# Finding the right pressure - Where now?



## loughrigg (Nov 10, 2008)

Over the last 15 months I have tried numerous times to understand what the tyre pressures should be for my MH - without real success.

For some inexplicable reason, I thought I'd have another go today, starting from basics. This is what I have, so far:

For the record, my MH was purchased second-hand fitted with Michelin 215/70 R15 109R tyres (I don't remember the exact tyre model) which had date codes consistent with the probable dates of conversion to an MH. As these tyres were badly cracked, they were immediately replaced with "C" tyres with the same size and indexes. *This tyre size is not listed in the manufacturers handbook in the schedule of recommended tyre pressures*. However, Peugeot say there is no reason why this size of tyre may not be fitted to a Boxer base vehicle.

1. Vehicle Converter - Autocruise (2002): No longer in existence and no records have survived from the time of conversion, so no point of reference.

2. Base Vehicle Manufacturer: Peugeot Boxer long wheelbase (Type 1400) first registered in 2002. A Peugeot commercial vehicle main agent has stated that the records of the vehicle show that the base was supplied as a "bare chassis for conversion". Accordingly, the advice from Peugeot is "*the tyre pressures specified in the handbook supplied with the vehicle or appearing on the door pillar should not be used as definitive statements of required tyre pressures as they relate to the vehicle only at the point of construction".* Peugeot's position is that as they have no knowledge of the manner or type of conversion, they cannot and will not recommend an appropriate tyre pressure for the vehicle. This is based upon the possibility of changes being made to the gearbox and/or suspension post-production (by the converter) that are unknown to Peugeot and which could change recommendations relating to the vehicle in standard (production) configuration.

When asked if they could confirm the size of tyre that should be fitted to the vehicle the answer was similar - without knowing full details of the conversion and any associated gearbox/suspension modifications, they could only confirm the standard tyres at production which might or might not be appropriate tyres post conversion.

Ultimately, Peugeot's advice was to refer to the vehicle converter who would be able to advise details of any post-production modifications and recommend the correct tyre pressures for the tyres they (the converter) deemed appropriate at the time of conversion. Unfortunately, Autocruise no longer exist and none of the company records survived the transfer of the brand name to Swift.
In short - a dead end.

2. Tyre Manufacturer (as currently fitted) - Vredestein. The tyre manufacturer says it does not maintain a set of load/pressure charts for each tyre and cannot recommend pressures based on axle loading alone. For C tyres their initial advice was to follow the manufacturers handbook recommendation and "add a few pounds". As established above, this is not an option because the manufacturer says the handbook does not necessarily apply.
A subsequent, long discussion with a Vredestein "technical specialist" has clarified the position a little, but has still not produced a definitive reference pressure for the tyre. In summary, the view expressed by Vredestein is to run the tyre as close to the maximum recommended pressure (for the tyre) as possible but having regard to the load on the tyre and the roadholding i.e. if running at lighter load and high pressure causes handling to deteriorate then adjust pressure until handling is acceptable.

So, if the base vehicle manufacturer can't (or won't) recommend a tyre pressure and says talk to the converter, the converter has gone out of business and the tyre manufacturer says they cannot recommend a definitive pressure for a specific tyre fitted to a specific vehicle, I think I have a very neat circle that I can plod around until the cows come home without ever finding an answer.

Ho Hum!!

Mike


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## peejay (May 10, 2005)

Hi;

Have a read of this flyer, if you have your individual axle weights, the info you need could be in there on the last few pages.....

http://www.tyresafe.org/data/files/motorhome 08.pdf

Pete


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## loughrigg (Nov 10, 2008)

peejay said:


> Hi;
> 
> Have a read of this flyer, if you have your individual axle weights, the info you need could be in there on the last few pages.....
> 
> ...


Thanks Pete - I saw that leaflet when it was first issued last year. Unfortunately, it doesn't list my tyre size.

Mike


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## Mrplodd (Mar 4, 2008)

The ONLY way of getting the correct pressures is as follows

Weigh your MH when in "going away" configuration (dont forget to include the driver and passenger(s) Axle weights are more important that Gross weight

Make a note of tyres size, type and make.

Contact the tyre manufacturer (not a tyre depot) Give them all of the above info and ask them what pressures you should be running. They make the tyre they are the experts !!

Simplez !!!!!


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## loughrigg (Nov 10, 2008)

Mrplodd said:


> The ONLY way of getting the correct pressures is as follows
> 
> Weigh your MH when in "going away" configuration (dont forget to include the driver and passenger(s) Axle weights are more important that Gross weight
> 
> ...


Point 2 in my post. I've already had a long conversation with the tyre manufacturer.

Not so simplez!!!!  

Mike


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## Jean-Luc (Jul 21, 2005)

loughrigg said:


> Point 2 in my post. I've already had a long conversation with the tyre manufacturer.
> 
> Not so simplez!!!!
> 
> Mike


What are the full specs. of your tyres and what are your MH's axle weights.
Even if you can't find the exact spec in the tyresafe leaflet you should be able to make an intelligent guestimate referencing tyres 'either side' of yours.


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## Wizzo (Dec 3, 2007)

loughrigg said:


> Thanks Pete - I saw that leaflet when it was first issued last year. Unfortunately, it doesn't list my tyre size.
> 
> Mike


Have another look Mike, Your tyre size is about 3/4 down on Page 5.

