tyre pressure reassurance

[Derek Uzzell]

Naittaw

 

As your Ravenna has Michelin XC Camping tyres - presumably as original equipment - almost certainly it will have 'metal' bolt-in tyre-valves rather than the 'rubber' snap-in variety.

 

If you swap your present tyres for Continentals Vanco-2s, it would be worth you checking what the tyre-fitter plans to do about the tyre-valves. When a tyre-valve is the snap-in type it is standard practice when replacing a discarded tyre to replace the valve too, but I'm not sure what fitters normally do regarding bolt-in valves.

 

The tyre-fitter who handles my own tyre requirements told me that, when he replaces a tyre on a wheel with a bolt-in tyre-valve, he just replaces the valve's core rather than the complete valve. His argument for doing this is that bolt-in valves are very robust and, as long as a valve is undamaged and not leaking at the base-seal, a change of core should be sufficient. He does not stock the types of bolt-in valves used on motorhome wheels, so swapping the valves wouldn't be an option anyway.

 

I only mention this as an afterthought in the unlikely event that the tyre-fitter you use planned to replace your bolt-in valves with common-or-garden 'car' snap-in tyre-valves that may prove unsuitable for use on a motorhome. After 6 years, it would probably be no bad thing to replace your present valves with new bolt-in ones, but you may well find that, to allow this to be done, the tyre-fitter would need to order in the replacement valves specially, or you would have to provide them yourself when your Ravenna's tyres are replaced.

[naittaw]

Thanks Derek. As it happens my tyre fitter has already discussed, and ordered, replacement bolt on valves. I have had an experience where an old valve failed, stationery and on site fortunately, but with the common problem of trying to jack up a 3 ton motorhome with a puny scissor jack. I know better now and carry a bottle jack. I've also had the experience of a motorway blow-out when on a previously unused, but elderly, spare. Hence my caution, and better safe than sorry approach, now !

[Brian Kirby]

The March edition of Le Monde du Camping-Car (No 209) has a review of the currently available "Camping" tyres from Michelin (Agilis Camping), Continental (Vanco Camper), and Pirelli (Chrono Camper).

In the same article, there is a recommendation to change the base seal of bolt-in valves when changing tyres.

Regarding when to replace, it seems the French consider tyres good for around 10 years, but recommend they should then be inspected for signs of ageing.  Funny, that!

[Derek Uzzell]

Brian Kirby - 2009-03-03 5:32 PM

 

Regarding when to replace, it seems the French consider tyres good for around 10 years, but recommend they should then be inspected for signs of ageing.  Funny, that!

 

I believe French politicians have a very similar principle for their mistresses.

[Mike Chapman]

Brian Kirby - 2009-03-03 5:32 PM

Regarding when to replace, it seems the French consider tyres good for around 10 years, but recommend they should then be inspected for signs of ageing.  Funny, that!

Hello Brian,And there was me thinking that Michelin were French, what do I know?Michelin technical advice "Tyres should be regularly inspected and changed within 5 to 7 years of manufacture". Even funnier that eh!Regards,Mike.

[roger20]

It seems that Continental are not particularly consistent with their advice.

 

For the 215/70/R15C/109R size and quoted weighbridge weights of 1560kgs front and 1740kgs rear (fully loaded) they suggested 50psi (front) but 65psi (rear).

 

After looking at my previous advice for Michelin Camping and the Tyresafe site I decided to err on the side of caution and run at up to 52 front (reflecting 10% weight transfer loading under braking) and 63 rear. On balance I would have said that 50 & 60 seemed about right but would always have a feel of the tyrewall after any sort of running at speed, particularly on a warm day.

 

My concern was that the rear seemed high and overall I would say that these tyres seem harsher than the Michelin, although this may be (apples and pears) because the Contis are now on the new X250 Fiat whereas the Michelins were on the previous generation Ducato.

[rupert123]

roger20 - 2009-03-05 5:42 PM

 

It seems that Continental are not particularly consistent with their advice.

 

For the 215/70/R15C/109R size and quoted weighbridge weights of 1560kgs front and 1740kgs rear (fully loaded) they suggested 50psi (front) but 65psi (rear).

 

After looking at my previous advice for Michelin Camping and the Tyresafe site I decided to err on the side of caution and run at up to 52 front (reflecting 10% weight transfer loading under braking) and 63 rear. On balance I would have said that 50 & 60 seemed about right but would always have a feel of the tyrewall after any sort of running at speed, particularly on a warm day.

 

My concern was that the rear seemed high and overall I would say that these tyres seem harsher than the Michelin, although this may be (apples and pears) because the Contis are now on the new X250 Fiat whereas the Michelins were on the previous generation Ducato.

 

If you go back to my original post this would seem to be close to what they advised me, although I have since enquired again, no answer yet.

[Brambles]

If those figures quoted are correct for what continental gave you then someone there does not know what they are doing. I suggest they have treated you as a commercial vehicle and allowed for occassional overloading of rear axle as is comon with delivery vans.

