 2010)

Martin[/quote]
________________________________________
Interesting isn't it on 3 different occasions Mich' with the axle weights given still said their tyre pressures should be 80psi, all tyres (Agilis camper tyres)......... Continental (Vanco) gave me the precise tyre pressure when given the axle weights...Mich' should, but won't give the same service I wonder why?? [/quote]

Michelin told me that the 80 psi at the rear comes from a "Trade" agreement and arrived at for safety reasons.


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## VanFlair (Nov 21, 2009)

*Re: Tyres*



acctutor said:


> Hi,
> 
> Thanks for all your replies - I think I can begin to see the light at the end of the tunnel (unless some bu**ger switches the torch off!!).
> 
> ...


I think you mean max pressure/by max load X actual load= actual pressure, only pulled you up because I tried to work it out for ours and had to modify the calcs, it comes pretty close to what we are running though.

Martin


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## nicholsong (May 26, 2009)

A couple of years ago Michelin only responded to my posting weights to them with the standard 80psi. One pressure cannot be right for all axle weights. Sounds like lawyers running the customer services rather than engineers. 

That was part of the reason I switched tyre manufacturer.

Geoff


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## mentaliss (Oct 23, 2012)

*Re: Tyres*



VanFlair said:


> acctutor said:
> 
> 
> > Hi,
> ...


 __________________________________________
Sorry were old  but cant seem to work out your formula perhaps you can advise?
Michelin Agilis camper tyres 225/70/15 load rating 112
maximum pressure 80 psi, rear axle weight is 1970 kg................

112 maximum load divided by maximum pressure 80 psi = 1.4000
rear axle weight (loaded) is 1950 kg half that is 975 kg
975 x 1.4000 =1365,000...????????
I'm obviously doing something wrong


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## VanFlair (Nov 21, 2009)

Hi mentaliss

Load index 112 means that at max pressure the tyre can support a load of 1120kg or 2240kg per axle.

Max pressure 80/max load 2240=0.357 x actual load 1950=69.6psi

The above calculation would suggest that the rear axle tyre pressure should be around 70psi.

I should also add that I am not telling you what pressure to run, but simply explaining the calculation.

Martin


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