JohnW


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## loughrigg (Nov 10, 2008)

Wizzo said:


> loughrigg said:
> 
> 
> > Thanks Pete - I saw that leaflet when it was first issued last year. Unfortunately, it doesn't list my tyre size.
> ...


Hi John

My tyres are "C" (light commercial) tyres rather than motorhome specific "CP" tyres - I 've not seen anything that indicates how pressure recommendations correlate across the the two tyre constructions (if at all), so a bit of guesswork might come into it.

The leaflet goes to the trouble of providing separate recommendations for C and CP tyres, so I've taken that to indicate that there is a significant difference. Where there is a tyre size that is common to the two tables, there does seem to be a marked difference in indicated load /pressure recommendations.

Mike


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## loughrigg (Nov 10, 2008)

Jean-Luc said:


> loughrigg said:
> 
> 
> > Point 2 in my post. I've already had a long conversation with the tyre manufacturer.
> ...


My tyres are Vredestein Comtrac Winter 215/70 R 15 109R. Maximum plated axle weights are 1850 (front) and 2000 (rear).

The closest on the leaflet (I think) would be 225/70 R 15 109 which would give around 48psi (front) and 52 psi (rear). The comments I already have from Vredestein indicate that those pressures would be considered (by them) to be too low for Comtrac Winter tyres.

Mike


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## Wizzo (Dec 3, 2007)

Hi Mike,

Sorry I misread your post I thought you were running CP's. I think I would agree with Vredestein. There's a big difference between the 225s and 195s and yours fall somewhere inbetween.

If I were in your shoes I think I would be trying 65psi all round for starters (that allows for an extra 10% in the fronts for the driving wheels).

JohnW


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## Wizzo (Dec 3, 2007)

Mike,

I've had further thought on this whilst travelling home from work. I would have thought that you could use the pressures given for the CP version as a guide, after all they do list the right size. Having looked again at the tyre leaflet it would seem that you are running quite close to the maximum load for the tyre size, especially for the rears. However it does bear out my thoughts that 65psi would not be too far off.

JohnW


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## loughrigg (Nov 10, 2008)

Wizzo said:


> Hi Mike,
> 
> Sorry I misread your post I thought you were running CP's. I think I would agree with Vredestein. There's a big difference between the 225s and 195s and yours fall somewhere inbetween.
> 
> ...


Hi John

I ran the MH (briefly) at 65 psi with a very light load (back axle at about 1,850kg) and it was very skittish - almost had a close encounter with a ditch. A full load might give more stability.

I've had a few runs with the pressure at 57 psi all round which gives bags of grip but might have cost me a little bit in fuel efficiency. I'm guessing that a better pressure (fully loaded) could be around 60 - 62 psi.

Mike


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## Jean-Luc (Jul 21, 2005)

One of the advantages of CP tyres over C is that CP's have stronger carcase and will take a max pressure of 5.5 bar (80psi).
Have you checked the max pressure stamped on your tyres. As has been said I would use the pressures quoted for your size for CP in the Tyresafe leaflet bearing in mind not to exceed the max for your tyres.
BTW do you know the actual axle weights as its that weight you need to reference against, not the max. permitted weight.


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## loughrigg (Nov 10, 2008)

Jean-Luc said:


> One of the advantages of CP tyres over C is that CP's have stronger carcase and will take a max pressure of 5.5 bar (80psi).


True enough, but I didn't want CP's.



Jean-Luc said:


> Have you checked the max pressure stamped on your tyres.


Max pressure for the Comtrac Winters is 65psi



Jean-Luc said:


> As has been said I would use the pressures quoted for your size for CP in the Tyresafe leaflet bearing in mind not to exceed the max for your tyres.


It is an option, but I was hoping for something a bit more fact based. As you have said, the construction of CP tyres is different.



Jean-Luc said:


> BTW do you know the actual axle weights as its that weight you need to reference against, not the max. permitted weight.


I do - several of them, at different load levels. It would be useful to have something to reference them against 

Mike


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## CliveMott (Mar 10, 2008)

Axle weights determine the minimum pressure that is suitable for the tyre, however that,s not the only criteria. Motorhomes, specifically coachbuilts generally have quite a high centre of gravity. Soft tyres mean more body roll on corners and roundabouts. So frequently the vehicle converters will specify a higher tyre pressure than the tyre manufacturers to improve the vehicles overall handling.

If anyone tells you to lower the pressures until the ride is comfortable they are talking out of their fundemental orifice!

Take care

C.


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## loughrigg (Nov 10, 2008)

CliveMott said:


> If anyone tells you to lower the pressures until the ride is comfortable they are talking out of their fundemental orifice!
> 
> Take care
> 
> C.


I would agree with that - no one has said that to me (yet).

Mike


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## Jean-Luc (Jul 21, 2005)

loughrigg said:


> Jean-Luc said:
> 
> 
> > One of the advantages of CP tyres over C is that CP's have stronger carcase and will take a max pressure of 5.5 bar (80psi).
> ...


As the max is 65psi, I would go for that. It's not too heigh, particularly at the back and its the highest you can go. If you find the front end too rough you can always experiment by lowering the pressures there a couple of psi at a time. Use the 'seat of your pants' :wink:


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## loughrigg (Nov 10, 2008)

Jean-Luc said:


> loughrigg said:
> 
> 
> > Jean-Luc said:
> ...


For now, I think I can only continue with the "trial and error" approach. The comment from Vredestein is sensible and I can appreciate the position taken by Peugeot - but it is still irritating that any element of guesswork has to be involved.

Mike


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