 

10% weight shift? Think there is a misunderstanding here. What Michelin do I believe is suggest adding 10% extra pressure to front to allow for weight shift when braking. That is not the same as adding 10% extra weight and working out tyre pressure. The weight shift when braking can be a lot more than 10%.

[TonyHilary]

Hi there, Can anyone advise if C and CP are the same tyres. I have been looking at some tyre pressure tables from an earlier thread and only 215/70R15 CP are quoted.... [ mine are just 215/70 R15 C ] --Autocruise Tempo. Thanks :'(

[Brambles]

No they are not the same. CP is a Motorhome specific tyre whereas the C is not. However this is recent. Some older tyres will only be marked C like the Michelin XC camping and pirelli camping. The later offerings from Continental (Vanco camping) and Michelin (Agilis camping) should be marked CP.

If your tyre is only marked C suffix and does not say camping then it will be a standard van tyre not a camping tyre.

If it says CP but not camping, it will still be a tyre designed specifically for Motorhomes.

 

 

[Derek Uzzell]

As Brambles says, Michelin's recommendation regarding motorhome tyre-pressures is to take the inflation pressure appropriate to a 'static' measured front-axle load and increase that figure by 10%. So, if the recommended pressure were 50psi for a weighbridge-measured front-axle load of 1560kg, Michelin would suggest that you use 55psi. This pressure increase is to offset weight transfer to the front tyres during hard cornering and braking.

 

The "C" or "CP" tyre marking is slightly more complicated than Brambles advises.

 

According to Michelin, the "C" marking shows that a tyre has been designed for use on a "Camionette" (ie. a van or delivery truck) and the English-language equivalent is "Commercial" (eg. a light-Commercial vehicle).

 

Until quite recently there was no agreed standard for tyres aimed particularly at the motorhome market, so even those tyre patterns designed with motorhomes in mind (Michelin XC Camping for example) just bore a "C" marking. Nowadays there is a standard for 8-ply rated motorhome tyres and, if a tyre complies with that standard, it can be marked "CP".

 

Michelin told me "CP" stands for "Camping Pneu". I'm not sure if there is an English equivalent, though I do recall the (bizarre) phrase "Commercial Pressure Warning" being trotted out once on a motorhome forum. As the motorhome-tyre standard seemingly applies only to 8-ply rated (8PR) tyres (ply-rating is an archaic way of indicating a tyre's load-bearing capability), non-8PR motorhome tyres will carry a "C" marking rather than "CP". An example of this is Continental's VancoCamper 215/75 R16 C 116/114R 10PR tyre.

 

(To further complicate matters, I've this vague feeling that Michelin began to mark some of the old XC Camping pattern tyres with "CP" when the new standard was ratified.)

[naittaw]

The Continental web site contains the following in the Camper Tyre section -

 

"As the CP standard acc. to ETRTO is only valid for 8 PR tires, the 10 PR size 215/75R16C marked only as a „C“ tire requires a front inflation with 5.25 bars and a recommended and permitted rear inflation with 6.0 bars. Despite its C marking this tire provides safety reserves like a CP tire."

 

 

[Brambles]

Derek, thanks for clarifying the CP and C markings.

[Patricia]

This thread has begun to worry me as the tyre pressure my husband always used (printed on the door) is much higher. Haven't got the vehicle handy at the moment so can't quote the axle weights but I think the tyre pressure is about 72 and the French tyre fitter put the same in anyway when I had new tyres at two different garages. I have also got Air-rides fitted which I usually keep at about 40. At least I can do those myself if they need topping up - not completely useless !!!

 

Forgot to say I have Michelin XC tyres.

[Brian Kirby]

Patricia

72psi is about 5 bar, which is about what was generally recommended by Fiat/most converters for their vehicles.  There is nothing wrong with using this pressure, it is the chassis/motorhome manufacturer's recommended pressure.  However, it is a pressure designed to be "idiot proof", that inflates the tyres to the maximum (in your case) Michelin recommend, at which pressure they can cope with some degree of overload.  Not all users check the laden weight of their vans, so provision for overload has been deemed wise.  Your tyres will also be more resistant to overheating at this pressure.

However, one consequence - because many users do not load their motorhomes to, or beyond, the maximum - is a rather harsh ride, some reduction in braking performance, and a tendency to wander a little, tending especially to follow irregularities on the road, shy off white lines etc.

To overcome this, if the fully laden vehicle is weighed and the two axle loads individually noted, the tyre manufacturers will advise lower pressures that are, in their opinion, the correct pressures for the actual load being carried.  Because this departs from the standard pressures recommended by the chassis manufacturer it is wise to make sure that the axle loads are recorded on a weighbridge ticket, and that your question to the tyre manufacturer, and their reply, both refer to these loads.

At the "correct" pressure, which will almost invariably be lower, the tyre walls will flex slightly more, allowing a wider contact patch with the ground, with benefits to braking performance and frequently directional stability: i.e. the van will cease wandering where previously it did.  If the difference in pressures is significant, there is also likely to be an improvement to the ride, as the now softer tyres will absorb some of the shocks before passing the remainder to the suspension, and ultimately the van, and you.  Final advantage is that at maximum pressure the tyre will tend to have become more rounded across the tread, with a smaller than desirable contact patch but also increased wear in the centre of the tread.

All that, just for the cost of a trip to a weighbridge!  :-)

[Patricia]

Brian thanks for the good advice; explained excellently as usual.

[duc3auto]

The 4 PSI Rule.

As a general rule for bitumen use the following.

Inflate tyre to recommended (Placard on vehicle)

Then to determine the correct pressure for the given load.

Check and note cold pressure (before driving).

Drive several kilometres.

Check tyre pressure.

If more than 4PSI above cold pressure ADD more air.

If less than 4 PSI below cold preesure DEFLATE.

Reason.  For over inflated tyres there is too much friction which builds up heat.

[Derek Uzzell]

duc3auto - 2009-03-08 5:49 AM

 

For over inflated tyres there is too much friction which builds up heat.

 

Don't you mean "For UNDER-inflated tyres there is too much friction which builds up heat"?

 

Otherwise it seems to conflict with your advice to increase the cold tyre-pressure if this has risen by more than 4psi after driving for several kilometres (presumably because the tyre is over-heating due to being run at too low a pressure), or to decrease the cold tyre-pressure if this has risen by less than 4psi (presumably because the over-inflated tyre has not attained its proper working temperature and is running unnecessarily 'cool').

[Brambles]

duc3auto - 2009-03-08 5:49 AM

The 4 PSI Rule.

As a general rule for bitumen use the following.

Inflate tyre to recommended (Placard on vehicle)

Then to determine the correct pressure for the given load.

Check and note cold pressure (before driving).

Drive several kilometres.

Check tyre pressure.

If more than 4PSI above cold pressure ADD more air.

If less than 4 PSI below cold preesure DEFLATE.

Reason.  For over inflated tyres there is too much friction which builds up heat.

Oh dear oh dear. This is full of misinformation and the whole story. I know you are trying to be helpful and much appreciated as highlights a rule many use around the world.Here is a more detailed explanation........Inflation and the 4 psi ruleTyre pressures vary for every vehicle and caravan depending on weight loads, size etc. An easy do-it-yourself method for checking you have the correct tyre pressure for your vehicle and caravan is known as the "4 psi rule".Firstly you need to inflate all tyres to the pressure recommended by the manufacturer, for a "cold tyre" reading and then tow your caravan for say a distance of 100 klm on bitumen.Upon stopping, immediately recheck your tyre pressure while the tyre is still warm. If your tyre pressure reading is greater than 4 psi from the "cold tyre" reading you took at the beginning of your trip, then the tyres are getting too hot and your starting tyre pressure was too low. Under inflated tyres wear unevenly and lead to increased fuel consumption. They can also impair the handling of the vehicle in the areas of braking and handling. If the tyre pressure reading is less than 4 psi from the "cold tyre" reading you took at the beginning of your trip, then your starting tyre pressure was too high. You will need to run the test again (once the tyre has completely cooled) with less pressure until you find the right balance.It is acceptable for larger 4WD's to have a 6 psi difference between the pressure at the commencement of the trip, and after 100 klm of travel.Remember to use the same accurate gauge for all pressure readings and always carry a gauge with you when possible.Always see your tyre dealer if you have any further concerns or questions regarding tyre maintenance or pressures....So from above we can see 100Km is mentioned not just a few.Also for Motorhome tyres, they are not car or caravan tyres and yet not SUV 4x4 tyres, so is it 4 psi, 6psi or indeed maybe 5psi. So in effect these rules are fine if but have no information, no way of knowing weights etc.Great for the Australian outback!!! There are too many variables which could affect this, temp of tarmac, sun shining on tyres while driving, heat from engine etc, so this could all affect results, and probably most of all speed you drive at. edit - a couple of typos!

[Brian Kirby]

Plus, of course, no clues as to how long to leave the vehicle standing before running the test again, and no indication by how much to increase or lower the pressure to compensate!  On the whole, however well intentioned, not much help and rather dangerous!  Sorry.

[duc3auto]

Apologies it was a typoo that said over instead of under inflated.

All that said I have used this principal for about 15 years. Yes it does change with load but by recording variations in constants like water tank levels, fuel levals etc I have been able to drive the Aussi main ways and byways with only two flat tyres.  I have used this on both a 4x4 and motorhome style Toyota 4.8 tonnes.  The shortest life span on Tyres were a set of Bridgestone 110,000 km. The longest B F Goodrich 153,000 km and a set of Cooper tyres about 98,000 km but then I sold the vehicle with many more km's left on the tyres.  So it might be said that my personal experience using this method has been successful at least in my eyes.  But each to his own.  I just repeat I found that 10 km was more than sufficient to test the difference in pressure and I did check them cold as in before I started the day. 

Agreed that the Australian outback is just a little different to England and Europe but for those in the know Desert temperatures vary from 45 + degrees celcius during the day to minus 15 - degrees at night.  Many of our roads could be considered as potholes surrounded by a bit of bitumen to world class freeways (although not many) as a consequence my tyres have also not always had the smoothest of rides.